Synthesis of Cobalt Based Nanoferrites and Study of Their ... · Dr. Muhammad Maqsood, Director,...

Synthesis of Cobalt Based Nanoferrites and

Study of Their Structural and Conduction

Properties

By

Muhammad Akram

CIIT/FA08-PPH-015/ISB

PhD Thesis

In

Physics

COMSATS Institute of Information Technology

Islamabad-Pakistan

Fall, 2014

ii

COMSATS Institute of Information Technology

Synthesis of Cobalt Based Nanoferrites and Study of

Their Structural and Conduction Properties

A Thesis Presented to

COMSATS Institute of Information Technology, Islamabad

In partial fulfillment

of the requirement for the degree of

PhD Physics

By

Muhammad Akram


Fall, 2014

iii



A Post Graduate Thesis submitted to the Department of Physics as partial

fulfillment of the requirement for the award of Degree of PhD (Physics).

Name Registration No.

Muhammad Akram CIIT/FA08-PPH-015/ISB

Supervisor

_________________________

Dr. Muhammad Anis-ur-Rehman

Associate Professor, Department of Physics,

COMSATS Institute of Information Technology (CIIT),

Islamabad.

February, 2015

iv

Final Approval

This thesis titled



By

Muhammad Akram


has been approved

For the COMSATS Institute of Information Technology, Islamabad

External Examiner:_____________________________________________________

Dr. Muhammad Munib Asim, CSO, KRL, Rawalpindi.

External Examiner:_____________________________________________________

Dr. Muhammad Maqsood, Director, NINVAST, NCP, Islamabad

Supervisor:___________________________________________________________

Dr. M. Anis-ur-Rehman, Department of Physics/ Islamabad.

Head of the Department: __________________________________________

Dr. Sadia Manzoor, Department of Physics/ Islamabad.

Chairman of the Department: _______________________________________

Prof. Sajid Qamar, Department of Physics/ Islamabad.

Dean, Faculty of Science: _________________________________________

Prof. Arshad Saleem Bhatti

v

Declaration

I Mr. Muhammad Akram, Reg. # CIIT/FA08-PPH-015/ISB, hereby declare that I

have produced the work presented in this thesis, during the scheduled period of study.

I also declare that I have not taken any material from any source except referred to

wherever due that amount of plagiarism is within acceptable range. If a violation of

HEC rules on research has occurred in this thesis, I shall be liable to punishable action

under the plagiarism rules of the HEC.

Date: _________________

___________________________

Muhammad Akram


vi

Certificate

It is certified that Mr. Muhammad Akram, CIIT/FA08-PPH-015/ISB has carried out

all the work related to this thesis under my supervision at the Department of Physics,

COMSATS Institute of Information Technology, Islamabad and the work fulfills the

requirement for award of PhD degree.

Date: _________________

Supervisor:

_________________________

Dr. Muhammad Anis-ur-Rehman,

Associate Professor

Head of the Department:

_______________________

Dr. Sadia Manzoor

Department of Physics

vii

I dedicate this thesis to my

parents and teachers

viii

Acknowledgements

All praise to Almighty Allah, the most gracious and the most merciful, whose

blessings are unlimited, and who blessed me with opportunity to pay my contribution

in efforts to explore some facts of HIS created striking and outstanding universe.

Without the help and blessing of my Allah, I was unable to complete my project.

Countless prayers for HIS holy prophet Muhammad (P.B.U.H), who is forever, a

light of guidance and wisdom for all humanity.

Special gratitude to my supervisor Dr. Muhammad Anis-ur-Rehman, for all kinds

of support he has provided me. I am sincerely obliged to him for providing the full

guidance and freedom with respect to the research activities. It was his

encouragement, that the project has been completed. He remains in my favor in all

hard times. I have really no words to express my thoughts for him. God bless him all

the time in every walk of life.

I would like to express my sincere thanks to my supervisory committee members Prof.

Arshad Saleem Bhatti (Dean of Sciences) and Prof. Ishaq Ahmad for their noble

cooperation and guidance. I am also grateful to Prof. Sajid Qammar, Chairman of

the Department of Physics and Dr. Sadia Manzoor Head of the Department of

Physics for providing support. I would especially like to thank Dr. Javed Akhtar from

PINSTECH, for sparing his time for discussion from his busy time schedule during

my visits to PINSTECH.

I am very thankful to all my teachers especially for their cooperation and healthy

suggestions during my study and research work at COMSATS Institute of

Information Technology Islamabad, Pakistan. Higher Education Commission

(HEC), Pakistan is highly acknowledged for providing financial support through

“National Research Program for Universities (NRPU)” and “Indigenous 5000

Scholarship Program”. This scholarship policy made it possible for me to do this

work.

ix

My special thanks goes to my lab fellows Ghulam Hasnain, Ali Abdullah, Ghulam

Asghar, Said Nasir Khisro, Muhammad Mubeen, Anwar-ul-Haq, Yasin Shami, Ms.

Zeb-un-Nisa and Ms. Mariam Ansari for their help in research work. I would also like

to thanks Ms. Fatima-tuz-Zahra, Arsalan Munir and Rafiq-ur-Rehman for helping me

when I felt stuck. I would also like to express my thanks to Muhammad Farooq Nasir,

Mohsin Rafiq and M. Hafeez. I would also like to thanks Mr. Tanveer Baig, Program

coordinator and his colleagues in the Physics Office for their nice cooperation

My deep gratitude goes to my parents and my family members. Without their

continuous encouragement and support it would have been impossible for me to

accomplish all of my study and research work. I am deeply grateful to my son

Muhammad Hassan and daughter Hadia Noor who missed me very much whenever

they wanted me with them.

Muhammad Akram


x

ABSTRACT

Synthesis of Cobalt Based Nanoferrites and Study of Their

Structural and Conduction Properties

Crystal structure and cation distribution at particular sites in crystal lattice play the

primary role in determining the properties of nanocrystalline transition metal oxide

materials. Spinel ferrites are a class of compounds with general formula MFe2O4 (M =

Mn, Co, Ni, Zn, Mg, etc.). The main focus of this research work was the synthesis of

nanocrystalline CoFe2O4 ferrite with different dopant elements like Zn, Mn and Cd

with different ratio by weight. The study of cation distribution in doped Co

nanocrystalline ferrites and dependence of structural and electrical properties on

dopant cations and distribution of these cations in certain interstitial sites in entire

crystal structure, was made. All compositions were synthesized and characterized

under same conditions so that a comparative analysis could be done.

In the first experiment nanocrystalline ferrite particles of Co1-xZnxFe2O4 (x= 0.0 to 1.0

with step of 0.2) were synthesized by co-precipitation synthesis technique. In the case

of CoZnFe2O4 relative concentration of Co and Zn with their particular site occupancy

plays a crucial role in deciding the ultimate material properties. Samples synthesized

at the reaction temperature of 70°C were sintered for 3 hours at 600°C. For the

selection of sintering temperature to have maximum crystallinity, thermal analysis

was done through differential scanning calorimetry (DSC) and thermogravimetry

analysis (TGA) techniques. The FCC spinel structure of the synthesized particles was

confirmed by X-ray diffraction (XRD) patterns. Lattice constants obtained were in the

range 8.36(1) to 8.44(1) Å. The crystallite sizes calculated from the most intense peak

(311) via the Scherrer equation, were found in the range of 10 nm to 35 nm. XANES

spectroscopy was used at Fe, Co and Zn K-edges to examine the cation distribution in

the crystal structure. Dependence of electrical transport properties on shift in crystal

structure due to the successive replacement of Co by Zn in CoFe2O4 was examined.

The dc electrical conduction measurements were taken as a function of temperature

ranging from 313 K to 700 K. Activation energy values (0.518-0.537 eV) indicated the

polaron hopping conduction mechanism. The ac electrical transport properties were

xi

studied by measuring the dielectric constant, dielectric loss tangent (tan δ) and ac

conductivity as a function of frequency. A regular shift in electrical properties is

observed depending upon the cation distribution. Jonscher power law and Maxwell-

Wagner two layer models were employed to investigate the conduction phenomenon.

Manganese substituted cobalt ferrites are promising materials for stress and torsion

sensor applications. Effects of Mn doping on the crystal structure and change in

electrical transport properties with the shift of cation distribution in CoFe2O4were

studied. Co1-xMnxFe2O4(x= 0.0 to 1.0 with step of 0.2) nanocrystallite particles with

stoichiometric proportion were synthesized via co-precipitation method at 70°C of

reaction temperature. The crystalline phase of FCC spinel structure, with lattice

constants in the range 8.36(1) to 8.46(8) Å, were confirmed by XRD patterns. Space

group was found to be Fd3m. The crystallite sizes were found to be in the range from

16 nm to 35 nm. X-ray absorption fine structure (XAFS) spectrometry is an elemental

specific technique and is sensitive to the local crystal structure. X-ray absorption near-

edge structure (XANES) spectroscopy is a prevailing tool for the structural study of

metal oxide materials. XANES spectroscopy is used at Fe K-edges to investigate the

cation distribution in the crystal structure. DC electrical resistivity measurements were

done at different temperatures by means of two-probe method from 370 K to 700 K.

AC electrical properties were also analyzed. Results are explained in terms of polaron

hopping model under the effects of cation distribution. Nyquist plots were done to

determine the equivalent circuits for grains and grain boundary conduction

mechanism.

Co contents were also replaced by Cd successively in CoFe2O4 nanocrystalline

particles. FCC spinel structure with space group Fd3m of Co1-xCdxFe2O4(x= 0.0 to 1.0

with step of 0.2) crystal was confirmed by XRD patterns. XANES spectroscopy was

used at Fe and Co K- edges to find the distribution of cations. The lattice constants

were in the range from 8.36(1) Å to 8.60(1) Å. The variation in dielectric properties

such as dielectric constant, dielectric loss tangent (tan δ) and ac conductivity (σac)

have been observed as a function of composition and frequency. Conduction

mechanism was correlated with cation type and hopping lengths for charge carriers. The

xii

results were discussed in terms of the polaron hopping model under the effects of

cation distribution.

A systematic replacement of Co, in CoFe2O4 ferrites by Zn, Mn and Cd is done.

Structural and electrical properties are correlated and explained. Deep structural

investigation by XAFS is rarely reported. Electrical properties could be controlled by

structure (cation distribution and site occupancy by specific element) for the desired

useful applications. Mn substituted Co ferrites are useful for relatively low resistance

demanding applications while Cd substituted Co ferrites are suggested for high

resistance requiring applications.

xiii

Table of Contents

1 Introduction ........................................................................................................ 1

1.1 Materials at nano length scale ...................................................................... 2

1.2 Ferrites ........................................................................................................ 4

1.2.1 Soft ferrites .......................................................................................... 5

1.2.2 Hard ferrites ......................................................................................... 5

1.3 Types of ferrites .......................................................................................... 5

1.4 Spinel ferrites .............................................................................................. 5

1.5 Crystal structure of the spinel ferrites .......................................................... 6

1.5.1 Tetrahedral A sites ............................................................................... 7

1.5.2 Octahedral B sites ................................................................................ 9

1.6 Classification in spinel ferrites .................................................................... 9

1.6.1 Normal spinel ferrites ........................................................................... 9

1.6.2 Inverse spinel ferrites ........................................................................... 9

1.6.3 Intermediate spinel ferrites .................................................................. 10

1.7 Importance and applications of spinel ferrites ............................................. 10

1.8 Literature review/Background .................................................................... 11

1.9 Aims and objectives ................................................................................... 11

1.10 Thesis Synopsis .......................................................................................... 14

2 Synthesis of ferrite nanoparticles ....................................................................... 19

xiv

2.1 Synthesis of nanocrystalline ferrites ........................................................... 20

2.2 Solid state reaction method......................................................................... 20

2.3 Chemical methods ...................................................................................... 21

2.3.1 Sol-gel method .................................................................................... 22

2.3.2 Spray Pyrolysis ................................................................................... 22

2.3.3 Freeze-drying method ......................................................................... 22

2.3.4 Micro emulsion method ...................................................................... 22

2.3.5 Combustion flame synthesis ................................................................ 23

2.3.6 Hydrothermal method ......................................................................... 23

2.3.7 Chemical co-precipitation method ....................................................... 23

2.4 Influence of different parameters on synthesis by co-precipitation method . 25

2.5 Synthesis of Zn, Mn and Cd doped CoFe2O4nanocrystalline ferrites particles

……………………………………………………………………………...27

Chapter 3.................................................................................................................. 30

3 Characterization techniques ............................................................................... 30

3.1 Thermal analysis ........................................................................................ 31

3.1.1 Differential scanning calorimetry (DSC) ............................................. 31

3.1.2 Thermogravimetry analysis (TGA) ...................................................... 32

3.2 Structural analysis ...................................................................................... 33

3.2.1 X-ray diffraction (XRD) analysis ........................................................ 33

xv

3.2.1.1 Structure determination ................................................................ 35

3.2.2 X-ray absorption fine structure (XAFS) spectroscopy.......................... 36

3.3 Electrical properties ................................................................................... 40

3.3.1 DC electrical properties ....................................................................... 40

3.3.2 Frequency dependent alternating current (ac) measurements ............... 43

4 Thermal, structural and electrical properties of nanocrystalline Co1-xZnxFe2O4

(x = 0.0 to 1.0) ......................................................................................................... 50


4.2 Structural properties ................................................................................... 54

4.2.1 X-ray diffraction (XRD) studies .......................................................... 54

4.2.2 X-Ray absorption fine structure (XAFS) studies .................................. 57

4.3 Electrical measurements ............................................................................. 59


4.3.2 AC electrical properties ....................................................................... 64

4.3.2.1 Dielectric constant ....................................................................... 64

4.3.2.2 Dielectric loss (tan δ) ................................................................... 66

4.3.2.3 AC conductivity ........................................................................... 67

4.4 Summary.................................................................................................... 73

5 Thermal, structural and electrical properties of nanocrystalline Co1-xMnxFe2O4

(x = 0.0 to 1.0) ......................................................................................................... 75

xvi



5.2.1 The X-ray diffraction (XRD) studies ................................................... 78

5.2.2 X-ray absorption fine structure (XAFS) measurements ........................ 80

5.3 Electrical measurements ............................................................................. 82


5.3.2 AC electrical properties ....................................................................... 86

5.3.2.1 Dielectric Constant ....................................................................... 86

5.3.2.2 Dielectric loss (tan δ) ................................................................... 88

5.3.2.3 AC conductivity ........................................................................... 89

6 Thermal, structural and electrical properties of nanocrystalline Co1-xCdxFe2O4

(x = 0.0 to 1.0) ......................................................................................................... 95



6.2.1 X-ray diffraction (XRD) studies .......................................................... 97

6.2.2 X-ray Absorption fine structure (XAFS) measurements..................... 100

6.3 Electrical properties ................................................................................. 102

6.3.1 DC electrical properties. .................................................................... 102

7 Comparison of doping effects .......................................................................... 117

7.1 Thermal studies ........................................................................................ 118

xvii

7.2 Structural changes .................................................................................... 120

7.2.1 Crystal structure ................................................................................ 120

7.2.2 Lattice constant ................................................................................. 120

7.2.3 Hopping lengths ................................................................................ 121

7.3 Occupancy of interstitial sites ................................................................... 122

7.4 DC electrical properties ............................................................................ 123

7.4.1 DC electrical resistivity ..................................................................... 123

7.4.2 Activation energies ........................................................................... 126

7.5 AC electrical properties ............................................................................ 127

7.5.1 Dielectric constant ............................................................................ 127

7.5.2 Dielectric loss (tan δ) ........................................................................ 129

7.5.3 AC conductivity (ac) ........................................................................ 131

8 Conclusions ……………………………………………….…………………….…134

8.1 Summary /Conclusions…………………….………………………….….135

8.2 Future work and recommendations……………………………….......136

9 References……………………………………………………………….......137

xviii

List of Figures

Figure 1.1: Typical materials with dimension in nano length scale [2]……................2

Figure 1.2: Calculated surfaces to bulk atomic ratio (for Fe) [6]……………...........3

Figure 1.3: A spinel unit cell structure showing oxygen atoms, tetrahedral A sites, and

octahedral B [21]. ...................................................................................................... 7

Figure 1.4: (a) Tetrahedral A sites (b) Octahedral B site (c) A complete cubic spinel

unit cell (d) Two alternate sub cells [23] ...................................................................... 8

Figure 2.1: Flow chart of co-precipitation method for the synthesis of Co1-xZnxFe2O4 .

……………………………………………………………………………………….29

Figure 3.1: A model DSC scan ................................................................................ 31

Figure 3.2: Schematic diagram of processes involving DSC .................................... 32

Figure 3.3: Α schematic thermo balance .................................................................. 33

Figure 3.4: Diffraction of scattered X-ray beam from atoms in a crystal .................. 34

Figure 3.5: (a) Energy levels of an electron in the crystal lattice for excitation

energies corresponding the single (EXAFS) and multiple scattering for (XANES) (b)

Corresponding to a free electron outgoing wave and scattered wave interference (A)

X-ray photon absorbing atom, (B) neighboring atom (c) dependence of energy of

absorption coefficient μ (left) in the absence of neighboring atoms (right)In the

presence of scattering from the neighboring atom [98]..……………………………….37

Figure 3.6: A typical XAFS spectrum ...................................................................... 38

Figure 3.7: Schematic of the XAFS experiment ....................................................... 39

xix

Figure 3.8: (a) Two-probe (b) Four-probe resistance measurement circuitry ........... 41

Figure 3.9: Circuitry of the apparatus used for dc electrical resistivity

measurements…. ..................................................................................................... 42

Figure 3.10: (a) Relaxation time (τ) of masses after turning off the applied electric

field. (b) Impedance plane of real capacitor (c) Response of real ε′ and imaginary ε′′

parts of permittivity ............................................................................................... 43

Figure 3.11: General polarization mechanisms ........................................................ 44

Figure 3.12: ε′ and ε″ as a function of frequency and different polarization

mechanisms ............................................................................................................. 46

Figure 3.13: A model complex plane plot for parallel RC equivalent

circuit………………………………………………………………………………..……….49

Figure 4.1(a): The Differential scanning calorimetriy (DSC) of CoFe2O4 ................ 51

Figure 4.1(b): The Differential scanning calorimetriy (DSC) of ZnFe2O4 ................ 52

Figure 4.2(a): The Thermogravimetric Analysis (TGA) of CoFe2O4 ........................ 52

Figure 4.2(b): The Thermogravimetric Analysis (TGA) of ZnFe2O4 ........................ 53

Figure 4.3: X-ray diffraction pattern of Co1-xZnxFe2O4(x = 0.0, 0.2, 0.4, 0.6, 0.8,

1.0)…………………………………………………………………………………...54

Figure 4.4: Change in lattice constants and crystallite size with Zn concentration in

Co1-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0)…………………...…………………...55

Figure 4.5: Change in hopping length between consecutive tetrahedral A sites (LA)

and consecutive octahedral B sites (LB) with Zn concentration (x) in Co1-xZnxFe2O4 (x

= 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)……………..……………………………………………56

Figure 4.6: XANES spectra after normalization at K-edge of Fe (a), K-edge of Co (b),

and K-edge of Zn (c) for Cox Zn1−x Fe2O4. ‘A’ indicates the pre edge peak…...…..58

xx

Figure 4.7: Variation of resistivity ρ (Ω-cm) with change in temperature for

Co1-xZnxFe2O4 .......................................................................................................... 60

Figure 4.8: Variation in dc electrical resistivity at different temperatures of

composition Co1-xZnxFe2O4 (x=0.0,0.2, 0.4, 0.6, 0.8, 1.0) ........................................ 61


Co1-xZnxFe2O4 with linear fit ..................................................................................... 61

Figure 4.10: Variation in activation energy (E) with change of Zn concentration (x)

in Co1-xZnxFe2O4. .................................................................................................... 63

Figure 4.11: Variation of drift mobility µd (cm2/Vs) with change in temperature for

Co1-xZnxFe2O4. ........................................................................................................ 63

Figure 4.12: Effect of Zn concentration and frequency of the applied electric field on

dielectric constants for Co1-xZnxFe2O4. .................................................................... 64

Figure 4.13: Effect of Zn concentration on dielectric constant at different frequencies

for Co1-xZnxFe2O4.................................................................................................... 66


tan δ for Co1-xZnxFe2O4 ........................................................................................... 66

Figure 4.15: Change in tangent loss for all compositions at different

frequencies……..…………………………………………………………………….67


ac conductivity for Co1-xZnxFe2O4 .......................................................................... 68

Figure 4.17: Variation in ac conductivity at different frequencies of composition

Co1-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0)…….…………………………………..69

Figure 4.18: Variation in ac conductivity with respect to power law with variation in

concentration of composition Co1-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0) .............. 69

xxi

Figure 4.19: Nyquist plots for all the samples of Co1-xZnxFe2O4

(x=0.0 -1.0)………….........................................................................................................71

Figure 5.1: The differential scanning calorimetriy (DSC) of CoFe2O4 and

MnFe2O4……………………………………………………………………………..76

Figure 5.2: The thermogravimetric analysis (TGA) of CoFe2O4 and MnFe2O4 ......... 77

Figure 5.3: XRD patterns of Co1-xMnxFe2O4(x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0)............... 78

Figure 5.4: Change in hopping length between tetrahedral A sites (LA) and octahedral

B sites (LB) with Mn concentration (x) in Co1-xMnxFe2O4 ........................................ 79

Figure 5.5: Change in theoretical density (ρx ) and measured density (ρm ) with Mn

concentration (x) in Co1-xMnxFe2O4 ......................................................................... 79

Figure 5.6: Normalized XANES spectra at Fe K-edge for the different composition of

Co1−x MnxFe2O4. Inset is the pre edge peak. ............................................................ 81

Figure 5.7: Variation of dc resistivity ρ (Ω-cm) with change in temperature for

Co1-xMnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0) ........................................................... 82

Figure 5.8: Variation of dc resistivity ρ (Ω-cm) at diffferent temperature for

Co1-xMnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0) ........................................................... 83


Co1-xMnxFe2O4. Lines are the Arrhenius relation fit. ................................................... 83

Figure 5.10: Variation in activation energy (E) with change of Mn concentration (x)

in Co1-xMnxFe2O4 .................................................................................................... 85

Figure 5.11: Variation of drift mobility µd (cm2/Vs) with change in temperature for

Co1-xMnxFe2O4. ........................................................................................................ 85

Figure 5.12: Change in dielectric constant with ln(f) in Co1-xMnxFe2O4.................. .86

Figure 5.13: Change in dielectric constant at different frequencies in Co1-xMnxFe2O4 ..

……………………………………………………………………………………..87

xxii

Figure 5.14: Effect of Mn concentration and frequency of the applied electric field on

tan δ for Co1-xMnxFe2O4. ......................................................................................... 88

Figure 5.15: Variation in dilectric loss (tan δ) at different frequencies of the applied

electric field for Co1-xMnxFe2O4. ............................................................................. 89

Figure 5.16: Effect of Mn concentration and frequency of the applied electric field on

ac conductivity for Co1-xMnxFe2O4. ........................................................................ 89

Figure 5.17: Change in ac conductivity at different frequencies for

Co1-xMnxFe2O4.……………………………………………………………………....90


concentration of composition Co1-xMnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0) ............. 91

Figure 5.19: Nyquist plots for all the compositions for Co1-xMnxFe2O4 (x=0.0, 0.2,

0.4, 0.6 0.8, 1.0) ...................................................................................................... 92

Figure 6.1: The differential scanning calorimetric (DSC) thermogram of CoFe2O4 and

CdFe2O4 .................................................................................................................. 96

Figure 6.2 The thermogravimetric analysis (TGA) thermogram of CoFe2O4 and

CdFe2O4 .................................................................................................................. 97

Figure 6.3: X-ray diffraction pattern of Co1-xCdxFe2O4 ............................................ 98

Figure 6.4: Change in lattice constants (a) and crystallite size with Cd concentration

in Co1-xCdxFe2O4 .................................................................................................... 99

Figure 6.5: Change in hopping length between tetrahedral A sites (LA) and octahedral

B sites (LB) with Cd concentration (x) in Co1-xCdxFe2O4 .......................................... 99

Figure 6.6: Normalized XANES spectra at Fe K-edge for the different compositions

of Co1−x Cdx Fe2O4. Inset shows the pre edge peak heights in enlarged view ........... 101

xxiii

Figure 6.7: Normalized XANES spectra at Cd K-edge for the different composition of

Co1−x Cdx Fe2 O4. Inset shows the pre edge peak heights in enlarged view.. ............ 101

Figure 6.8: Change in dc resistivity with Cd concentration (x) and temperature in

Co1-xCdxFe2O4. ...................................................................................................... 102

Figure 6.9: Change in dc resistivity with Cd concentration (x) at different

temperatures in Co1-xCdxFe2O4. ............................................................................. 103

Figure 6.10: Change in dc resistivity with Cd concentration (x) and temperature in

Co1-xCdxFe2O4. Lines are the Arrhenius relation fit..………………………………103

Figure 6.11: Effect of Cd concentration on activation energyin Co1-xCdxFe2O4 ...... 104

Figure 6.12: Effect of Cd concentration and temperature on drift mobility of

Co1-xCdxFe2O4 ....................................................................................................... 105

Figure 6.13: Effects of Cd concentration and frequency of the applied electric field on

dielectric constants for Co1-xCdxFe2O4 .................................................................... 106

Figure 6.14: Effects of Cd concentration at different frequencies of the applied

electric field on dielectric constants for Co1-xCdxFe2O4........................................... 106

Figure 6.15: Effects of Cd concentration (x) and frequency of the applied electric

field on dielectric loss (tan δ) for Co1-xCdxFe2O4 ................................................... 108


field on dielectric loss factor () for Co1-xCdxFe2O4............................................... 109

Figure 6.17: Effects of Cd concentration (x), at different frequency of the applied

electric field on dielectric loss (tan δ) for Co1-xCdxFe2O4 ........................................ 110


field on ac conductivity (ac) for Co1-xCdxFe2O4 ..................................................... 111

xxiv

Figure 6.19: Effects of Cd concentration (x) at different frequencies of the applied

electric field on ac conductivity (ac) for Co1-xCdxFe2O4 ........................................ 112


concentration of composition Co1-xCdxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0) ............ 113

Figure 6.21: Nyquist plots for Co1-xCdxFe2O4 ....................................................... 114

Figure 7.1: The differential scanning calorimetric (DSC) thermogram of AFe2O4

(A= Co, Zn, Mn, Cd) ............................................................................................. 119

Figure 7.2: The thermogravimetric Analysis (TGA) thermogram of AFe2O4

(A= Co, Zn, Mn, Cd) ............................................................................................. 120

Figure 7.3: Variation in dc resistivity with composition at 423K as a function of

concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). .............................................. 125







Figure 7.7: Variation in activation energy with composition as a function of

concentration x in Co1-xAxFe2O4 (A= Zn, Mn, Cd). ............................................... 127

Figure 7.8: Variation in dielectric constant with composition at 100 kHz frequency as

a function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). ......................... 128

Figure 7.9: Variation in dielectric constant with composition at 1.2 MHz frequency

as a function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). ..................... 129

xxv

Figure 7.10: Variation in dielectric constant with composition at 3MHz frequency as

a function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). ......................... 129

Figure 7.11: Variation in dielectric loss with composition at 100 kHz frequency as a

function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). ............................ 130

Figure 7.12: Variation in dielectric loss with composition at 1.2 MHz frequency as a


Figure 7.13: Variation in dielectric loss with composition at 3 MHz frequency as a


Figure 7.14: Variation in ac conductivity with composition at 100 kHz frequency as a


Figure 7.15: Variation in ac conductivity with composition at 1.2 MHz frequency as a


Figure 7.16: Variation in ac conductivity with composition at 3 MHz frequency as a



concentration of composition Co1-xAxFe2O4 (A=Zn, Mn, Cd) (x=0.0, 0.2, 0.4, 0.6, 0.8,

1.0)………………………………………………………………….….…………….134

xxvi

List of Tables

Table 4.1 Crystallite size d(311), lattice constant (a), Cell volume (V), hopping lengths

LA and LB, X-ray density (ρx), measured density (ρm), porosity (P) drift mobility (µd) at

different temperatures and activation energy E for different values of ‘x’ in

Co1-xZnxFe2O4 .......................................................................................................... 72

Table 5.1: Crystallite size d(311), lattice constant (a), hopping lengths LA (Å) and LB (Å)

Cell volume (V), X-ray density(ρx), measured density (ρm), porosity (P) for different

values of ‘x’ in Co1-xMnxFe2O4................................................................................. 93

Table 6.1: Crystallite size d(311), lattice constant (a), Cell volume (V), popping lengths

LA (Å) and LB (Å) X-ray, density (ρx), measured density (ρm), and porosity (P) for

different values of ‘x’ in Co1-xCdxFe2O4 ................................................................. 115

Table 7.1: Lattice constants “a” in Angstroms (Å) for all compositions

Co 1-xMx Fe2O4 (M = Zn, Mn, Cd & x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)......................... 122

Table 7.2: Hopping Lengths between cations at tetrahedral A sites (LA) in Angstroms

(Å) and between octahedral B sites (LB) in Angstroms (Å) for all sample compositions

Co1-xAx Fe2O4 (A= Zn, Mn, Cd & x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) ........................... .123

xxvii

LIST OF ABBREVIATIONS

XRD X-ray diffraction

DSC Differential Scanning Calorimetry

TGA Thermogravimetric Analysis

XAFS X-ray Absorption Fine Structure

XANES X-ray Absorption Near Edge Fine Structure

EXAFS Extended X-ray Absorption Fine Structure

FWHM Full-width at half-maximum

ρx Theoretical density

ρm Measured density

ρ Resistivity as a function of temperature

Å Angstrom

Ω Ohm

μ Micro

θ Theta

α Alpha

β Beta

ε/ Dielectric constant

γ Gamma

ω Omega

π Pi

δ Delta

σac ac conductivity

tanδ Loss tangent

xxviii

List of Publications

1. Dependence of Site Occupancy and Structural and Electrical Properties on

Successive Replacement of Co by Zn in CoFe2O4

M. Akram and M. Anis-ur-Rehman

Journal of Electronic Materials: 43, 2 (2014), 485-492

2. Structural and Magnetic Properties of Nanocrystalline Mg-Co Ferrites

M. Anis-ur-Rehman, Muhammad Ali Malik, M. Akram, M. Kamran, Kishwar Khan

and Asghari Maqsood

J. Supercond Nov Magn 25, 8 (2012), 2691-2696

3. Structural, Dielectric, and Magnetic Characterization of Nanocrystalline Ni–

Co Ferrites

Kishwar Khan, Asghari Maqsood, M. Anis-ur-Rehman, Muhammad Ali Malik, M.

Akram

J. Supercond. Nov. Magn. 25, 8 (2012), 2707-2711

4. Proficient magnesium nanoferrites: synthesis and characterization

M. Anis-ur-Rehman, Muhammad Ali Malik, M Akram, Kishwar Khan and Asghari

Maqsood

Phys. Scr. 83 (2011) 015602- 015607

5. Microstructures, Electrical and Magnetic Properties of Zn Doped Co

Nanoferrites

M. Akram, M. Anis-ur-Rehman, M. Mubeen and M. Ali

Key Engineering Materials Vols. 510-511 (2012) 221-226

6. Role of Synthesis Parameters on Structural and Electrical Properties of

M-type Hexa-Ferrites

G. Asghar, S. Nasir, M.S. Awan, G.H. Tariq, M. Akram and M. Anis-ur-Rehman


7. Proficient Ni-Zn-Cr Ferrites: Synthesis and Characterization

S. Nasir, M.A. Malik, G. Asghar, G.H. Tariq, M. Akram and M. Anis-ur-Rehman


8. Correlation of Microstructures with Electrical Transport Properties in Mn

Doped Co Nanoferrite Particles

M. Akram, M. Anis-ur-Rehman, S. Nasir and G. Asghar


1

Chapter 1

1 Introduction

2

1.1 Materials at nano length scale

In the technology and science; the prefix ‘nano’ symbolizes a billionth part of a unit.

One billionth (10−9) of a meter indicates a nanometer. An idea about the length of

some typical objects is given in Figure 1.1. Nanoscale is usually referred to the range

beginning from the atomic size to 100 nanometers, and the materials with smallest

dimension below 100 nanometers are called nanomaterial [1]. Study of the materials

at nano length scale is the natural evolution of science exploring the characteristics of

matter with such a small dimension.

Figure 1.1 Typical materials with dimension in nano length scale [2]

One of the basic questions is “what is the size of a material where it begins to act

more like an atom or molecule?” Progression in instrumentation, particularly in

spectroscopy, has allowed examining the matter and phenomena with resolution of

angstrom-level [3]. It is leading towards the profound understanding of nano

structured materials.

Nanoscience probes the objects having size in nano length scale and playing an

imperative role for coming technological era. Nanotechnology is considered as the

technology of designing, manufacturing and application of nanostructures and

devices. Materials with nano length scale reveal quite unusual characteristics as

compared to their bulk counterpart for two reasons [4, 5]. First the large surface to

volume ratio which in turn produces a large fraction of coordinately unsaturated sites

at the surface and second, the quantum effects. The ratio of low coordinated atoms at

the surface increase largely when the size falls less than 35 nanometers as shown in

Figure 1.2 [6]. A nanocrystalline has crystalline order in along with nanoscale length

3

size. A significant feature of nanotechnology is that by having control over chemical

composition, material size and crystal structure, at the nanoscale one can tailor the

material for desired properties [7]. Material properties depend on chemistry and

pattern of crystal structure. Characteristic dimensions where material properties turn

into size receptive are between 1nm and 100 nm. Research on nano structured

materials is intensively studied area nowadays [8, 9].

As per theory of quantum confinement, holes of valence band and conduction band

electrons are restrained spatially by surface potential barrier. Regarding electronic and

optical characteristics, nano solids show the subsequent properties different from their

bulk counterpart. Spontaneous contraction occurs in chemical bonds in surface atoms

which causes the increase in binding energy. Properties such as dielectrics get

changed with the reduction of particle size because when the band gap expands the

energy levels of the core bands shift towards higher binding energy [10]. For the

materials with band gap lying in the visible spectrum, a modification in band gap with

change in size causes the variation in color. Magnetic properties of materials such as

Ni, Fe, Co, and Fe3O4, etc are size dependent [11]. The ‘coercive force’ or magnetic

Figure 1.2 Calculated surfaces to bulk atomic ratio (for Fe) [6]

4

memories have size dependence. In recent years nanocrystalline ferrite materials are

gaining interest for their diverse technological applications.

1.2 Ferrites

Ferromagnetic materials which are primarily composed of ferric oxide (Fe2O3) are

known as “ferrite materials.” The ferrites history began centuries prior to the Christ

birth with the finding of stones having attraction to iron. The vast deposits of these

stones were found at Magnesia in Asia Minor. These were used initially as pigments

in paints at some stage in the Paleolithic. Later in China these were used as the

magnetic compass. Ferrites have the general chemical formula MeFe2O4, owning the

structure of the mineral spinel, MgAl2O4. Here Me stands for a divalent metal ion

having an ionic radius ranges from 0.6 to 1Å. For simple ferrites, Me may be the

Co2+, Mn2+, Cu2+, Zn2+, Fe2+, Ni2+, Mg2+ or Cd2+. In case of mixed spinel ferrites the

combination of these transition metals is also possible. This substitutional potential

has led to the broad technological utilization of ferrites [12].

Ferrite crystal structure can be considered as the interconnected arrangement of

divalent oxygen anions and metal cations [13]. Even though the majority of ferrites

include iron oxide however there exist some ferrites based on Cr, Mn and some other

elements. Although Cr and Mn did not belong to ferromagnetic family of elements but

by the grouping with elements such as oxygen and with different metal ions, they can

perform as magnetic ions. Ferrites are examined broadly due to owing high electrical

resistivity as compared to the metals. Ferrites have high values of resistivity ranging

from~102-cm to 1011-cm at room temperature depending upon their chemical

composition [14]. Another important factor, which is of considerable importance in

ferrites and is completely insignificant in metals, is porosity. Because of possessing

small eddy current losses with high electric resistivity these materials have better

properties as compared to the other magnetic materials. Ferrites also show very useful

dielectric properties. It grants them an advantage over the nickel, iron and other

transition metals. Depending upon the magnetic properties, ferrites are divided as

"soft" and "hard" which refers to their low or high coercivity.

5

1.2.1 Soft ferrites

Soft ferrites have low coercivity (Hc< 10 A/Cm) and reveal ferrimagnetic behavior

[15]. There is a net magnetic moment which is caused by two sets of unpaired inner

electron spin moments in opposite directions which do not terminate each other.

Ferrites containing manganese, nickel or zinc compounds are used in transformer or

electromagnetic cores. These materials are widely used in inductors, RF transformers

and in the Switched-Mode Power Supply (SMPS) as a core because of having

relatively low losses even at high frequencies [16].

1.2.2 Hard ferrites

In contrast to soft ferrites hard ferrites have high coercivity (Hc> 300 A/Cm) [15].

These materials are composed of barium, iron and strontium oxides. After

magnetization hard ferrites retain high remanence. They have large magnetic

permeability and can conduct magnetic flux very well after magnetic saturation. This

is why ceramic magnets can store strong magnetic field. In radios these magnets are

used most commonly [16].

1.3 Types of ferrites

Ferrites are generally divided in three groups for their crystal structure foundations

Spinel ferrites (Cubic ferrites)

Garnets (Ortho ferrites)

Hexa ferrites (Hexagonal ferrites)

1.4 Spinel ferrites

The word spinel originates from Italian “spinella”, diminutive of spine. These are also

called cubic ferrites and most of the spinel compounds have space group Fd3m.

Spinel ferrites make a great group of oxides which acquires the structure of the natural

spinel MgAl2O4[17]. Several of the technologically important spinels are synthetic,

however one of the most significant and perhaps the oldest with useful applications

the magnetite Fe3O4, is a natural oxide. Their presence in abundance indicates the

6

stability of spinel crystal structure. Spinels are mostly ionic. The meticulous site

occupancy by cations is influenced by many factors such as covalent bonding effects

and crystal field stabilization energies of transition-metal cations. Several different

types of cation grouping may exist in a spinel structure with a net charge eight to

equalize the anion’s net charge. Spinel ferrites make the most widely useable ferrites

group [18].

1.5 Crystal structure of the spinel ferrites

Bragg and Nishikawa determined spinel structure for the first time in 1915 [19, 20].

The spinel unit cell is composed with two kinds of alternately repeating sub cells in

the three dimensional arrays. Eight such sub cells form a complete repeating unit cell.

Spinel ferrites have a cubic close-packed arrangement of oxygen anions with divalent

metal cations and trivalent Fe3+ at two different kinds of crystallographic sites called

tetrahedral sites (A sites) and octahedral sites (B sites) respectively as given in Figure

1.3 [21]. The formula can be written as M8Fe16O32. Total 96 interstices are formed by

oxygen anions in the unit cell, 64 A sites and 32 B sites.

The cubic unit cell with spinel crystal structure ferrites is shaped by 56 atoms, 32 of

oxygen atoms spread in a cubic close packed configuration (CCP) whereas 24 metal

cations are distributed among 8 A sites and 16 B sites. Total available A sites are 64

but only 8 are occupied while available B sites are 32 from which only 16 are

occupied for charge neutrality [22]. Depending upon the distribution of cations the

structural relation for a spinel structure could be (Me2+δ Fe3+

1-δ) A [Me2+1-δ Fe3+

1+δ] B O4

where Me stands for a divalent metal cation and δ for inversion parameter. The ideal

molar ratio of trivalent to divalent ions is 2:1.With reference to cation allocation

among these A sites and B sites, spinel structure can be normal, inverse, or partially

inverse. For complete normal spinel δ =1, for complete inverse δ = 0 and for mixed

ferrite δ ranges between these two extreme values [21].

7

1.5.1 Tetrahedral A sites

All interstitial sites in the spinel ferrites are not same; sites which are called A sites

are coordinated by 4 nearest neighboring oxygen ions. Hence A sites are called

tetrahedral sites. There are 3 atoms in a plane touching each other and the fourth atom

sits on top with symmetrical position to generate a tetrahedral site. Only 8 out of 64 A

sites are occupied to maintain the charge neutrality in the crystal system (Figure 1.4)

[23].

Figure 1.3 A spinel unit cell structure showing oxygen atoms, tetrahedral A sites,

and octahedral B sites [21].

8

Figure 1.4 (a) Tetrahedral A sites (b) Octahedral B site (c) A complete cubic spinel unit

cell (d) Two alternate sub cells [23]

9

1.5.2 Octahedral B sites

The interstitial sites formed by the coordination of 6 nearest neighboring oxygen ions

in cubic spinel crystal structure are called octahedral B sites. The oxygen ions around

the octahedral B site occupy the corners of an octahedron (Figure 1.3 b). Four atoms

out of 6 lies in a plane whereas each one of other 2 lies above and below of the plane

in the symmetrical position. Out of 32 octahedral B sites only16 are filled to maintain

the charge neutrality with in a spinel structure (Figure 1.4) [23].

1.6 Classification in spinel ferrites

On the basis of cation allocation at interstices the spinel ferrites are placed in three

groups.

Normal spinel ferrites

Inverse spinel ferrites

Intermediate spinel ferrites

1.6.1 Normal spinel ferrites

In normal spinel ferrites, B sites are filled by only one type of cations. In these ferrites

the A sites are occupied by the divalent cations while the trivalent cations lie at B

sites. In order to represent the ionic allocation; square brackets are used to indicate the

B sites. ZnFe2O4 is a typical example of normal spinel ferrites [22].

1.6.2 Inverse spinel ferrites

In these ferrites A site is occupied by half of the trivalent metal ions and other half of

trivalent metal ions lies at B sites. All the divalent metal ions lie on octahedral B sites.

The formula (Me3+)A [M+2Me3+]BO4 represents these ferrites. Fe3O4 has a typical

pattern of inverse spinel ferrites where divalent cations of Fe occupy the B sites [22].

10

1.6.3 Intermediate spinel ferrites

When the distribution of metal cations is intermediate between normal and inverse

conditions, the ferrites are considered as mixed spinel. Generally this condition can be

expressed as (Mδ2+Me1-δ

3+)A [M1-δ2+Me1+δ

3+]B O4. Where δ stands for inversion

parameter. Quantity δ depends on the synthesis method and nature of the constituents

of the ferrites. For complete normal spinel δ =1, for complete inverse δ = 0 and for

mixed ferrite, this δ ranges between 0 and 1. If there is unequal number of each kind

of cations on octahedral sites, the spinel is called mixed spinel. Typical examples of

mixed spinel ferrites are MgFe2O4 and MnFe2O4 [22].

1.7 Importance and applications of spinel ferrites

Ferrites make a large group of oxide materials with amazing combination of electric

and magnetic properties. They have a vast field of applications covering a notable

range from circuitry of millimeter wave integrated to handling power. Ferrites

comprise two important aspects, magnetic phenomena along with ceramic

microstructures [24]. Applications of ferrites materials are based on the very

fundamental properties of ferrites, a considerable saturation magnetization and high

values of electrical resistivity, low eddy current electrical losses, and high chemical

stability. The synthesis of ferrites can be done by several methods. The possibility to

produce a vast number of solid solutions opens the path for tailoring their properties

for a number of applications. In several applications, ferromagnetic metals cannot

substitute ferrites and for many other applications often ferrites compete metals for

cost-effective reasons [25, 26]. The opportunity of ferrites synthesis in the form of

nanoparticles has unlocked a new research field, with innovatory applications not only

in the electronics industry but also in biotechnology. Here are some important features

making ferrites most widely used material [27].

Useful materials for high frequencies applications

Microwave frequency applications

Mechanical strength

Durability (time and temperature stability)

Wide range of material selection

11

Useful dielectric properties

Low cost materials and also low cost of synthesis

On the surface, the uncompensated spins produce a phenomenon known as “exchange

bias” this property is used in sensors for magnetic field. There is also a great

possibility for magnetic nanostructures that electric field can affect their magnetic

order. It is called “current-induced magnetization dynamics”. Cobalt is well known

because of its ability to reduce magnetic losses at high frequencies. Among spinel

ferrites, cobalt ferrite CoFe2O4 is especially interesting because of its applications in

biotechnology [28], magnetic storage appliances [29] and in high frequency devices

[30]. CoFe2O4 has high cubic magneto crystalline anisotropy, high coercivity and

moderate saturation magnetization. Cobalt ferrite nanocrystalline particles as a photo-

magnetic material show interesting light-induced coercivity change [31]. Many

biotechnological applications have been developed depending upon biogenic and

synthetic nanoparticles [32]. In magnetic resonance imaging (MRI), super

paramagnetic particles are selectively associated with healthy tissues because these

particles change the rate of proton decay from the excited to the ground state.

Consequently, a disparate and darker contrast is obtained from these healthy regions

of tissue [33, 34]. Thermal energy obtained from hysteresis loss of ferrites is used in

heating of specific tissues for treatment of cancer (hyperthermia) [35]. Grain

dimensions, chemical composition, crystallinity, and cations allocation play an

influential role in defining the properties of nano ferrite particles. Furthermore, the

structural, magnetic and electrical properties of ferrites are very much sensitive to

synthesis conditions.

1.8 Literature review/Background

An active research area in physics, materials engineering, chemistry and in

biomedical engineering is the exploitation of nanocrystalline materials. A striking

significance is the tunability of the ultimate properties by miniaturizing the materials.

Due to the accessibility of new and precise techniques for preparation and

characterization of nano ferrite particles, the research has got a boost in present years

[36-38]. The nanocrystalline spinel ferrite materials have unusual characteristics as

12

compared to bulk due to large fraction of atoms which lies at the surface. Surface has

a large number of defects as all the atoms present at the surface are under-

coordinated.

Different features of processing, effects due to additives and properties and

applications of soft ferrites have been explored by several researchers [39-41]. To

explore the useful properties of spinel ferrites materials for different applications

many researchers have done a large quantity of work. A number of synthesis methods

have been employed up till now to produce nanocrystalline materials, like as

mechanical alloying [42], co-precipitation [43, 44], citrate precursor [45],

hydrothermal [46], sonochemical [47], sol-gel [48], shock wave [49], forced

hydrolysis in a polyol [50], reverse micelle [51], and even by the use of egg white for

aqueous medium [52].

In spinel ferrites trivalent and bivalent cations are located at octahedral sites and

tetrahedral sites which exist in close packing crystal structure of oxygen anions.

Properties like catalytic, electric and magnetic could be influenced significantly by the

variation in distribution of cations between these interstitial sites 52]. Several probes

such as Mossbauer spectroscopy, X-ray and neutron diffraction have been employed

to collect the information about cation distribution [53]. Nevertheless, the XRD has

limited effectiveness due to the analogous scattering factors for Fe and Co and also

due to the peak broadness for nano dimensions of the particles [54]. Nano crystals of

ferromagnetic zinc ferrites have been synthesized by thermal decomposition of metal

surfactant complexes at ambient temperature [55]. It has been shown that properties

of ZnFe2O4 depends upon particle size while the electrical conductivity is temperature

dependent.

Electrical transport properties of Mn-Zn ferrites have been examined by Ravinder et

al. [56]. Co1-xMnxFe2O4 was synthesized by Lee et al. as a bulk phase in air[57]. With

increasing contents of Mn lattice constant increases which is directly related to the

Mn2+ cation doping. The electrical conduction properties of Ni ferrite nanoparticles

with Zn substitution have also been considered by Islam et al. [58]. The magnetic

performance of ZnFe2O4 has drawn a lot of interest and remains a subject of serious

studies. ZnFe2O4 synthesized by co-precipitation route has been investigated and has

13

been verified that ZnFe2O4 could be used as gas-sensing material which has a high

value of sensitivity and better selectivity for C2H5OH. Lu et al. has reported recently

the magnetic and electrical transport properties of ZnxFe3-xO4 (x= 0.0 to 1.0) nano

crystalline ferrites synthesized via sol-gel method [59]. Saturation magnetization was

found to increase first until x=0.2 then diminish with further increase in x i.e. Zn

concentration. The dependence of the Curie temperature (TC) upon the x was also

investigated. Nissato and Yamamoto [60] studied the physical and magnetic

properties of Co spinel ferrite particles with NiO substitution doping. They have

reported that Co-Ni spinel ferrite fine particles with single-phase were achieved

without post annealing by co-precipitation method. Nanoparticles of cobalt ferrites

with lanthanide ions as dopant element CoLn0.12Fe1.88O4 were synthesized by Khan

and Zhang using oil in water micelle method [61]. Magnetic properties were

modulated due to lanthanide ions doping. A considerable change in coercivity and

blocking temperature was observed. Nanoparticles of cobalt ferrite were synthesized

by Li et al. in water and oil by using microemulsions reverse micelles by changing

composition of cations [62]. Particle size was estimated by electron microscopy and

was found to be in the range of 12 - 18 nm. XRD studies revealed that at small

Co2+:Fe2+precursor ratio 1.10 and 1.5 in the precursor, the formed particles maintain

the structure γ - Fe2O3. But with the further increase in the Co2+ contents CoFe2O4 is

formed.

Wei et al. synthesized NiMnxFe2-xO4, (x = 0.0 to 1) spinel ferrite by ceramic method

[63]. The allocation of cations was analytically calculated for the first time by XRD

with computer computation. It was shown that both Mn3+ and Ni2+ ions take up

octahedral sites predominantly with rise in manganese ion. It was due to their

preference for large site energy in octahedral coordination. Han et al. synthesized

cubic spinel with single phase Cu0.5-xNi0.5ZnxFe2O4 (x = 0.0 to 0.5) by

co-precipitation method [64]. By increasing Zn contents saturation magnetization was

found to increase. A quantum mechanical method, magnetic moments fitting, was

adopted to estimate the cation distribution. For XRD patterns the Rietveld fitting was

made by using the cation distribution thus obtained by quantum mechanical method

and was found reasonable. Mohan et al. studied cation distribution in CoFe2O4 by

Raman spectroscopy and Mossbauer method [61]. Particle size was in range 6 nm to

14

500 nm. Studies were done at room temperature and also at higher temperature. The

dependence of cation distribution on temperature and size was confirmed

conclusively. Akhtar et al. synthesized perovskite based SrFe0.5Nb0.5O3 and SrFeO3

materials by solid state reaction method. Investigations for structural studies were

done by XAFS and XRD techniques [65]. It was demonstrated by XAFS data that

SrFeO3 has valence state “4” but it converted in to “3” with the substitution of 50%

Nb5+ at Fe site. Yao-Jen Tu et al. used XANES to study the oxidation state of As(III).

The XANES analysis of K-edge proved that the As (V) adsorbed on ferrite was not

converted to toxic As (III) by Fe2+ in entire ferrite structure [66]. Kravtsova et al.

studied the electronic and atomic structure of nanoclusters of free niobium on the

basis of XANES [67]. Trail et al. investigated the potential of zircon for continuous

recording and the study of conditions evolving in magmatic redox was done by

quantifying Ce valence in synthetic crystals and in natural by XANES [68]. Bilovol et

al examined the Electronic structure of firstly quenched ribbons of composition

NdyFe(86-y-x)B14Tix (x = 0, 2, 4; y= 7,8 at %) by X –ray absorption (Fe and Ti K-edge)

was studied by X-ray absorption (Fe and Ti K-edge) and X-ray photoelectron

spectroscopy [69].

Exact determination of site occupancy in these complicated systems is currently the

topic of interest. Determination of precise structural information is the key to the

controlled properties particularly the electrical response, which is the main aim of the

current research.

1.9 Aims and objectives

Among ferrites, spinel ferrites have the most exciting crystal system. These materials

are especially interesting for addressing the basic correlation between material

properties and crystal structure. Crystal chemistry of any material reveals how we can

link together the chemical composition, internal crystal structure, microstructures and

ultimate physical properties of that material [70, 71]. The spinel ferrites have

advantage over other electromagnetic materials due to their inherent high resistivity

which results in low losses over wide frequency range and high thermal stability for

wide temperature range.

15

CoFe2O4 having spinel crystal structure is a distinguishing ferrimagnetic oxide

material. Being a magnetic material, with a Curie temperature (Tc) about 793 K,

CoFe2O4 is well known to have large magnetic anisotropy. It has good chemical

stability, mechanical hardness and moderate saturation magnetization. For several

years, CoFe2O4 has been investigated for fundamental research and potential

applications as well. As a potential predominant magnetic and electrically resistive

material, it has been applied in high-density magnetic recording media, high

performance electromagnetic and spintronic devices. Cobalt fine powders and films

have attracted considerable attention for their wide range of technological applications

such as transformer cores, recording heads, antenna rods, memory, ferrofluids,

biomedical application and sensors. Cobalt ferrites have a strong crystal field. The

useful properties of the spinel ferrites are firmly connected to the allocation of cations

at interstitial sites (A and B sites) in the spinel crystal system, internal crystal structure

and synthesis method. Also when materials are synthesized at nano length scale the

properties are governed by the size [72]. Crystal structure of spinel ferrites offer

opportunity to tailor their properties by specific cation doping at particular interstitial

crystal sites. Control over the cation distribution and crystal structure provides a way

to tailor the properties. So we selected cobalt ferrites as a base material and then it

was doped with Zn, Mn and Cd. Zn, Mn and Cd has considerably larger ionic radius

as compared to Co. A certain variation in the crystal system and crystal field was

expected to occur. So we were aiming to synthesize the many compositions of doped

nanocrystalline cobalt ferrites with certain dopant elements like Zn, Mn and Cd.

Therefore a range of compositions of Co ferrites with these dopant elements was

synthesized by co-precipitation method and explored for variation in crystal structure

and electrical conduction properties. A number of synthesis methods have been

explored over years. But co-precipitation method is one of the mostly used synthesis

route.

CoFe2O4 is considered to have inverse spinel structure whereas ZnFe2O4 and CdFe2O4

are known to have normal spinel structure [22]. MnFe2O4 is known for having partial

inverse spinel structure. So we replace Co from CoFe2O4 gradually with Zn, Mn and

Cd to observe the change in crystal structure for the whole range of transformation

from inverse spinel to normal spinel structure. Mostly these materials have been

16

studied for their magnetic properties but sequential studies for electrical properties of

these materials are rare in literature. We were aiming to investigate the dependence of

structural and electrical properties of our synthesized materials on many aspects

including dopant type, dopant concentration and cation distribution in particular

crystal system of spinel ferrites.

In reality the electrical properties of these materials are more complicated than an

ideal situation because cationic distribution over A and B sites vary with chemical

composition of compounds. A precise study of the cationic distribution in the entire

crystal structure is of great importance in understanding the material properties. There

are reports in the literature about the cation occupancy depending on mathematical

calculations based on Mossbauer spectroscopy and XRD [70]. In finding the local

surroundings of Fe ions Mossbauer spectroscopy is efficient but it could not give

information for Co cations. It is also not much effective in case of dilute samples [73,

74]. For the manipulation and understanding of material properties, it requires a

precise study of site occupancy and cation distribution. Therefore, we make use of

XAFS spectroscopy because it has revealed itself the most capable technique for

examining the allocation of cation sites in nanocrystalline particles of doped cobalt

ferrites. In the XANES region the absorption modulations are powerful because of

manifold scattering [75-78]. The shift in edge position can elaborate the oxidation

state [79]. The electrical properties of these materials strictly depend on the entire

crystal structure and cation allocation within the crystal structure. Therefore a precise

study for cation distribution in the crystal structure is carried out. Finally the DC and

AC electrical conduction properties of the prepared materials have been investigated

in this research work. It helped us in finding the dependence of electrical conduction

properties on the cation allocation and the entire crystal structure.

1.10 Thesis Synopsis

Thesis consists on an introduction to nanotechnology, synthesis techniques,

characterization techniques, characterizations and discussion of results. Chapter wise

detail is given as under.

17

Chapter 1:

The introduction to the nanoscience, a brief history of ferrites, crystal structure of

spinels and objectives of this research work are discussed. Very inserting crystal

structure and wide range of applications motivates for this research work.

Chapter 2:

Synthesis method plays a very important role in deciding the final material properties.

The fundamental requirement from the synthesis route is to have phase pure material

with desired properties. Synthesis methods can be categorized on the bases of

technique followed. For synthesis of materials at nano length scale there are two main

approaches i.e. bottom-up and top-down approach. Wet chemical methods provide

better options for synthesis of nano ferrites materials. In this chapter different

synthesis techniques have been discussed. Detailed procedure of preparation of

ferrites sample material by co-precipitation method for this research work is discussed

in this chapter.

Chapter 3:

The progress in scientific research based upon the characterizing tools. Every material

has its own specific physical and chemical properties. There are many experimental

techniques to investigate different properties of cobalt based nanocrystalline ferrite

materials. In this chapter the brief introduction to experimental tools and techniques

is given. Also the theories and formulae used in evaluating the results are given.

Chapter 4:

In this chapter the Zn doped CoFe2O4 has been discussed. Zinc is recognized for

playing a vital role in deciding the final properties when it is used as a dopant element

in ferrites [41].By using co-precipitation method nanocrystalline ferrite particles of

Co1-xZnxFe2O4 with x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 were synthesized. Synthesized ferrite

particles were investigated very precisely for their thermal, structural and electrical

properties. XANES spectroscopy was used to study the site occupancy and cation

distribution.

18

Chapter 5:


sensor applications. The study the cobalt based nanoferrite particles with Mn as a

dopant element with different ratio by weight is presented in chapter 5. Synthesized

samples were characterized to study the effects of successive replacement of Co with

Mn on cation distribution, structural and conduction (electrical) properties. A very

precise technique (XANES) was applied to study the cation shift.

Chapter 6:

CdFe2O4 is generally understood as normal spinel in which all Fe3+ ions occupy B-

sites and all Cd2+ ions occupies A sites. The gradual replacement of nonmagnetic Cd

ions in cobalt ferrite is expected to create considerable distortion in (CoCd)Fe2O4

system. In chapter 6 the study of thermal, structural and electrical properties of

Co1-xCdxFe2O4(x = 0.0 to 1.0) is presented.

Chapter 7:

We have replaced Co from the CoFe2O4 gradually by three different metal cation ions.

The variations in the different properties on account of dopant type and dopant

concentration were observed. A comparison of results thus obtained is given here in

this chapter

Chapter 8:

Conclusions about the results obtained from this research work are given in this

chapter.

Chapter 9:

References and citations are given here in the chapter

19

Chapter 2

2 Synthesis of ferrite nanoparticles

20

2.1 Synthesis of nanocrystalline ferrites

An overlook of the scientific literature plainly signifies that the synthesis is the critical

step in the preparation of a material with desired properties. Components of the

modern age devices need increasing control over the characteristics of materials for

their particular applications. The ultimate material properties deeply depend upon

preparation method. Also the microstructures such as crystallite size distribution,

aggregates, porosity, vacancies, morphology, stresses and defects etc. plays a vital

role in deciding the final performance. These microstructures are mostly affected by

the synthesis route and post synthesis treatments. Moreover, the synthesis technique is

of fundamental importance for the addition of specific chemical elements in the

conventional compounds to modify the properties as per need. There are two main

approaches to synthesize materials at nano length scale [76]:

Top-down technique

Bottom-up technique

Top-down technique involves reduction of size by mechanical process like milling

and grinding. It is a traditional way to manufacture nano materials and have been

employed to produce nanoparticles of minerals for example coal, clay, and metals.

Bottom-up synthesis technique initiates at the molecular stage to generate

nanoparticles. Nano ferrite particles may be produced by a range of methods using

solid, liquid and gas phase processes. These techniques may be classified as

i. Solid state reaction method (Dry method)

ii. Chemical methods (Wet methods)

2.2 Solid state reaction method

Solid state reaction method is a conventional route for the synthesis of ferrites

particles [68]. However, this method is often slow and difficult to carry out to

completion unless performed at high temperatures typically above than 1100 °C, so

that reacting atoms can diffuse through solid materials to the reaction front. For this,

the precursors of metal oxides or carbonates in their stoichiometric proportions are

21

mixed and ground for having homogeneous mixture. This powder is kept at elevated

temperatures for long time. The benefits of solid state reaction method is low cost of

synthesis at industrial scale but short comings of this synthesis route includes the

deprived chemical homogeneity, large grain sizes and broad size division and often

undesirable secondary phases. To surmount these confines, milling and calcinations

are done repeatedly. However, these repeated cycles of milling introduce considerable

quantity of impurities. Therefore, ultimate properties are compromised. Hence,

chemical synthesis routes are now gaining attention.

2.3 Chemical methods

Presently, particular focus has been given to unconventional synthesis routes [80, 81].

Especially wet chemical preparation methods have some certain advantages for

producing fine ferrite nanoparticles. Wet chemical methods have potential to attain

better chemical homogeneity at the molecular level. It is particularly, significant for

electro-ceramic materials because their properties are usually modified by addition of

a precise amount of particular dopant elements. Moreover, several chemical synthesis

routes have ability to produce powder with nano sized grains which could be sintered

at lower temperatures [82, 83]. A basic knowledge of thermodynamics, crystal

chemistry, reaction kinetics and phase equilibrium is vital for obtaining the advantage

of various benefits of chemical synthesis to form new desired materials. There are

different chemical methods for synthesis of ferrites among them a few are described

below.

i. Sol-gel method

ii. Spray pyrolysis

iii. Freeze-drying method

iv. Micro emulsion method

v. Combustion flame synthesis

vi. Hydrothermal method

vii. Chemical co-precipitation method

22

2.3.1 Sol-gel method

It is also known as chemical solution deposition method [80]. In sol-gel technique,

metal oxides are prepared via hydrolysis of reacting precursors then their hydroxides

are produced. The removal of water condensation of hydroxide molecules causes the

development of network of metal hydroxide. By linking of hydroxide species in a

network structure, a dense porous gel is obtained. Finally solvents are removed by

heating which gives a fine powder of the metal oxide [73]. Microstructure like

porosity and crystallinity depends upon the pH, chemical composition and the rate of

removal of solvent. Templates or surfactants are not required in this method [84].

2.3.2 Spray pyrolysis

In this process droplets of aerosol are produced by atomization of the starting

suspension or solution. Evaporation of these droplets occurs by the condensation of

solute within these droplets. Drying of the precipitates with heat treatment gives

porous particles and then by sintering we can achieve dense material. It is a useful

synthesis route for having homogenized pure nanoparticles. However, a large quantity

of uses of solvent increases the cost.

2.3.3 Freeze-drying method

In synthesis by this technique, fine droplets are produced from concentrated solution

and then freezing of these droplets is done by keeping them in a bath at very low

temperature like as ice-acetone or liquid nitrogen. These particles are dried by ice

sublimation in vacuum.

2.3.4 Micro emulsion method

This is one of the famous techniques for the synthesis of nanoparticles [85]. Water

and oil remains in two separate phases on their mixing together. Energy is required

for the combination of these two phases because of interfacial tension between the

water and oil (30-50 dynes/cm). To overcome this, surface active molecules are used

which are called surfactants. It is a simple technique as compared to the normal

emulsion because higher shear conditions are not needed here. Lipophilic (oil-loving)

23

and hydrophilic (water loving) moieties both are present in surfactants. These

surfactant molecules form an interface linking the oil and water by reducing the

interfacial tension [77]. The continuity of phase depends upon the nature of surfactant.

Emulsions are stable kinetically, but are unstable thermodynamically.

2.3.5 Combustion flame synthesis

In this route of synthesis for precursor decomposition the energy is supplied with

burning of precursor by air-fuel mixture. The suitable precursor (gas/vapor) is mixed

with a fuel like methane or acetylene and oxygen. By keeping the temperature

constant this mixture is burnt in a flame. Temperature must be kept constant because

variation in temperature can cause the growth of particles with different size. Nano

particles are formed with the decomposition of precursors.

2.3.6 Hydrothermal method

Hydrothermal is a useful synthesis technique to produce nanostructures [86, 87]. In

this method the synthesis is carried out at high pressure and temperature. Synthesis is

done within a closed container having water as a solvent. Precursors in form of

chlorides or nitrates are typically dissolved in a solvent and placed in an autoclave.

High pressure and temperature is produced inside the autoclave by external heating.

Heating is done by a conventional oven or microwave. The reaction occurring in such

conditions of high pressure and temperature is recognized as a hydrothermal reaction.

A liquid having temperature above of its critical point is known as supercritical and

can acts like both gas and liquid. At this stage the surface tension becomes very low

and it can dissolve the compound efficiently. To increase the reaction rate we can use

electric field, ultrasonic or microwaves. This method has some disadvantages such as

large distribution of particle size, not appropriate for every material and non-

expectable morphology of the material thus produced.

2.3.7 Chemical co-precipitation method

A famous technique for the preparation of fine ferrite nanoparticles is precipitation of

solid particles in a solution. Common process involves simultaneous occurrence of

24

reactions, nucleation, growth and agglomeration process [88]. Heterogeneous or

homogeneous nucleation causes the formation of precipitates. Nuclei formation

generally proceeds by diffusion. Growth rate of the particles is defined by the

concentration of reactants and reaction temperature. For having unagglomerated nano

ferrite particles along with a very small size distribution, nucleation must start at the

same time. And without any particle agglomeration, subsequent growth must occur. In

general the reaction kinetics can affect the physical properties like crystallinity,

microstructures, particle size and size distribution [89]. Furthermore, the pH and rate

of mixing of the solutions are also important.

From synthesis methods discussed above, we employed the chemical co-precipitation

method successfully for obtaining nanocrystalline ferrite particles with different

chemical compositions. This synthesis route is an economical technique to make out

ultrafine particles [86]. Co-precipitation is broadly used to synthesize ferrites

nanoparticles. Co-precipitation synthesis is comparatively simple to scale up and very

high processing temperature or complicated procedures are not required in this

method. Co-precipitation has a superior feature in comparison to other synthesis

routes that it discards most of the impurities to the solution during the formation of

solid product. We used co-precipitation method to synthesize nanocrystalline ferrites

such as Co-Zn ferrites, Co-Mn ferrites and Co-Cd ferrites. The primary molar

proportion (M2+:Fe3+) is always taken1:2 as the stoichiometry. Where M2+ stands for

Fe2+, Co2+, Zn2+, Mn2+, and Cd2+.

There are two steps which are involved in co-precipitation method [90, 91]:

Co-precipitation step

In the initial phase highly dispersed solid hydroxides of metals in the shape of

colloidal particles are acquired by simultaneous precipitation of metal cations in

alkaline medium. The reaction for the synthesis of Co-Zn mix ferrites take place as

25

Fe(NO3)3.9H2O (1-x) + Co (NO3)2.6H2O + xZn (NO3)2.6H2O +8NaOH

↓

(1-x) Co (OH)2.xZn(OH)2.2Fe(OH)3

Ferritization step

In ferritization step, the product obtained from the precipitation step is heated in

alkaline solution to transform metal hydroxides solid solution in to Co-Zn ferrites,

(1-x)Co(OH)2.xZn(OH)2.2Fe(OH)3

↓

Co(1-x)ZnxFe2O4.nH2O + (4-n)H2O

Here ‘n’ is an integer. A particular aspect of the "co-precipitation method" is that even

after many hours of heating in alkaline solution a certain amount of connected water

molecules remains there.

2.4 Influence of different parameters on synthesis by

co-precipitation method

Some important parameters which can influence the synthesis and the ultimate

properties of prepared nano crystalline ferrites particles are given here.

Role of anions

Anions used in the chemical reaction affect the ultimate properties of acquired

particles. However, typically in the synthesis of Co based nanocrystalline ferrite

particles salts are used in the form of chloride or nitrates. Hence, to achieve more

same conditions for all three metals, we used salts in the form of their nitrates [91].

26

Mixing rate of reagents

In deciding the size of the produced particles, mixing rate of reagents plays an

important role. Particle formation in co-precipitation involves two consecutive

processes which are nucleation and succeeding particle growth. Particle size and size

distribution depends on the relative rates of these two processes. We can have small

size distribution by creation of new nuclei and growth of the earlier created particles

simultaneously. Small sized particles can be obtained by fast nucleation formation and

low growth rate. This condition is achieved by quick addition with dynamic mixing of

reagents. On the other hand, slow mixing rate leads to the large size distribution.

Therefore, to have small size with thin size dispersion and chemically homogeneous

particles, we need the fast mixing of reagents.

Influence of temperature

For a chemical reaction the required activation energy is different for different

reagents. Activation energy computed from the reaction kinetics for three diverse

ferrites increases with the sequence as given below in the temperature range 20°C -

100°C. EA (Ni ferrite) > EA(Co ferrite) > EA(Zn ferrite). The increase in temperature

in this range increases the formation rate of ferrites particles.

Role of pH in the reaction

In synthesis of the ferrites by co-precipitation method the yield amount grows with

increase of pH from 6.8 to 8.6. Further raise of pH by 8.6 to 10 results in to only

minor increase in yield but further raise in pH from 12.5 to 14 gives a considerable

increase in the yield. Also, at higher pH values the particle size remains smaller

because of the simultaneous creation of nucleation sites.

Concentration of reagents

For better mixing of the reacting solution, 0.2-0.4 molar solutions are generally taken

for ferrite particles synthesis. Low-viscose suspension is essential for higher mixing

rate. Whereas, with concentration higher than 0.5 molar extensive mixing by stirrer

becomes difficult because of higher viscosity.

27

Role of co-precipitation base

The choice of co-precipitation base is very significant in the synthesis processes for

the preparation of nanocrystalline ferrite particles. Typically NaOH is employed for

co-precipitating. Effort with NH4OH did not proved to be useful [92, 93].

2.5 Synthesis of Zn, Mn and Cd doped CoFe2O4 nanocrystalline

ferrites particles

Co1-xMxFe2O4 (M=Zn, Mn, Cd ) nanocrystalline spinel fine particles were synthesized

by co-precipitating route with x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0. The chemicals reagents

used in our work were ferric nitrate (hydrated) Fe(NO3)3.9H2O, zinc nitrate (hydrated)

Zn(NO3)2.6H2O, cobalt nitrate (hydrated) Co(NO3)2.6H2O, Cadmium nitrate

(hydrated) Cd(NO3)2.6H2O and Manganese nitrate (hydrated) Mn(NO3)2.6H2O. All

reagents used were of analytical grade and used without further purification. To have

Co1-xZnxFe2O4,aqueous solutions (0.4 M) of the chemicals Fe(NO3)3.9H2O,

Zn(NO3)2.6H2O and Co(NO3)2.6H2O, were mixed with steady stirring by a magnetic

stirrer, until a clear solution was obtained. 1.5 M aqueous solution of (NaOH) as a

precipitating reagent was added quickly in metal solutions, kept in a beaker, at room

temperature with steady stirring.

The equation,

(2.1)

was used to measure the chemicals for their stoichiometric quantities. Similar method

was adopted for other compositions like Co1-xMnxFe2O4 and Co1-xCdxFe2O4for all

values of x. To have maximum homogeneity, solution was stirred initially for

30minutes at room temperature and then temperature was raised up to70°C under

steady stirring. To convert hydroxides into ferrites the solution was kept for 45

minutes at 70°C. Rearrangement of atoms and dehydration involves in obtaining

ferrites from intermediate hydroxide phase. By the management of the

co-precipitation steps we can control the particle size during synthesis processes. The

pH value during the reaction was maintained between 12.5 and 13. To get rid of

28

contaminations deionized water was used in washing of product thoroughly. The

particles thus obtained kept at 105°C temperature for overnight in an electric oven to

eliminate water contents. Homogeneous mix of dried powder was achieved by

grinding. A hydraulic press was used to for making pellets of grinded powder. The 50

lb/in2load was applied on each pellet of for 5 minutes. Sintering of these pellets was

done at 600 °C for 3h. The samples were cooled by slow cooling to room temperature

and were used for further purification. The flow chart for synthesis processes for

Co1-xZnxFe2O4is given in Figure 2.1. Similar method was adopted for other

composition like Co1-xMnxFe2O4 and Co1-xCdxFe2O4 for all values of x (x=0.0, 0.2,

0.4, 0.6, 0.8, 1.0).

29

Co-precipitation method

Figure 2.1 Flow chart of co-precipitation method for the synthesis of Co1-xZnxFe2O4

Fe(NO3)

3.9H

2O Co(NO

3)

2.6H

2O Zn(NO

3)

2.6H

2O

Aq. sol NaOH

pH = 12.5

0.1

+ + Aqueous solution

Aqueous solution

Aqueous solution

All well stirred solutions are mixed together

Washing

& Drying

Sintering at

600±5oC for

3hrs

Pellets

Characterizations

Stirred at 70 0C

30

Chapter 3

3 Characterization techniques

31

Each material has its specific physical and chemical properties. There are many

experimental techniques to investigate different properties of cobalt based

nanocrystalline ferrite materials. To investigate the structural, thermal and electrical

properties of the synthesized materials in the present research work following

techniques were used.

3.1 Thermal analysis

To know the specific transition temperatures and information about the suitable

sintering temperature for having pure phase crystalline material, we use Differential

Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) techniques. We

employed DSC and TGA techniques on, as prepared powder samples before any

further heat treatment. This analysis was done from room temperature to 1000°C.

3.1.1 Differential scanning calorimetry (DSC)

DSC is applied to examine the loss of solvents, glass transition crystallization

temperature, melting temperature and other processes concerning energy changes

[93]. We measured the energy produced (exothermic) or absorbed (endothermic) as a

function of temperature and time. A model DSC scan is shown here in Figure 3.1.

Figure 3.1 A model DSC scan

32

Simultaneous heat flow to the sample and a reference at the same temperature is

recorded. A reference material was used for this purpose. The temperature of the

sample material and reference are raised at a constant rate. Because the DSC analysis

is carried out at a same pressure the heat flow is same to enthalpy. The differential

rate of heat flow dH/dt between sample and reference may be either positive or

negative consistent with exothermic or endothermic reaction. In a thermogram the

area below the peak is directly proportional to the heat evolved or absorbed in the

reaction as shown in the Figure 3.2. The curve height is directly proportional to the

reaction rate. We analyzed our samples from room temperature to 1000°C with

calorimetric accuracy of ±2%.

Figure 3.2 Schematic diagram of processes involving DSC

3.1.2 Thermogravimetry analysis (TGA)

TGA is an analytical technique used for finding the materials thermal stability and

fraction of its portion of volatile material. It is done by observing the weight change

which arises when the sample is heated. Usually this measurement is taken in air or in

33

an inert gas like Argon or Helium [94]. The weight is recorded as a function of

increasing temperature (Figure 3.3). The weight changes may occur due to absorption,

adsorption, desorption decomposition, oxidation or dehydration of a specimen as a

function of temperature or time. These specific characteristics are associated with the

chemical crystal structure of the material. The thermogravimeteric analyses of

synthesized sample materials were done from room temperature to 1000°C with

accuracy of ±2%.

Figure 3.3 Α schematic thermo balance

3.2 Structural analysis

3.2.1 X-ray diffraction (XRD) analysis

XRD is a versatile technique which is non-destructive and can explore many details

regarding the crystallographic structure of the sample material. Atoms allocated in a

regular order in the space forms a crystal lattice. These atoms are in a pattern to form

a sequence of parallel planes apart from each other by a specific distance (inter-planer

distance) d which depends on the nature of the material. These planes of atoms in a

crystal lattice can be associated to the unit cell [95].

34

While X-ray shines at a powder specimen, the atomic layers in the crystal perform

like mirrors which reflect the beam of X-ray. The electric field associated with X-ray

gets scattered after interacting with that of charged entities like electrons in an atom

with amplitude given as 𝐹(𝑞) = ∫ 𝑓(𝑟) 𝑒𝑖𝑞−𝑟𝑑𝑟; here ‘f(r)’ is directly proportional to

electron density and ‘q’ represents the variation in the incident and scattered wave

vectors. Constructive or destructive interference may occur between the scattered

waves. Diffraction peaks are obtained for constructive interference. In this way the

obtained interplanar distances are used for identification of the crystal structure. The

width of the line and peak shape depends upon the crystal structure of the sample

material. The schematic illustration of the Bragg’s Law is given in Figure 3.4.

2dsinθ = nλ

Where (n = 1, 2, 3…)

Figure 3.4 Diffraction of scattered X-ray beam from atoms in a crystal

In this research work, the XRD machine (PANalytical X'pertpro MPD) with radiation

Cu-Kα was used. Data obtained from XRD was used to have information about the

crystal chemistry, phase identification, phase purity and crystallite size. The X-ray

35

used were CuKα (λ=1.5406 Å) while the tube was operated at 30mA and 40kV. The

X-ray patterns were recorded by varying 2θ from 20ο to 80ο. The accuracy in θ

measurements is better than 0.01degree.

3.2.1.1 Structure determination

The crystallite sizes were calculated through the Scherrer equation [96, 97] by using

the full-width at half-maximum (FWHM) value of the most intense peak, which is

(311).

0.89

cost

(3.1)

In this formula, β is the FWHM of the intensity versus 2θ profile, “λ” is the

wavelength of the Cu Kα (incident radiation X-ray, λ=1.5406 Å), and θ is the Bragg

diffraction angle. Scherrer equation assumes approximation and give the crystallite

size if the particle size distribution is narrow and strain induced effects are quite

negligible.

The lattice constant ‘a’ was determined by the check cell software using (h k l) values

obtained from XRD data. For the calculation of the theoretical density for the

synthesized samples we use the relation.

3x

nM

Na

(3.2)

Here M is the molecular weight of the sample, n is the number of molecules per unit

cell, 'a’ is the lattice parameter, and N is the Avogadro's number. The measured

density, ρm was determined by using the relation

2m

m

r h

(3.3)

36

Here r represents the radius, m is the mass, and h is the thickness of the sample pellet.

For the determination of porosity (P) of the synthesized samples for all compositions

we use the relation.

1 m

x

P

(3.4)

Where ρm is measured density and ρx is theoretical density.

Using lattice constant the distances between cation ions i.e. hopping length in

tetrahedral A sites (LA) and in octahedral sites B sites (LB) were calculated by the

following relations [88]

𝐿𝐴 =𝑎√3

4 (3.5)

𝐿𝐵 =𝑎√2

4 (3.6)

3.2.2 X-ray absorption fine structure (XAFS) spectroscopy

When an assembly of atoms is exposed to X-ray, at certain energy, a sharp rise in the

absorption is observed. This sharp rise is called the absorption edge. The binding

energy of a core level determines the absorption edge energy. There is a range of

information in the X-ray absorption edges about the local structure and the chemical

state of the absorbing atom. Extended X-ray absorption fine structure (EXAFS) and

X-ray absorption near-edge structure (XANES) spectroscopy are powerful tools for

the structural study of metal oxide materials [93]. These techniques are element

specific and sensitive to the local structure.

In X ray absorption the primary process is the photoionization of an atom. Excitation

of inner or outer electron is achieved which is defined by the photon energy ħω. In

XAFS electrons are made ejected from K-shell (inner-shell). It requires a minimum of

energy (ħω = EB, here EB represents the binding energy of the K-shell electrons). K

edge absorption is observed in the XAFS spectrum at the first excitation threshold

when photon energy equals to the K shell binding energy. Now it has been established

37

that XANES region extends by 50-100 eV ahead of the absorption edge [94]. It is

defined by the local vacant states density in an absorbing atom and also due to

multiple scattering. XANES spectrum is highly responsive to coordination chemistry

(tetrahedral coordination and octahedral coordination) and to oxidation state of the

absorbing atom. In the presence of the neighboring atom, the photo electron will

scatter by the electrons of the neighboring atom. In this way scattered photo electron

will return back to absorbing atom. In Figure 3.5 the photons absorption by an atom in

a solid is illustrated [94].

Figure 3.5 (a) Energy levels of an electron in the crystal lattice for excitation energies

corresponding the single (EXAFS) and multiple scattering for (XANES) (b)

Corresponding to a free electron outgoing wave and scattered wave interference (A)

X-ray photon absorbing atom, (B) neighboring atom (c) dependence of energy of

absorption coefficient μ (left) in the absence of neighboring atoms (right) In the

presence of scattering from the neighboring atom [98].

38

Because of the dependence of absorption coefficient on an available electronic state

the existence of photo-electron scattered back from the neighboring atom will change

the absorption coefficient. Thus X-ray absorption fine structure (XAFS) spectrum can

be divided into two different parts XANES and EXAFS depending on the energy

range of the X-ray beam compared to the absorption edge. A typical XAFS spectrum

is shown in Figure 3.6 [99].

Figure 3.6 A typical XAFS spectrum

To have X-ray absorption fine structure (XAFS) measurements pellets were prepared

by pressing the mix of 100 mg boron nitride powder and 5to10 mg of sample powder

(nanometric). The EXAFS measurements were taken in transmission mode at room

temperature for Fe K-edge, at the XAFS beam line 11.1 in ELETTRA synchrotron

light source with a double crystal Si (111) as a monochromator. The K-edge and pre-

edge regions were scanned at uniform energy steps ΔE of 0.2 and 5 eV, respectively,

while the scanning of EXAFS region was done at uniform photoelectron wave

number steps of Δk = 0.03 Å−1. The time for data acquisition was 2 s/point. For each

stoichiometric sample two spectra were collected for every edge in order to get better

results with least statistical noise.

For data acquisition the most commonly used method is transmission mode. The

schematic of the XAFS experiment is given in Figure 3.7.

39

Figure3.7 Schematic of the XAFS experiment

The beam intensity is measured before and after interaction with sample. The process

has accordance with the Bear-Lambert’s law of dilute solutions.

I = Ioexp(-µd) (3.7)

Io represents the photons intensity before their interaction with the sample of thickness

d, and I1, I2 are the intensities after interaction with the samples 1 and 2 respectively.

Ionization chambers filled by a noble gas or N2 are usually used to measure the X-ray

intensity. The sample is placed amid two chambers, which are linked with a normal

current measuring device. The current achieved is proportional to X-ray intensity. In

such a way the obtained physical quantity is the total inelastic photo absorption cross

section. Energy calibration was done by locating the first inflection point of the

absorption edge of metallic Fe to 7112 eV. For the purpose of energy-scale

calibration, a metallic Fe foil was placed after the second ionization chamber. The

edge of metallic Fe foil spectra was aligned within 0.01 eV. The relation for EXAFS

signals is χ(k) = (μ − μ0)/μ0, where μ represents the absorption coefficient

(experimental) and μ0 is a smooth spline symbolizing the fixed-atom absorption

background [92].

40

To infer the results from the experimental data the normalization procedure was done

by Xanda VIPER (Visual Processing in EXAFS Research) program. For the

normalization procedure for XANES spectra the smooth pre-edge absorption was

subtracted from the experimental spectra and taking unity as edge jump height. In the

XANES spectrum the transitions from 1s to 4p gives main peak and observed small

pre-edge peak is due to the transition from 1s to 3d [100, 101]. Both the quadrupole

transitions from 1s to 3d and dipole transitions from 1s to 4p are allowed for the Fe K-

edge, although the quadrupole transition intensity remains very low generally [102].

The rise in the pre-edge peak intensity is attributed to the local mixing of 3d and 4p

orbitals, which is permitted in case of tetrahedral symmetry while it is not allowed in

case of octahedral symmetry, because of the presence of inversion symmetry. The

peak height of this pre-edge peak gives the information about the site occupancy of

cations. So with the help of XANES study we can estimate the degree of inversion in

spinel ferrites.

3.3 Electrical properties

3.3.1 DC electrical properties

One of the most significant properties of the soft ferrites is their resistivity. Electrical

properties of ferrites depend upon chemical composition, synthesis method and

various heat treatments during the preparation. While doing resistivity measurements,

temperature was also factor to consider. The dc electrical resistivity is given by

𝜌 =𝑅𝐴

𝐿 (3.8)

Where A = Cross section area of the sample pellet

R = Material resistance

L = Thickness of the sample pellet

Resistance occurs due to the following reasons

i. In case of metals with increase in temperature, vibrations of lattice ions cause

the increase in resistance. But in the materials such as semiconductor where

41

charge carriers movement increases with increase in temperature, the

resistance decreases.

ii. Imperfections and dislocation in a crystal lattice is also a cause of resistivity

which is lower than that due to lattice vibrations.

Electrical properties of ferrites are influenced by their method of preparation and their

composition. The most common methods to determine the electrical resistivity are the

following [103],

i. Two probe method

ii. Four probe method

Four probe method is used for the samples with low resistance. Two probe method is

used most commonly for the measurement of samples with high resistivity. For

semiconductors and insulators when the resistivity of the samples itself is very high,

the contact resistance becomes negligible and two probe method is applicable. To

measure the dc resistivity of our samples, we used two probe method. The schematic

diagram for both the two probe and four probe method is given in Figure 3.8.

Figure 3.8 (a) Two-probe (b) Four-probe resistance measurement circuitry

In two probe method the contact resistance is not included because it is negligible as

compared to the sample resistance. The circuit diagram of the system used to measure

the dc resistivity is shown in Figure 3.9. The sample was placed between two

42

electrodes in pressure contacts. The electrodes with sample specimen kept in the tube

furnace. The temperature in the furnace was controlled by a power regulator. The

pressure contacts are made between electrodes and pellet. A dc source is employed

across the sample.

Figure 3.9 Circuitry of the apparatus used for dc electrical resistivity measurements

The change in current flowing through the sample due to known value of applied

voltage is measured as a function of temperature at step size of 5C. The accuracy in

resistance measurements was 0.1 %. The data acquired in such a way was used to

evaluate the temperature dependent dc resistivity and activation energy of the

synthesized samples. Arrhenius relation was used to find the activation energy [100].

𝜌 = 𝜌0 𝑒𝑥𝑝 (∆𝐸

𝑘𝐵𝑇) (3.9)

where ‘ΔE’, ‘kB’ ‘ρ’,‘T’ and ‘ρ0’, are the activation energy, Boltzmann’s constant,

resistivity at temperature T, temperature, and resistivity respectively.

For all the samples drift mobility (µd) was determined using the following relation

1d

ne

(3.10)

43

Here ρ is the electrical resistivity at a particular temperature, e is the charge of an

electron and n is the concentration of charge carrier, which can be calculated from

the relation,

a m FeN Pn

M

(3.11)

Where Na is the Avogadro's number, M is the molecular weight, ρm the measured

density of sample and PFe is the number of iron atom in the chemical formula unit of

the ferrites.

3.3.2 Frequency dependent alternating current (ac) measurements

Precision component analyzer (6400B) was used to measure ac electrical properties. It

offers both two probe and four probe methods to measure different ac quantities in 20

Hz to 3 MHz frequency range. The derive level of this apparatus to measure ac

quantities ranges from 1mV to 10V rms. It measures real as well as the imaginary

component of the impedance vector. Which is a significant parameter to characterize.

The impedance vector can be written as Z=R+jX, where R is the real part and X is the

imaginary part. The response of the real and imaginary factors is shown in Figure

3.10.

Figure 3.10 (a) Relaxation time (τ) of masses after turning off the applied electric

field. (b) Impedance plane of real capacitor (c) Response of real ε′ and imaginary ε′′

parts of permittivity

44

Admittance is the reciprocal of the impedance. The reactance purity is the quality

factor Q. For capacitors the dissipation factor (D) is generally spoken in terms of tan

δ. Tan δ is the ratio of the energy dissipated by a component to the energy stored in

that component. Accuracy in capacitance measurements was 0.2 % whereas the

accuracy in measurements of dissipation factor was 0.02 %.

The dielectric characteristics of ferrites normally emerge from hopping of charge

carriers (electrons or polarons) between cationic sites and across the grain boundaries.

Charge polarization owing to applied external electric field is the source of dielectric

behavior. Polarization occurs by the following four basic mechanisms. These

processes are shown in the Figure 3.11.

Figure 3.11 General polarization mechanisms

45

A. Interfacial polarization or space charge polarization

Placement of charges at grain boundaries, inter phase boundaries and at the

surfaces skin contribute to the dielectric response of the material when

external field is applied.

B. Dipolar or orientation polarization

Some materials are naturally dipolar such as water. The orientation of

dipoles of these materials against the applied field also contributes in

polarization.

C. Ionic polarization

Ionic crystals have incorporated dipoles which just revoke one another. The

slight displacement from their mean position due to external applied fields

can make a net polarization.

D. Electronic or atomic polarization

Usually all materials have this kind of polarization. In general an atom has

spherical symmetry with its electrons. An applied electric displace the

electrons slightly at s side from the nucleus and as a result dipole moment is

formed.

All of the four mechanisms of polarization take part towards the dielectric constant at

lower frequencies for an applied electric field.

46

Figure 3.12 ε′ and ε″ as a function of frequency and different polarization mechanisms

Overall volume polarization is the vector sum of all above mentioned dipole moments

per unit volume. The movement of charges in polarization mechanisms cannot follow

the higher frequencies and their contribution vanishes one by one. The dielectric

constant decreases with rise in frequency and at each relaxation frequency, there is a

peak of dissipation factor as shown in the Figure 3.12 [101].

The complex dielectric constant can be expressed as

휀∗ = 휀′ − 𝑗휀′′ (3.12)

where ‘ ’ ‘ ’ ‘* ’, are imaginary part, real part and complex part of the dielectric

constant respectively.

The dielectric behavior can be interpreted on the foundation of Koop’s theory [102],

which stands on the Maxwell-Wagner’s double layer model [103]. As said by

Maxwell-Wagner’s model the dielectric structure of the material is composed by two

layers. Well conducting grains make first layer which are separated by poor

conducting grain boundaries making the second thin layer. At lower frequencies,

47

grain boundaries are found to be more effective and relatively low resistive grains are

more effective on higher frequencies. One can derive complex dielectric constant by

using Maxwell-Wagner two layer model, which is given by,

휀∗ = 휀∞ +휀𝑠 − 휀∞

1 + 𝑗𝜔𝜏− 𝑗

𝜎𝑑𝑐

𝜔휀0 (3.13)

where‘0’ ‘’ ‘dc’, ‘’,and ‘s’ are dielectric constant of free space, dielectric

constant for electronic polarization, conductivity, relaxation time , dielectric constant

at dc field and =2f.

From the above equation the real part of dielectric constant is given by

휀 ′ = 휀∞ +휀𝑠 − 휀∞

1 + 𝜔2𝜏2 (3.14)

The dielectric constant () was calculated from capacitance (C) values found from

precision component analyzer in all frequency range using equation [102]

휀′ =𝐶𝑑

𝐴𝜀0 (3.15)

Here ‘d’, ‘0’ and ‘A’ are the thickness, permittivity of free space and face area of the

pellet respectively. The dielectric loss factor (ε'') and alternating current conductivity

(ac) was also calculated by using the following appropriate relations [103]

= 𝑡𝑎𝑛 𝛿 (3.16)

𝑎𝑐 = 𝜔휀0휀′𝑡𝑎𝑛𝛿 (3.17)

where ω=2πf.

AC conductivity in nanocrystalline material is frequency dependent. In the low

frequency region the change in conductivity is due to the interfacial polarization

effects. As the frequency reduces the interfacial charge accumulation occurs and

ultimately conductivity drops. At the higher frequency values the conductivity

increases. The dependence of conductivity is simply related by the Jonscher’s law as

48

ac = 0 + A 𝜔n (3.18)

Here pre-exponential constant is represented by A and 𝜔 = 2πf represents the

angular frequency, n stands for the exponent of the power law and its value lies

between 0 and 1. We employed the Jonscher’s power law to find the conductivity

mechanism. ac represents the ac conductivity and 0 stands for the limiting

conductivity at zero frequency.

An impedance analysis is an important way to explore the electrical characteristics of

nanocrystalline ferrites materials. We can separate the resistance of grains from the

other resistance sources like grain boundaries. The Nyquist plots give us useful

information about the reactive “imaginary part” and resistive “real part” components

of the synthesized material. To measure the dispersion, an alternating field with

varying frequency is applied on material and response is observed as a function of

frequency. Complex plane graphs (Nyquist plots) can be drawn in form of any one of

the four achievable complex relations, the impedance (Z), the permittivity (ε), the

admittance (Y), the electric modulus (M) and dielectric loss (tan δ). These are related

to each other [102] as

𝑡𝑎𝑛 𝛿 =

=

Z

Z=

Y

Y=

M

M (3.19)

For differentiating between the contribution of grains and grain boundaries the

complex plane plots has been drawn in the frequency range of 20 Hz to 3M Hz at

room temperature. Nyquist plots were attained by plotting the () against ().

Usually, two semicircles can be observed in these plots; first semicircle for low

frequency region signifies the resistance of the grain boundary whereas the second

semicircle achieved at high values of frequencies indicates the resistance of grains.

Two semicircles trend, which is attributed to the resistance of the grain and grain

boundaries can be modeled approximately with an ideal equivalent circuit consisting

on two parallel R–C elements in series. A model complex plane plot for parallel RC

equivalent circuit is shown in Figure 3.13. Errors in measurements are indicated in

graphs, usually these are within the plot symbols.

49

Figure 3.13 A model complex plane plot for parallel RC equivalent circuit

50

Chapter 4

4 Thermal, structural and electrical properties

of nanocrystalline Co1-xZnxFe2O4

(x = 0.0 to 1.0)

51

Crystal structure and cation distribution at particular sites in crystal lattice plays

primary role in deciding the properties of nanocrystalline transition metal oxide

materials. Zinc is well known for playing a vital role in influencing the ferrite

properties [104]; thus the composition was varied by successive replacement of Co by

Zn in CoFe2O4. Nanocrystalline ferrite particles of Co1-xZnxFe2O4 with (x = 0.0, 0.2,

0.4, 0.6, 0.8, 1.0) were synthesized by co-precipitation method as explained in

synthesis method in section 2.5. Synthesized ferrite particles were investigated very

precisely for their thermal, structural and electrical properties. Site occupancy and

cation distribution is determined by x-ray absorption near edge spectroscopy

(XANES).

4.1 Thermal Analysis

Specific transition temperatures were investigated by differential scanning calorimetry

(DSC) and thermogravimetric analysis (TGA). DSC analysis was employed on, as

prepared powder samples before any further heat treatment. Thermograms for DSC

and TGA of CoFe2O4 and ZnFe2O4 from ambient temperature to 1000°C are shown

in Figures 4.1 and 4.2 respectively.

Figure 4.1 (a) The differential scanning calorimetry (DSC) of CoFe2O4

52

Figure 4.1 (b) The differential scanning calorimetry (DSC) of ZnFe2O4

Figure 4.2(a) The thermogravimetric analysis (TGA) of CoFe2O4

53

Figure 4.2(b) The thermogravimetric analysis (TGA) of ZnFe2O4

In DSC graphs for both samples; a wide exothermic peak without any sharp peak for

any kind of transition indicates the temperature range for crystal recovery

temperature. In TGA plot all major weight loss processes in both compositions has

been completed before 600°C. Weight loss processes begins from room temperature.

Initially from room temperature to about 100°C the weight loss is due to dehydration

of the material. At the next step, the major weight loss occurs from 100°C to 300°C.

This weight loss can be attributed to the decomposition of nitrates, removal of OH

ions and some organic residues that were left in the sample. Also at this stage the

crystallization of sample materials is started. After 300°C the weight loss is due to the

removal of structural water. After 600°C the samples becomes stable by weight loss

[105]. The composition ZnFe2O4 is more thermally stable than CoFe2O4, keeping in

view the lesser weight loss. Considering the DSC and TGA graphs of CoFe2O4 and

ZnFe2O4, temperature of 600°C was selected for sintering of samples to obtain the

maximum crystalline phase.

54

4.2 Structural properties

4.2.1 X-ray diffraction (XRD) studies

The X-ray diffraction (XRD) patterns confirmed the FCC spinel structure for all the

samples of Co1-xZnxFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) nanocrystalline particles with

‘x’ changing from 0.0 to 1.0 as shown in Figure 4.3.

Figure 4.3 X-ray diffraction pattern of Co1-xZnxFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)

The crystallite sizes were calculated by the data obtained from the XRD. The

crystallite sizes were in the range from 102 nm to 354 nm evaluated by Scherrer

equation through the full-width at half-maximum (FWHM) value of the most intense

peak which is (311). The lattice constant 'a' is found to increase with increase of Zn

concentration. It is due to the fact that the radius of Zn2+ ions (0.82 Å) is larger than

that of the Co2+ ions (0.78Å). The addition of Zn at the expense of Co in the

composition is expected to increase the lattice constant. The linear increase in lattice

20 30 40 50 60 70 80

(3 3 1)

(5 3 3)(5 1 1)

(4 2 2)(4 0 0)

(3 1 1)

(2 2 0)

x = 0.0

x = 1.0

x = 0.8

x = 0.6

x = 0.4

x = 0.2

Inte

nsit

y (

a.

u)

2Degree)

(4 4 0)

55

constant with increase in zinc contents indicate that lattice expands without upsetting

the symmetry of lattice. It is in accordance with Vegard’s Law [106]. The values of

lattice constants and crystallite sizes with possible errors are given in table 4.1 and are

also shown in Figure 4.4.

Figure 4.4 Change in lattice constants and crystallite size with Zn concentration in

Co1-xZnxFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)

Lattice constant values are in good agreement with the reported values [107].

Increases in crystallite sizes come about due to coalescence. During sintering two or

more particles seems to fuse together by melting of their surfaces. In nano sized

particles, surfaces melts below the melting point of their bulk counter part because of

large fraction of low coordinated atoms that lies at surface skin. The values of

theoretical density (ρx), measured density (ρm), and porosity (P), are given in table 4.1.

It was observed that due to increase of Zn concentration, no significant change

occurred in theoretical density. Because with increase of Zn concentration lattice

constant increased, this decreased the density but at the same time the Zn atoms which

56

replace Co atoms, have greater atomic mass as compared to that of cobalt atoms and

compensate the decrease in density.

The distance between cations at two consecutive tetrahedral A sites (LA) and between

two consecutive octahedral B sites (LB) plays an important role in the electrical

conduction phenomenon. These are determined by using the equations 3.5 and 3.6.

For the conduction mechanism, electrons hop between ions of the same element

existing in different valence states on equivalent lattice sites. The change in hopping

lengths LA and LB is shown in Figure. 4.5 and are given in table 4.1 with possible

errors. Both LA and LB increases with increase in Zn content. This is due to the

increase in lattice constant in the crystal geometry of the prepared samples.

Figure 4.5 Change in hopping length between consecutive tetrahedral A sites (LA) and

consecutive octahedral B sites (LB) with Zn concentration (x) in Co1-xZnxFe2O4

(x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)

0.0 0.2 0.4 0.6 0.8 1.0

3.60

3.61

3.62

3.63

3.64

3.65

3.66

LA

LB

x ( Zn Concentration )

LA

(Ho

pp

ing

len

gth

) Å

2.95

2.96

2.97

2.98

2.99

LB

(Ho

pp

ing

Len

gth

)Å

57

4.2.2 X-Ray absorption fine structure (XAFS) studies

We are concerned primarily to absorption coefficient µ in X-ray absorption

spectroscopy. µ gives the probability that according to the Beer’s law the X-ray may

be absorbed. When X-ray having energy equal to the core level electron’s binding

energy shines on that atoms, we get a sharp increase in absorption, which is called

absorption edge and the core level promoted to continuum. Measurements of XAFS is

actually a measure of energy dependence of µ above and at the binding energy of core

level. The XAFS measurements were taken at room temperature in transmission mode

at Fe, Zn, and Co K-edges, at the XAFS beam line of the ELETTRA synchrotron light

source. After normalization the absorption spectra of the near-edge region (XANES)

for Fe, Zn, and Co K-edges for all compositions of CoxZn1−xFe2O4 are given in Figure

4.6. XANES region of the XAFS spectra provides information for the oxidation state

of the absorbing atom, its local atomic surroundings, especially on the site symmetry.

Typically the oxidation state changes by +2 to +3 when the edge position gets rises

about 3 eV [108]. There are some characteristic features in XANES spectrum about

the transition metal oxides. A rise in the valence of a metal atom causes a shift to

higher energies. Absorption edge shoulders and the main peak correspond to the sharp

resonance of the environment of the metal atom and excitation to 4p states. The d-

states splitting which is dissimilar for octahedral and tetrahedral site symmetry also

influence the shape of peak for pre-edge. The broadening in main peak for a certain

environment occurs due to multiple scattering by surrounding atom shells. With

decrease in centrosymmetry degree of the environment the intensity of the pre-edge

peak increases.

There is an eminent behavior of the pre-edge peak for sites with octahedral and

tetrahedral symmetry and these are present in the spinel crystal structure. The pre-

edge peak height is more intense for the tetrahedral symmetry and less intense for

octahedral symmetry. This is mainly because of highly non centrosymmetric

tetrahedral symmetry. This enables the transition from p to d state and it contributes to

height of the pre-edge peak. The pre-edge peak becomes the addition of these

contributions, when both octahedral and tetrahedral sites are occupied; the peak

intensity increases directly with increase in proportion of tetrahedral sites. In the Fe

58

K-edge data, the intensity of the pre-edge peak increases relatively with increasing Co

content. This shows that an increasing fraction of Fe occupies the tetrahedral sites as a

function of Co. Investigation of the XAFS spectrum showed that end member

CoFe2O4 in the series CoxZn1−xFe2O4 has an inverse spinel structure while ZnFe2O4

has a normal spinel structure depending upon the cation distribution. For K-edge of Fe

the intensity of the pre-edge peak increases with increasing Co contents (Figure 4.6)

[109]. It shows that rising Fe fraction goes to tetrahedral sites as a function of Co. In

the intermediate compositions of CoxZn1−xFe2O4 when 0.2 ≤x ≤ 0.8 Co and Zn atoms

occupy always the octahedral and tetrahedral sites, respectively.

Figure 4.6 XANES spectra after normalization at K-edge of Fe (a), K-edge of Co (b),

and K-edge of Zn (c) for Cox Zn1−x Fe2 O4.‘A’ indicates the pre-edge peak

59

These results clearly suggest that we can change the degree of inversion successively

from a complete normal ferrite (ZnFe2O4) to an inverse ferrite (CoFe2O4) by changing

the stoichiometries. Hence the material properties can be controlled by selecting a

certain composition.

4.3 Electrical measurements


Hopping of electrons between ions of the same element existing in different valence

state at equivalent lattice sites causes the conduction in ferrites occurs [110]. For all

these samples the dc electrical resistivity was measured by two probe method from

313 K to 700 K temperature range. It was observed that the dc electrical resistivity

rises notably with increase in Zn content as shown in Figure 4.7. Accuracy in

measurements has been mentioned in section 3.3.1. The change in resistivity at

different temperature values for all the compositions is shown in Figure 4.8. At all the

temperature values the resistivity increases with increase in Zn concentration. The

resistivity is maximum at lowest temperature. The variation in resistivity can be

elucidated on the basis of actual location of cations in the spinel structure and also by

microstructures of the material. The spinel cubic structure of ferrites consists of two

interpenetrating sub-lattice shaping two kinds of interstitial sites where metal ions are

placed, the A-sites and B-sites. Now it is an established fact and also suggested by

Hosseinpour et al. (2007) that the general distribution of cations in the mix spinel

ferrites is [110]

(M2+1-(δ+γ) Fe3+

(δ+γ)) 8[M2+

δ Fe2+γFe3+

2-(δ+γ)] O2-

32

There are two possible electron hopping processes, the first process is

Fe2+ + Fe3+ ↔ Fe3+ + Fe2+

By this process, an electron can jump from Fe2+ the divalent ferrous ion at an

octahedral B site to the Fe3+ a trivalent ferric ion at similar site. By this hopping

mechanism the ionic state and charge neutrality of the crystal stay unaffected as

under.

60

(Fe3+ ) [Fe2+ + Fe3+] O2-4 = (Fe3+ ) [Fe3+ + Fe2+] O2-

4

The second possible reaction is

M2+ + Fe3+ ↔ M3+ + Fe2+

But if the electron hopping occurs in this way, the initial and final charge state of the

crystal will become changed. So the hopping of electrons between Fe2+ and Fe3+

existing at the equivalent lattice sites is the most favorable mode of conduction

mechanism. Therefore in CoZnFe2O4 conduction mechanism is considered as the

electron hopping between Fe2+and Fe3+in (B) sites and mobility of holes between Co2+

and Co3+ ions [111]. The tetrahedral A site is strongly preferred site of occupancy for

Zn in Zn Fe2O4 as shown in the XANES analyses above.

Figure 4.7 Variation of resistivity ρ (Ω-cm) with change in temperature for

Co1-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0)

300 400 500 600 700

0.0

2.0x107

4.0x107

6.0x107

8.0x107

1.0x108

-cm

)

Temperature (K)

x=0.0

x=0.2

x=0.4

x=0.6

x=0.8

x=1.0

61

Figure 4.8 Variation in dc electrical resistivity at different temperatures of

composition Co1-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0)


Co1-xZnxFe2O4 with linear fit

0.0 0.2 0.4 0.6 0.8 1.0

-5.0x105

0.0

5.0x105

1.0x106

1.5x106

2.0x106

2.5x106

3.0x106

3.5x106

4.0x106

4.5x106

cm

)

Concentration x

373 K

473 K

0.0

2.0x103

4.0x103

6.0x103

8.0x103

1.0x104

1.2x104

1.4x104

573 K

673 K

15 20 25 30 35 40

4

6

8

10

12

14

16

18

20

ln

(

-cm

)

1/kBT (eV)

-1

X = 1.0

X = 0.8

X = 0.6

X = 0.4

X = 0.2

X = 0.0

62

The addition of Zn ions at the expense of Co leads to increase of Fe3+ and

corresponding decrease of Fe2+ ions in octahedral site and a simultaneous decrease of

Co2+ ions. By this the available Fe2+ and Fe3+ pairs at octahedral B-sites decreases and

hence conduction phenomenon decreases. In Figure 4.8 the variation in dc electrical

resistivity at different temperatures for all the compositions is shown.

With increase of Zn concentration the lattice expands and jump length for hopping of

electrons increases as given in table 4.1 and shown in Figure 4.5 which increase the

required activation energy, therefore dc resistivity increases with increase of Zn

concentration. At higher temperature at about 400 K conductivity is attributed to the

hopping of holes between Co2+ and Co3+ ions. So the placement of zinc at the expense

of cobalt will consequently decrease the process and as a result conductivity decreases

[110]. The structure of the prepared material consists of conducting grains separated

by highly resistive thin layers (grain boundaries). Decrease in resistivity in the

temperature range from 313 K to 700 K, with increase of temperature showed the

semiconducting nature of the material. The values of activation energies (E)

evaluated through the slopes of the linear plots of dc electrical resistivity are shown in

Figure 4.10.

Activation energies increase with increase of Zn content. It can be attributed to

increase in hopping length and lattice expansion with increase of Zn concentration.

E ranges from 0.518 eV to 0.537 eV clearly shows that the conduction is occurring

here due to polaron hopping [110]. The hopping depends upon the activation energy,

which is associated with the electrical energy barrier experienced by the electrons

during hopping. The plots for drift mobility are shown in Figure 4.11. Drift mobility

is decreased with increase in Zn concentration which increases resistivity. Drift

mobility is low at low temperatures and increases sharply with increase in

temperature.

63

Figure 4.10 Variation in activation energy (E) with change of Zn concentration (x)

in Co1-xZnxFe2O4

Figure 4.11Variation of drift mobility µd (cm2/Vs) with change in temperature for

Co1-xZnxFe2O4

0.0 0.2 0.4 0.6 0.8 1.00.515

0.520

0.525

0.530

0.535

0.540

(

eV)

x ( Zn concentration)

15 20 25 30 35 40

-2.0x10-6

0.0

2.0x10-6

4.0x10-6

6.0x10-6

8.0x10-6

1.0x10-5

1.2x10-5

1.4x10-5

1.6x10-5

1.8x10-5

2.0x10-5

d

(cm

2/

Vs)

1/kBT(eV)

-1

X = 0.0

X = 0.2

X = 0.4

X = 0.6

X = 0.8

X = 1.0

64

4.3.2 AC electrical properties

4.3.2.1 Dielectric constant

The dielectric constants () of zinc substituted cobalt ferrites have been measured at

room temperature in the frequency range from 100 Hz to 3 MHz. The plots of

dielectric constant versus frequency show normal dielectric behavior of the spinel

ferrites. The variations of dielectric constant as a function of frequency for mixed Co-

Zn ferrites with different compositions are shown in Figure 4.12. Accuracy in

measurements has been mentioned in section 3.3.2. The characteristic features of the

dielectric properties have been known to arise from distribution and type of the

cations among the tetrahedral A-site and octahedral B-sites in the spinel structure.

2 4 6 8 10 12 14 16

0

10000

20000

30000

40000

50000

60000

Die

lectr

ic c

on

sta

nt (

)

ln (f)

X = 0.0

X = 0.2

X = 0.4

X = 0.6

X = 0.8

X = 1.0

Figure 4.12 Effect of Zn concentration and frequency of the applied electric field on

dielectric constants () for Co1-xZnxFe2O4

The graphs shown in Figure 4.12 show that the dielectric constant is maximum for

CoFe2O4 and it decreases with an increase in zinc content at lower frequencies but is

65

higher at higher frequencies for x = 0.4. The dielectric constant decreases with

increase of frequency. All the samples show the frequency dependent behavior. As the

frequency of the externally applied electric field increases gradually, the dielectric

constant decreases. This reduction occurs because beyond a certain frequency of the

externally applied electric field the movements of charges cannot follow the

frequency of the external applied electric field and as a result dielectric constant ()

decreases. The dielectric distribution curves can be explained by Koop’s theory which

is based on the Maxwell Wagner model considering the homogeneous double layer

structure [103]. There are conducting grains and insulating grain boundary regions

inside the medium. These regions form capacitors and resistors parallel circuits.

According to Rabkin and Novikova [112], the polarization phenomenon in the ferrites

occurs by a mechanism comparable to the conduction process. Polarization

phenomena occur due to local displacement of electrons, which determines the

dielectric constant in ferrites. The compositional dependence of the dielectric

constants of mixed Co-Zn ferrites can be explained by considering the site

occupation. The number of ferrous ions on the octahedral sites which take part in the

electron exchange interaction between Fe2+ and Fe3+ and are responsible for the

polarization is maximum in the case of cobalt ferrite (CoFe2O4), therefore a high

value of the dielectric constant is expected and is observed. It has been already

observed for spinel structure that the electron hopping between Fe3+ and Fe2+ leads to

the large dielectric constant () and dielectric relaxation [113]. As the zinc content in

the mixed Co-Zn ferrites was gradually increased at the expense of cobalt, the number

of ferrous ions on the octahedral sites decreases resulting in a continuous decrease in

the dielectric constant (). The variation in dielectric constant at different frequencies

for Co1-xZnxFe2O4 is given in Figure 4.13. From the Figure 4.13 it is clear that the

dielectric constant gradually decreases with increase in frequency. The dielectric constant

has the maximum value at x = 0.4 that is for Co0.6Zn0.4Fe2O4.

66

0.0 0.2 0.4 0.6 0.8 1.0

-400

-200

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Die

lect

ric

con

sta

nt (

)

Concentration (X)

Freq 100k

Freq 500k

Freq 1.2 M

Freq 3 M

Figure 4.13 Effect of Zn concentration on dielectric constant () at different

frequencies for Co1-xZnxFe2O4

4.3.2.2 Dielectric loss (tan δ)

The dissipation factor (D) is typically expressed in terms of tan δ which may be

defined as the ratio of the energy dissipated to the energy stored in a component

[114]. The change of dielectric loss (tan δ) as a function of frequency for all

compositions at room temperature is shown in Figure 4.14. The possible errors in

measurements are given in section 3.3.2.


tan δ for Co1-xZnxFe2O4

2 4 6 8 10 12 14 16

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

tan

ln (f)

X = 0.0

X = 0.2

X = 0.4

X = 0.6

X = 0.8

X = 1.0

67

In plots for (tan δ) the peaks emerges when hopping frequency of localized electron

matches with the externally applied alternating electric field [115]. For different

polarization mechanisms, resonance occurs at different frequencies. Here the first

peak at lower frequencies corresponds to the interfacial polarization and the second

peak corresponds to the dipole polarization. The shifting of these peaks towards

higher frequency region is attributed to the increase of the hopping rate of the charge

carriers. The tangent loss is observed to be higher for higher frequencies.

0.0 0.2 0.4 0.6 0.8 1.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

tan

Concentration (X)

Freq 100k

Freq 500k

Freq 1.2 M

Freq 3 M

Figure 4.15 Change in tangent loss with Zn concentration for all compositions at

different frequencies

4.3.2.3 AC conductivity

Generally, conduction phenomenon is due to electron hopping among the ions of the

same element existing in the different valence states [116]. As explained earlier in the

cubic close packed crystal lattice of spinel ferrites the most favorable electron

hopping process at B-B sites. Jump length calculations shown in a previous section

68

demonstrate that the hopping length between two metal ions on (B) sites is less than

that of the hopping length between two metals ions at (A) site. Hence the hopping

probably between A to A site is very small. Also the hopping conduction among A-A

sites is not possible because preferred site occupancy for iron ions is B-site [117]. As

shown in the Figure 4.16, the ac conductivity increases [113].


ac conductivity for Co1-xZnxFe2O4

At higher frequencies the value of ac conductivity is maximum it is due to higher

pumping force provided to charge carriers by high frequency and also the density of

material plays a role [118]. In Figure 4.17 variation in ac conductivity at different

frequencies for Co1-xZnxFe2O4 is given. AC conductivity is higher for higher frequencies

of applied electric field. Also for the composition x=0.4 the ac conductivity has highest

value. The n (frequency exponent) is related with the correlation degree among

moving ions. For ion conducting material in dispersive region of frequency the non-

zero vale of n is for the virtue of energy stored in the collective motion of ions. A

higher value of n implies that large energy is stored in such collective motion of ions.

The particular values of ‘n’ between ‘0’ and ‘1’show the hopping mechanism of ac

2 4 6 8 10 12 14 16

0.000

0.005

0.010

0.015

0.020

0.025

0.030

ac c

on

du

ctivity (

-cm

)-1

ln (f)

X = 0.0

X = 0.2

X = 0.4

X = 0.6

X = 0.8

X = 1.0

69

conduction. The graphs for change in ac conductivity with respect to the Jonscher’s

power law with variation in concentration of composition Co1-xZnxFe2O4 (x=0.0, 0.2,

0.4, 0.6 0.8, 1.0) are shown in Figure 4.18. The ac conductivity is observed to increase

with increase in cobalt contents.

0.0 0.2 0.4 0.6 0.8 1.0

0.000

0.005

0.010

0.015

0.020

0.025

0.030

a

c (

S-c

m-1)

Concentration x

1 kHz

10 kHz

100 kHz

1 MHz

Figure 4.17 Variation in ac conductivity at different frequencies of composition

Co1-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0)

0.0 0.2 0.4 0.6 0.8 1.0

0.2

0.3

0.4

0.5

0.6

0.7

0.8

po

we

r n

Concentration x

Figure 4.18 Variation in ac conductivity with respect to power law with variation in

concentration of composition Co1-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0)

70

Nyquist plots for all composition are shown in Figure 4.19. An important way to

examine the electrical features of ferrite materials is the impedance analysis. In this

way we can separate the resistance of grains from the other resistive sources such as

grain boundaries. The Nyquist plots provide very useful information about the

reactive and real components of the material. For dispersion measurements, an

alternating electrical field with changing frequency is applied on sample material and

the response is observed as a function of frequency. To differentiate the contribution

of grains and grain boundaries the complex plane plots has been drawn in the

frequency range of 100 Hz to 3 MHz at room temperature. Nyquist plots were

attained by plotting the () against (). Two semicircles trend, which are attributed

to the resistance of the grain and grain boundaries can be modeled approximately with

an ideal equivalent circuit consisting on two parallel R–C elements in series. The plots

shown in Figure 4.19 indicate that the resistance of the grain boundaries is higher than

entire grains. Nyquist plots for all compositions are shown in Figure 4.19

71

0 10000 20000 30000 40000 50000

0

20000

40000

60000

80000

100000

"

'

x=0.2

-5000 0 5000 10000 15000 20000 25000 30000 35000 40000

-5000

0

5000

10000

15000

20000

25000

30000

35000

40000

"

'

x=0.4

0 5000 10000 15000 20000 25000 30000

-5000

0

5000

10000

15000

20000

25000

30000

35000

"

'

x=0.6

0 2000 4000 6000 8000 10000 12000 14000

-2000

0

2000

4000

6000

8000

10000

12000

14000

16000

"

'

x=0.8

0 100 200 300 400 500

0

100

200

300

400

500

600

"

'

x=1

Figure 4.19. Nyquist plots for all the samples of Co1-xZnxFe2O4 (x=0.0 – 1.0)

0 50 100 150 200 250 300 350 400

0

100

200

300

400

500

600

Die

lectr

ic L

oss F

acto

r (

'')

Dielectric Constant (')

x = 0.0

72

Table 4.1 Crystallite size d(311), lattice constant(a), Cell volume (V), hopping lengths

LA and LB, X-ray density(ρx), measured density (ρm), porosity (P) drift mobility (µd) at

different temperatures and activation energy E for different values of ‘x’ in

Co1-xZnxFe2O4

Parameter x = 0.0 x = 0.2 x = 0.4 x = 0.6 x = 0.8 x = 1.0

d(3 11)(nm) 35(4) 10(2) 15(2) 19(2) 26(3) 30(3)

a(Å) 8.362(1) 8.368 (5) 8.393(1) 8.411(3) 8.425(1) 8.443(1)

V(Å3) 584 586 590 595 605 601

LA(Å) 3.619(1) 3.624(5) 3.632(1) 3.641(3) 3.645(1) 3.654(1)

LB(Å) 2.955(1) 2.959(5) 2.966(1) 2.973(3) 2.976(1) 2.983(1)

ρx(g/cm3) 5.34 5.37 5.38 5.37 5.26 5.33

ρm(g/cm3) 3.57 2.83 2.89 2.86 2.95 3.10

P (fraction) 0.33 0.47 0.46 0.44 0.43 0.43

µd(cm2/Vs) at

313K

3.55x10-10

2.15x10-10

1.21x10-10

8.06x10-11

4.58x-11

3.27x10-11

µd(cm2/Vs) at

500K 3.79x10-07 3.07x10-07 1.61 x10-07 9.84x10-08 6.60 x10-08 5.00 x10-08

µd(cm2/Vs) at

700K 1.52 x10-05 1.04 x10-05 6.16 x10-06 3.79 x10-06 2.73 x10-06 2.59 x10-06

E(eV) 0.518 0.521 0.523 0.525 0.531 0.537

73

4.4 Summary

It is important to acquire powder of high density, small grain size and narrow size

distribution with controlled stoichiometry, to have material with required properties.

Co1-xZnxFe2O4 nanocrystalline particles (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared

by chemical co-precipitation method. Specific thermal transition temperatures were

studied by differential scanning calorimetry (DSC) and thermogravimetric analysis

(TGA) to estimate the sintering temperature. After analysis of DSC and TGA plots the

600°C temperature was chosen for sintering. Sintering at 600°C gave us maximum

crystalline spinel phase. FCC spinel crystal structure was confirmed by X-ray

diffraction analysis. Lattice constant calculations show that lattice constant increases

with increase in Zn concentration. Cation distribution was found by XAFS analysis

which is a powerful technique for determination of local crystal structure with

elemental specification. XANES studies showed that CoFe2O4 has an inverse spinel

structure while ZnFe2O4 has normal spinel structure. In the midway CoxZn1−xFe2O4

compounds when 0.2 ≤x ≤ 0.8, Co and Zn atoms always occupy the octahedral sites

and tetrahedral sites, respectively whereas crystallite site sizes were in range 10 nm to

35nm. DC electrical resistivity was measured by two probe method. It was found that

dc electrical resistivity decreases with increase in temperature. With increase in

dopant concentration (Zn), dc electrical resistivity increased. Activation energies

calculated by linear fitting of resistivity measurements for all the samples ensured that

the conduction in Zn doped Co nanoferrites is due to polaron hopping. The drift

mobility of charge carriers increased with increase in temperature but decreased with

increase in Zn concentration. In ac studies the dielectric constant decreased with

increase of frequency. Depending upon the composition the dielectric constant

increased up to x = 0.4 (40 % of Zn) and then decreased with further increase in Zn

concentration. The dielectric losses were found to be lower for higher frequencies. AC

conductivity increases with increase in frequencies. Depending upon the composition

the ac conductivity is maximum for x = 0.4 (40 % of Zn). In graphs for change in ac

conductivity with respect to the Jonscher’s power law the values of ‘n’ between 0 and

1 indicates the polaron hopping mechanism of ac conduction. This study ascertains a

fact that properties of Co1-xZnxFe2O4 ferrites with spinel structure strongly depend

upon the elemental composition and site occupancy by cation. The conversion of

74

normal spinel structure of ZnFe2O4 into inverse spinel structure of CoFe2O4 pursues a

regular pattern with gradual replacement of Co by Zn even in nanocrystalline

materials as proved by XANES studies.

75

Chapter 5


of nanocrystalline Co1-xMnxFe2O4

(x = 0.0 to 1.0)

76


sensor applications. In the present study cobalt based nanoferrite particles with Mn as

a dopant element with different ratio by weight are synthesized. Mn has larger ionic

radius as compared to the cobalt. The replacement of Co by Mn was expected to

modify the structural parameter which could affect the ultimate material properties.

Synthesized samples were characterized to study the effects of successive replacement

of Co with Mn on cation distribution, structural and conduction (electrical) properties.

A very precise technique (XANES) was applied to study the cation shift.

5.1 Thermal analysis

Specific transition temperatures were studied by differential scanning calorimetry

(DSC) and thermogravimetric analysis (TGA). We employed DSC analysis on, as

prepared powder samples before any further heat treatment. Thermograms for (DSC)

and (TGA) of CoFe2O4 and MnFe2O4 from room temperature to 1000°C are shown in

Figures 5.1-5.2 respectively.

0 200 400 600 800 1000

-0.02

-0.01

0.00

0.01

He

at F

low

(W

/g)

Temperature (oC)

CoFe2O

4

-0.02

-0.01

0.00

0.01

0.02

MnFe2O

4

Figure 5.1 The differential scanning calorimetry (DSC) of CoFe2O4 and MnFe2O4

77

0 200 400 600 800 1000

60

65

70

75

80

85

90

95

100

We

igh

t %

Temperature (oC)

CoFe2O

4

MnFe2O

4

Figure 5.2 The thermogravimetric analysis (TGA) of CoFe2O4and MnFe2O4

In DSC graph a wide exothermic peak without any sharp peak for any kind of

transition indicates the temperature range for crystal recovery temperature for both

compositions. In TGA plot all major weight loss processes have been completed

before 600°C for CoFe2O4 whereas for MnFe2O4, the processes of weight loss

proceeds up to about 900°C. Initial weight loss up to about 100°C is due to the

dehydration of the samples. From 100 to about 400 °C weight loss can be associated

to the decomposition of nitrates and removal of OH ions. At the next step the weight

loss occurs due to the removal of structural water. After the gradual weight loss up to

800°C, the rate of weight loss increased in case of MnFe2O4, this weight loss may be

due to dehydration, and some organic residues left in the sample [119]. In DSC graph

a wide exothermic peak without any sharp peak for any kind of transition indicated

the temperature range for crystal recovery temperature. Considering the DSC and

TGA graphs, 600°C was the selected temperature for sintering to obtain the maximum

crystalline phase.

78

5.2 Structural analysis

5.2.1 The X-ray diffraction (XRD) studies

The X-ray diffraction (XRD) patterns confirmed FCC spinel structure for all

synthesized samples of Co1-xMnxFe2O4 nanocrystalline particles with ‘x’ changing

from 0.0 to 1.0 and are shown in Figure 5.3. The XRD patterns were indexed using

the JCPDS card numbers 22-1086 and 74-2403. The crystallite sizes were calculated

by Scherrer equation (3.1) using the full width at half maximum (FWHM) value of

the most intense (311) peak. The data obtained from XRD was used for this purpose.

The crystallite sizes were in the range from 16nm to 35nm.

Figure 5.3 XRD patterns of Co1-xMnxFe2O4(x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0)

The crystal structure is found to be cubic spinel with space group Fd3m and the lattice

constant ranges from 8.362(1) Å to 8.465(8) Å. Lattice constant increases with

increase in Mn concentration within error. Increase in lattice constant occurs because

the Mn has larger ionic radius as compared to the Co which was replaced by the Mn

79

ion. MnFe2O4 is a mixed spinel with 20% degree of inversion. When Mn goes on A-

sites, lattice constant increases. The distance between cations at tetrahedral A sites

(LA) and between octahedral B sites (LB) was calculated using the relations 3.5 and

3.6, are shown in Figure 5.4.

Figure 5.4 Change in hopping length between tetrahedral A sites (LA) and octahedral

B sites (LB) with Mn concentration (x) in Co1-xMnxFe2O4

Figure 5.5 Change in theoretical density (x ) and measured density(m ) with Mn

concentration (x) in Co1-xMnxFe2O4

0.0 0.2 0.4 0.6 0.8 1.0

3.45

3.50

3.55

3.60

3.65

3.70

3.75

LA

LB

X (concentration)

LA(H

opp

ing L

en

gth

) Å

2.80

2.85

2.90

2.95

3.00

3.05

3.10

3.15

3.20

LB(H

op

pin

g L

en

gth

)Å

0.0 0.2 0.4 0.6 0.8 1.0

4.8

4.9

5.0

5.1

5.2

5.3

5.4

5.5

5.6

5.7

5.8

5.9

6.0

6.1

6.2

rx

rm

Concentration (x)

r x (

g /cm

3)

0.0

0.8

1.6

2.4

3.2

4.0

4.8

5.6

rm (g

/cm

3)

80

Values are also given in table 5.1 with possible errors. For conduction mechanism

electrons have to hop between ions of the same element existing in different valence

state on equivalent lattice sites. The change in theoretical density (x) and measured

density (m) with Mn concentration (x) in Co1-xMnxFe2O4 is given in Figure 5.5. The

values of theoretical density (ρx), measured density (ρm), and porosity (P), are given in

table 5.1. It was observed that due to increase of Mn concentration both of the

theoretical density (ρx), and measured density (ρm) decrease. X-ray densities were

higher than corresponding physical measured densities. The lower values of measured

densities can be attributed to the porosity of the samples.

5.2.2 X-ray absorption fine structure (XAFS) measurements

CoFe2O4, ZnFe2O4, MnFe2O4, CdFe2O4 and their mix, they all are having spinel

crystal structure, but entirely they all differ from each other depending upon the

allocation of cations. To investigate the cationic distribution in CoMnFe2O4 crystal

system the XAFS spectroscopy was employed. The normalized absorption spectra in

the near-edge region (XANES) at Fe, K-edges for different compositions of

Co1−x Mnx Fe2 O4 are shown in Figure 5.6.

The X-ray absorption fine structure (XAFS) measurements for all compositions of

Co1−x Mnx Fe2O4 was taken in transmission mode at Fe K-edges, at the XAFS beam

line of the ELETTRA synchrotron light source using a double crystal Si (111)

monochromator. To have XAFS measurements sample pellets were prepared by

pressing 5-10 mg of sample powder (nanometric) with 100 mg of boron nitride

powder.

Normalization procedure was done by Xanda VIPER (Visual Processing in EXAFS

Research) Program. The normalized XANES spectra were obtained by subtracting the

smooth pre-edge absorption from the experimental spectra and taking the edge jump

height as unity. After normalization the absorption spectra of the near-edge region

XANES are shown in Figure 5.6. A small pre-edge peak shown in inset of the graph

(Figure 5.6) is observed which is due to the 1s to 3d transition, while the main peak is

attributed to 1s to 4p transitions [109]. As explained in the previous chapter an

eminent behavior of the pre-edge peak is for sites with octahedral and tetrahedral

81

symmetry which are present in the spinel crystal structure. This peak height is higher

for the tetrahedral symmetry and has low intensity for octahedral symmetry. Both 1s

to 4p dipole transitions and 1s to 3d quadrupole transitions and are allowed for the Fe

K-edge, although quadrupole transition intensity is generally very low [120].

Figure 5.6 Normalized XANES spectra at Fe K-edge for the different composition of

Co1−x Mnx Fe2 O4. Inset is the pre edge peak.

For Fe K-edge data, the intensity of the pre edge peak increases relatively with

increasing Co content. It shows that an increasing fraction of Fe goes to tetrahedral

sites as a function of Co and Co occupies the octahedral B sites. Know it is well

known that spinel structure of MnFe2O4 is a partially inverted (Inversion degree is

low) because manganese ions predominantly occupies tetrahedral sites. The degree of

inversion in MnFe2O4 is 0.20 because only 20% Mn goes to octahedral sites.

Therefore in the Co Mn Fe2O4 mix system the degree of inversion depends upon the

relative concentration of Co and Mn ions. These results clearly showed that the degree

of inversion can successively be changed the stoichiometries. Since the ultimate

electric and magnetic properties of spinels depend upon the particular cations at

82

particular sites in the crystal structure. Hence, the material properties can be

controlled by selecting an appropriate composition.

5.3 Electrical measurements


The dc electrical resistivity for all these samples were measured by two probe method

from 370 K to 690 K as shown in Figure 5.7. The change in resistivity at different

temperature values for all the compositions is shown in Figure 5.8 and also with

Arrhenius linear fit in figure 5.9.

300 400 500 600 700

0.0

2.0x107

4.0x107

6.0x107

8.0x107

1.0x108

(

-cm

)

Temperature (K)

x=0.0

x=0.2

x=0.4

x=0.6

x=0.8

x=1.0

Figure 5.7 Variation of dc resistivity ρ (Ω-cm) with change in temperature for

Co1-xMnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0)

83

0.0 0.2 0.4 0.6 0.8 1.0

-5.0x105

0.0

5.0x105

1.0x106

1.5x106

2.0x106

2.5x106

3.0x106

3.5x106

(

-cm

)

(

-cm

)

Concentration x

373 K

473 K

573 K

673 K

0.0

5.0x103

1.0x104

Figure 5.8 Variation of dc resistivity ρ (Ω-cm) at different temperature for

Co1-xMnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0)


Co1-xMnxFe2O4. Lines are the Arrhenius relation fit.

15 20 25 30 35 40

4

6

8

10

12

14

16

18

20

ln

(cm

)

1/kBT (eV)

-1

X = 0.0

X = 0.2

X = 0.4

X = 0.6

X = 0.8

X = 1.0

84

The resistivity plots in Figure 5.7 show that with decrease in temperature resistivity

increases. At all the temperature values the resistivity increases with increase in Mn

concentration. The dc electrical resistivity of the ferrite nanoparticles was found to

vary from 2.26×106 Ω cm to 7.74×106 Ω cm at 373K with changing Mn concentration

from x = 0.0 to1.0 in Co1-xMnxFe2O4. It is observed that dc electrical resistivity

increases significantly with increase of Mn concentration. With increase in

temperature resistivity decreased nonlinearly. It is also observed that resistivity

increases with decrease in crystallite size. The spinel structure of ferrites consists of

two interpenetrating sub-lattice forming two types of sites where metal ions are

located, the tetrahedral (A) and octahedral (B) sites. The jump length between B-B

sites is less as compared to A-A or A-B sites. In spinel structure the most favorable

sites for hopping of charge carriers are from B to B sites [121].

The increase in resistivity does not strictly mean that the number of charge carriers is

decreasing but it is the rate of hopping of charges which does affect. The variation of

resistivity can be explained on the basis of actual location of cations in the spinel

structure and also by microstructures of the material. In CoMnFe2O4 conduction

mechanism is considered as the electron hopping between Fe2+and Fe3+in (B) sites

and mobility of holes between Co2+ and Co3+ ions. Addition of Mn+2 ions causes the

increase in Fe2+ ions on octahedral site and decrease in Co2+ ions. Above 400K

conduction decreases with increase in Mn concentration, because in this range

conductivity is due to the jumping of holes between Co2+ and Co3+ions. So

replacement of Mn at the expense of Co will consequently decrease the conductivity.

From collected data it is observed that conductivity increases with increase in

temperature because at high temperature hopping of electrons between Fe2+ and Fe3+

and holes is maximized. Therefore with increase in temperature conductivity

increases and resistivity decreases.

The values of activation energies (E) evaluated from the slopes of the linear plots of

dc electrical resistivity are shown in Figure 5.10. Activation energies increase with

increase of Mn content and it may be attributed to increase in hopping length and

lattice expansion with increase of Mn concentration. E ranges from 0.516 eV to

0.557 eV clearly suggests that the conduction is due to polaron hopping [111].

85

Figure 5.10 Variation in activation energy (E) with change of Mn concentration (x)

in Co1-xMnxFe2O4

The hopping depends upon the activation energy, which is associated with the

electrical energy barrier experienced by the electrons during hopping. The plots for

drift mobility are shown in Figure 5.11.

Figure 5.11 Variation of drift mobility µd (cm2/Vs) with change in temperature for

Co1-xMnxFe2O4

0.0 0.2 0.4 0.6 0.8 1.0

0.46

0.48

0.50

0.52

0.54

0.56

0.58

0.60

E

(e

V)

x ( Mn Concentration)

15 20 25 30 35 40

0.0

4.0x10-6

8.0x10-6

1.2x10-5

1.6x10-5

2.0x10-5

Dri

ft m

obili

ty

d(c

m2

V-1

s-1)

1/kBT (eV) -1

X = 0.0

X = 0.2

X = 0.4

X = 0.6

X = 0.8

X = 1.0

86

Drift mobility decreased with increase in Mn concentration which in turn increases

resistivity. Drift mobility is low at low temperature and increases sharply with

increase in temperature due to ease in hopping process.


5.3.2.1 Dielectric Constant

The dielectric properties of manganese substituted cobalt ferrites have been measured

at room temperature in the frequency range from 100 Hz to 3 MHz. The graphs of

dielectric constant versus frequency in Figure 5.12 show a normal dielectric behavior

for the synthesized materials. The change in the values of dielectric constant as a

function of frequency for mixed Co-Mn ferrites with different compositions was

observed.

Figure 5.12 Change in dielectric constant with ln(f) in Co1-xMnxFe2O4.

The characteristic features of the dielectric behavior have been known to arise from

distribution and type of the cations among the tetrahedral A-sites and octahedral B-

sites in the structure [110]. The fall in dielectric constant is speedy at low frequencies

and becomes slow for higher frequencies and then approach to frequency independent

2 4 6 8 10 12 14 16

0

10000

20000

30000

40000

50000

60000

Die

lectr

ic c

on

sta

nt (

)

ln (f)

X = 0.0

X = 0.2

X = 0.4

X = 0.6

X = 0.8

X = 1.0

87

behavior. The dielectric distribution curves can be explained by Koop’s theory.

Koop’s theory based on the Maxwell Wagner model considering the homogeneous

double layer structure [103]. There are conducting grains and insulating grain

boundary regions inside the medium. These regions form capacitors and resistors.

Hence, equivalent capacitor resistor parallel circuits made up. According to Rabkin

and Novikova [28], the polarization phenomenon in the ferrites occurs by a

mechanism comparable to the conduction process. The local displacement of

electrons gives the phenomena of polarization, which determines the dielectric

constant of ferrites. The fall in polarization which means reduction in dielectric

constant as a function of frequency takes place when hopping frequency of charge

carriers cannot follow the externally applied alternating electric field [112].

The variations in dielectric constant at different frequencies for all compositions of

Co1-xMnxFe2O4 are given in Figure 5.13. It is clear from plots that the dielectric constant

gradually decreases with increase in frequency and has decreasing trend with increase in

Mn concentration.

0.0 0.2 0.4 0.6 0.8 1.0

0

50

100

150

200

250

300

350

D

iele

ctr

ic c

on

sta

nt (

)

Concentration (X)

Freq 100k

Freq 500k

Freq 1.2 M

Freq 3 M

Figure 5.13 Change in dielectric constant at different frequencies in Co1-xMnxFe2O4

88

The compositional dependence of the dielectric constants of mixed Co-Mn ferrites

can be explained by site occupancy. In the case of CoFe2O4 the ferrous ions content is

higher than in other mixed Co-Mn ferrites. As a consequence, it is possible for these

ions to be polarized to the maximum possible extent and give rise to a higher value of

dielectric constant. As the Mn content in the mixed Co-Mn ferrites gradually

increased at the expense of cobalt the ferrous and cobalt ions decreased. Also the

jump length increased as explained earlier. Therefore, the dielectric constant

decreases with increase in Mn content.

5.3.2.2 Dielectric loss (tan δ)

The change of dielectric loss (tan δ) as a function of frequency for all compositions at

room temperature is shown in Figure 5.14. In Figure 5.15 the variation in tangent loss

at different frequencies as a function of composition (x) for Co1-xMnxFe2O4 is given.

The dissipation factor (D) is typically expressed in terms of tan δ which may be

defined as the ratio of the energy dissipated to the energy stored in a component. In

plots for tan δ the peaks emerge when hopping frequency of localized electron

matches with the externally applied alternating electric field. The shifting of these

peaks towards higher frequency region is attributed to the increase of the hopping rate

of the charge carriers [122].

Figure 5.14 Effect of Mn concentration and frequency of the applied electric field on

tan δ for Co1-xMnxFe2O4.

2 4 6 8 10 12 14 16

0

2

4

6

8

10

12

14

16

tan

ln (f)

X= 0.0

X= 0.2

X= 0.4

X= 0.6

X= 0.8

X= 1.0

89

0.0 0.2 0.4 0.6 0.8 1.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

tan

Concentration (X)

Freq 100k

Freq 500k

Freq 1.2 M

Freq 3 M

Figure 5.15 Variation in dielectric loss (tan δ) at different frequencies of the applied

electric field for Co1-xMnxFe2O4.

5.3.2.3 AC conductivity

The variation in ac conductivity is shown in Figure 5.16. In Figure 5.17 variation in ac

conductivity at different frequencies as a function of composition for Co1-xMnxFe2O4

is given. AC conductivity is higher for higher frequencies of applied electric field.

Figure 5.16 Effect of Mn concentration and frequency of the applied electric field

on ac conductivity for Co1-xMnxFe2O4

2 4 6 8 10 12 14 16

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

ac c

on

du

ctivity (

-cm

)-1

ln (f)

X= 0.0

X= 0.2

X= 0.4

X= 0.6

X= 0.8

X= 1.0

90

Generally, conduction phenomenon is due to electron hopping among the ions of the

same element existing in the different valence state. As explained earlier in the cubic

close packed crystal lattice of spinel ferrites the most favorable electron hopping

process at B-B sites. Jump length calculations shown in a previous section

demonstrate that the hopping length between two metal ions on (B) sites is less than

that of the hopping length between two metals ions at (A) site.

Hence, the hopping probably between A to A site is very small. Also the hopping

conduction among A-A sites is not possible because preferred site occupancy for iron

ions is B-site [39]. A significant increase in magnitude with increase in cobalt

contents was observed. The variation in composition dependent behavior also arises

due to the variation in material density. At higher frequencies the value of ac

conductivity is maximum it is due to the pumping force of the externally applied

frequency which helps in pushing the charge carriers for hopping in between localized

cation sites and also in making free the electrons trapped in various trapping centers in

the material.

0.0 0.2 0.4 0.6 0.8 1.0

-0.006

-0.003

0.000

0.003

0.006

0.009

a

c (S

-cm

-1)

Concentration x

1 kHz

10 kHz

100 kHz

1 MHz

Figure 5.17 Change in ac conductivity at different frequencies for Co1-xMnxFe2O4

91

The graphs for change in ac conductivity with respect to the Jonscher’s power law are

shown in Figure 5.18. The particular values of ‘n’ between ‘0’ and ‘1’show the

hopping mechanism of ac conduction.

For all the composition Nyquist plots are shown in Figure 5.19. The Nyquist plots

provide very useful information about the reactive and real components of the

material. To differentiate the contribution of grains and grain boundaries the complex

plane plots has been drawn in the frequency range of 20 Hz to 3 MHz at room

temperature. Nyquist plots were attained by plotting the () against (). Two

semicircles trend, which are attributed to the resistance of the grain and grain


on two parallel R-C elements in series. The plots indicate that the resistance of the

grain boundaries is higher than entire grains

0.0 0.2 0.4 0.6 0.8 1.0

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

Po

we

r n

Concentration x


concentration of composition Co1-xMnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0)

92

0 5000 10000 15000 20000 25000 30000

-5000

0

5000

10000

15000

20000

25000

30000

35000

40000

"

'

x=0.2

-2000 0 2000 4000 6000 8000 10000 12000 14000

0

10000

20000

30000

40000

50000

"

'

x=0.4

0 500 1000 1500 2000 2500

0

2000

4000

6000

8000

10000

"

'

x=0.6

-500 0 500 1000 1500 2000 2500 3000 3500 4000

0

5000

10000

15000

20000

25000

30000

"

'

x=0.8

0 200 400 600 800 1000

0

2000

4000

6000

8000

10000

12000

14000

"

'

x=1.0

Figure 5.19 Nyquist plots for all the compositions for Co1-xMnxFe2O4 (x=0.0, 0.2,

0.4, 0.6 0.8, 1.0)

0 50 100 150 200 250 300 350 400

0

100

200

300

400

500

600

Die

lectr

ic L

oss F

acto

r (

'')


x = 0.0

93

Table 5.1 Crystallite size d(311), lattice constant(a), hopping lengths LA(Å) and LB(Å)

Cell volume (V), X-ray density(ρx), measured density (ρm), resistivity (ρ) and porosity

(P) for different values of ‘x’ in Co1-xMnxFe2O4


d(3 11)(nm) 35(4) 16(2) 28(3) 18(2) 35(4) 27(3)

a(Å) 8.362(1) 8.364(1) 8.373(2) 8.384(8) 8.465(8) 8.408(1)

V(Å3) 584 584 586 588 605 592

LA(Å) 3.619(1) 3.619(1) 3.624(2) 3.629(8) 3.663(8) 3.637(1)

LB(Å) 2.955(1) 2.955(1) 2.959(2) 2.963(8) 2.991(8) 2.970(1)

ρx(g/cm3) 5.336 5.318 5.281 5.244 5.079 5.171

ρm(g/cm3) 3.570 2.703 3.095 2.981 2.125 2,544

ρ (Ω-cm) 2.74×106 2.88×106 2.9×106 3.4×106 3.68×106 7.74×106

P 0.331 0.492 0.414 0.431 0.582 0.508

94

Summary

Co1-xMnxFe2O4 nanocrystallite particles for x = 0.0, 0.2, 0.3, 0.4, 0.6, 0.8 and 1.0, were

prepared successfully by co-precipitation method. To examine the specific thermal

transition temperatures, differential scanning calorimetry and thermogravimetric

analysis were done. The temperature of 600°C was selected for sintering. The crystal

structure is found to be an inverse cubic spinel with a space group of Fd3m and the

lattice constant ranges from 8.362(1) Å to 8.465(8) Å. Lattice constant increases with

increase of Mn concentration in CoFe2O4. XANES shows that an increasing fraction of

Fe goes to tetrahedral sites as a function of Co and Co occupies the octahedral B sites.

It is observed that dc electrical resistivity increases significantly with increase of Mn

concentration. The maximum value was found to be 7.74×106 Ω cm at 373K.

Activation energies increase with increase of Mn content and it may be attributed to

increase in hopping lengths. E ranges from 0.516eV to 0.557eV which clearly

suggest that the conduction is due to polaron hopping. Drift mobility decreases with

increasing Mn concentration. The dielectric constant decreases with the increase of

frequency. At higher frequencies the value of ac conductivity was found to be

maximum. The ac conductivity has decreasing trend with decrease in cobalt content

although microstructures are also affecting this trend. The dielectric losses decreases

with increase in frequency. In plots for variation in ac conductivity with respect to the

Jonscher’s power law the values of ‘n’ between 0 and 1 indicates the polaron hopping

mechanism of ac conduction. This study reveals a fact that properties of

Co1-xMnxFe2O4 ferrites with spinel structure strongly depend upon the elemental

composition and site occupancy by cations. The site occupancy can be controlled by

controlling the elemental composition.

95

Chapter 6


of nanocrystalline Co1-xCdxFe2O4

(x = 0.0 to 1.0)

96

CdFe2O4 is generally understood as normal spinel in which all Fe3+ ions occupies B-

sites and all Cd2+ ions occupies A-sites [112]. CoFe2O4 has an inverse spinel structure.

We replace Co from the CoFe2O4 to investigate the whole transformation from inverse

to normal spinel. Also the Cd has quite larger ionic radius as compared that of cobalt.

The gradual replacement of nonmagnetic Cd ions in cobalt ferrite is expected to create

considerable distortion in CoCdFe2O4 system. A little work was found on mixed Co-

Cd ferrites [117]. CoFe2O4 nanocrystalline particles were prepared by co-precipitation

method and Co contents were replaced by Cd successively. A study of thermal,

structural and electrical properties of Co1-xCdxFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) is

presented here.

6.1 Thermal Analysis

Particular transition temperatures were studied by differential scanning calorimetry

(DSC) and thermogravimetric analysis (TGA). We employed DSC analysis for as

synthesized powder before any further heat treatment. Plots of thermal studies (DSC

and TGA) for CoFe2O4and CdFe2O4 from room temperature to 1000°C are shown in

Figure 6.1 and Figure 6.2 respectively.

Figure 6.1The differential scanning calorimetric (DSC) thermogram of

CoFe2O4and CdFe2O4

0 200 400 600 800 1000

-0.02

-0.01

0.00

0.01

0.02

Exo Up

CdFe2O4

Heat

Flo

w (

W/g

)

Temprature (°C)

CoFe2O

4

97

In DSC graphs a wide exothermic peak without any sharp peak for any kind of

transition indicates the temperature range for crystal recovery temperature. In TGA

graphs all major weight losing phenomena have been completed before 600°C. After

dehydration, removal of nitrates or any other organic residual (if present) in the

sample occurs. At the next step weight loss can be attributed to the removal of

structural water for both end compositions in Co1-xCdxFe2O4 (x = 0.0 to 1.0). Taking

into account these facts, 600°C was the chosen temperature for sintering to obtain the

maximum crystalline phase of Cd doped CoFe2O4.

6.2 Structural Analysis

6.2.1 X-ray diffraction (XRD) studies

The X-ray diffraction (XRD) patterns confirmed FCC spinel structure for all the

prepared samples of Co1-xCdxFe2O4 nanocrystalline particles with ‘x’ changing from

0.0 to 1.0 as shown in Figure 6.3.

Figure 6.2 The thermogravimetric analysis (TGA) thermogram of CoFe2O4 and CdFe2O4

0 200 400 600 800 1000

80

82

84

86

88

90

92

94

96

98

100

CoFe2O

4

CdFe2O

4

We

igh

t (

)

Temprature (°C)

98

The crystallite sizes of the samples were in the range from 162 nm to 354 nm. It is

difficult to relate the crystallite size distribution with any single parameter. The

formation of nucleation sites at different times, coalescence and lattice constants may

affect the crystallite size. During sintering, two or more particles seem to fuse

together by melting of their surfaces. In our prepared samples the crystallite size

decreases up to x = 0.4 and then increased with further increase of Cd content which

may be due to increase in lattice constants. The lattice constants were in range from

8.362(1) Å to 8.602(2) Å. The lattice constant 'a' was found to increase with an

increase of Cd concentration at the expense of Co. It is due to the fact that the radius

of Cd2+ ion (0.97 Å) is larger than that of the Co2+ ions (0.78Å). A linear increase in

the lattice constant with an increase in Cd contents indicates that the lattice expands

without upsetting the symmetry of the lattice. It is in accordance with Vegard’s Law

[106]. The values of lattice constants and grain sizes are given in the table 6.1 and are

shown in

Figure 6.3 X-ray diffraction pattern of Co1-xCdxFe2O4

20 30 40 50 60 70 80

(3 3 1)(5 3 3)(4 4 0)(5 5 1)

(4 0 0)

(3 1 1)(2 2 0)

X = 1.0

X = 0.8

X = 0.6

X = 0.4

X = 0.2

X = 0.0

Inte

nsity (

a.

u)

2 (Degree)

99

Figure 6.4 Change in lattice constants (a) and crystallite size with Cd concentration

in Co1-xCdxFe2O4

Figure 6.5 Change in hopping length between tetrahedral A sites (LA) and

octahedral B sites (LB) with Cd concentration (x) in Co1-xCdxFe2O4

0.0 0.2 0.4 0.6 0.8 1.0

8.35

8.40

8.45

8.50

8.55

8.60

Lattice Constant

Crystallite Size

X (Concentration)

La

ttic

e C

on

sta

nt (Å

)

16

18

20

22

24

26

28

30

32

34

36

38

40

Cry

sta

llite S

ize

(nm

)

0.0 0.2 0.4 0.6 0.8 1.0

3.62

3.64

3.66

3.68

3.70

3.72

3.74

LA

LB

X (Concentration)

LA

(H

op

pin

g le

ng

th)Å

2.94

2.96

2.98

3.00

3.02

3.04

LB

(Ho

pp

ing

len

th) Å

100

Figure 6.4. The distance between cation ions at tetrahedral A-sites (LA) and between

octahedral B-sites (LB) was calculated using the relations (3.5) and (3.6) respectively.

For conduction mechanism, electrons hop between ions of the same element existing

in different valence states on equivalent lattice sites. The change in hopping lengths

LA and LB is shown in Figure 6.5, values are also given in table 6.1 with possible

errors. Both LA and LB increase with an increase of Cd content. It is due to the increase

in lattice constant with increasing Cd content in the crystal geometry of the samples.

6.2.2 X-ray Absorption fine structure (XAFS) measurements

For Co1−x Cdx Fe2 O4 series the XAFS spectroscopy was also done at XAFS beam line

of the ELETTRA synchrotron light source. The XAFS measurements were taken at

room temperature in transmission mode at Fe, and Co K-edges, Cd has well known

site occupancy of tetrahedral symmetry. The normalized absorption spectra in the

near-edge region (XANES) at Fe K-edges and Co K-edges for different compositions

of Co1−xCdxFe2O4 are shown in Figure 6.6 and in Figure 6.7 respectively.

Normalization procedure was done by Xanda VIPER (Visual Processing in EXAFS

Research) Program. The normalized XANES spectra were obtained by subtracting the

smooth pre-edge absorption from the experimental spectra and taking the edge jump

height as unity. A small pre-edge peak shown in inset of the graph (Figure 6.6 and

Figure 6.7) is observed which is due to the 1s to 3d transition, while the main peak is

attributed to 1s to 4p transitions [107]. For the Fe K-edge, both 1s to 3d quadrupole

transitions and 1s to 4p dipole transitions are allowed [108], although the intensity of

the quadrupole transition is generally very low. The pre-edge peak becomes the

addition of these contributions, when both octahedral and tetrahedral sites are

occupied, the peak intensity increases directly with increase the proportion of

tetrahedral sites. For K-edge data of Fe, the intensity of the pre-edge peak increases

with rising Co content. It shows that a rising fraction of Fe goes to tetrahedral sites as

a function of Co and Co occupies the octahedral B sites. Cd has well known

occupancy of octahedral B site. These results clearly suggested that we can change the

101

.

Figure 6.7 Normalized XANES spectra at Cd K-edge for the different composition of

Co1−x Cdx Fe2 O4. Inset shows the pre edge peak heights in enlarged view.

Figure 6.6 Normalized XANES spectra at Co K-edge for the different compositions of

Co1−x Cdx Fe2 O4. Inset shows the pre edge peak heights in enlarged view

7000 7200 7400 7600

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d a

bso

rptio

n

Energy (eV)

X = 0.0

X = 0.2

X = 0.4

X = 0.6

X = 0.8

X = 1.0

7110 7111 7112 7113 7114 7115 7116 71170.00

0.02

0.04

0.06

0.08

0.10

X = 0.0

7700 7800 7900 8000 8100 8200 8300 8400 8500

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Norm

aliz

ed a

bsorp

tion

Energy (eV)

X = 0.0

X = 0.2

X = 0.4

X = 0.6

X = 0.8

7705 7710 7715 7720

0.00

0.05

X = 0.0

102

degree of inversion successively by changing the stoichiometry. Hence we can control

the material properties by selecting an appropriate composition

6.3 Electrical properties

6.3.1 DC electrical properties.

Conduction in ferrites occurs as a result of electron hopping between ions of the same

element existing in different valence state at equivalent lattice sites [108]. The dc

electrical resistivity for all these samples was measured by two probe method from

313 K to 700 K temperature range. It is observed that dc electrical resistivity increases

significantly with increase in Cd concentration as shown in the Figure 6.8. The change

in resistivity at different temperature values for all the compositions is shown in Figure

6.9 and 6.10. The resistivity plots in Figure 6.8 show that with decrease in temperature

resistivity increases. At all the temperature values the resistivity increases with increase in

Cd concentration. The rate of resistivity after x = 0.6 is higher which can be attributed to

the large expansion in lattice and increase in hopping lengths.

400 450 500 550 600 650 700 750

-2.0x109

0.0

2.0x109

4.0x109

6.0x109

8.0x109

1.0x1010

1.2x1010

1.4x1010

1.6x1010

(

-cm

)

Temperature (K)

x=0.0

x=0.2

x=0.4

x=0.6

x=0.8

x=1.0

Figure 6.8 Change in dc resistivity with Cd concentration (x) and temperature in

Co1-xCdxFe2O4

103

In Figure 6.9 the variation in dc electrical resistivity at different temperatures for all

the composition is shown.

0.0 0.2 0.4 0.6 0.8 1.0

0.0

3.0x109

6.0x109

9.0x109

1.2x1010

1.5x1010

(

-cm

)

(

-cm

)

Concentration x

423K

473K

0.0

5.0x106

1.0x107

1.5x107

2.0x107

2.5x107

573K

673K

Figure 6.9 Change in dc resistivity with Cd concentration (x) at different temperatures

in Co1-xCdxFe2O4

Figure 6.10 Change in dc resistivity with Cd concentration (x) and temperature in

Co1-xCdxFe2O4. Lines are the Arrhenius relation fit.

16 18 20 22 24 26 28

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

ln

(cm

)

(1/kBT (eV)

-1

X = 0.0

X = 0.2

X = 0.4

X = 0.6

X = 0.8

X = 1.0

104

Conduction mechanism in CoCdFe2O4 is considered as electron hopping between

Fe2+and Fe3+in (B) sites. The tetrahedral A site is strongly preferred site of occupancy

for Cd. The addition of Cd ions at the expense of Co leads to increase of Fe3+ and a

corresponding decrease of Fe2+ ions in the octahedral B site and a simultaneous

decrease in Co2+ ions. Due to this mechanism, the available Fe2+ and Fe3+ pairs at

octahedral B sites decrease and hence conduction phenomenon decreases. Also with

an increase of Cd concentration the lattice expands and hopping length of electrons

increased as given in the table 6.1 and shown in Figure 6.5. It increases the required

activation energy; therefore dc resistivity increases with increase of Cd concentration

(Figure 6.10). The values of activation energies (E) evaluated from the slopes of the

linear plots of dc electrical resistivity are shown in Figure 6.11.

Activation energies increase with an increase of Cd content and it is attributed to

increase in the hopping length and lattice expansion. E ranges from 0.54 eV to

0.98 eV clearly suggested that the conduction is due to polaron hopping [111]. The

hopping depends upon the activation energy, which is associated with the electrical

energy barrier experienced by the electrons during hopping. The plots for drift

mobility are shown in Figure 6.12. Drift mobility decreases with increase in Cd

Figure 6.11 Effect of Cd concentration on activation energy in Co1-xCdxFe2O4

0.0 0.2 0.4 0.6 0.8 1.00.5

0.6

0.7

0.8

0.9

1.0

E

(e

V)

x ( Cd Concentration)

105

concentration which correspondingly increases resistivity. Drift mobility varies

directly with temperature. With increase of temperature hopping processes of charge

carriers increases which consequently increases drift mobility of charge carrier.


6.3.2.1 The dielectric constant (ε')

The dielectric characteristics of ferrites normally arise from hopping of charge

carriers between cationic sites and across the grain boundaries. The Variation in

dielectric constant at different frequencies for Co1-xCdxFe2O4 is given in Figure 6.13.

From the Figure 6.13 it is clear that the dielectric constant gradually decreases with

increase in frequency. At all temperatures the dielectric constant has higher values for x =

0.6. Dielectric constant initially increases up to x = 0.6 and then decreases with further

increase in Cd concentration. The variation in dielectric constant for all samples with

change in frequency and composition is shown in Figure 6.14.

16 18 20 22 24 26 28

0.0

4.0x10-6

8.0x10-6

1.2x10-5

1.6x10-5

2.0x10-5

Dri

ft m

obili

ty

d(c

m2

V-1

s-1

)

1/kBT (eV) -1

X = 0.0

X = 0.2

X = 0.4

X = 0.6

X = 0.8

X = 1.0

Figure 6.12 Effect of Cd concentration and temperature on drift mobility

of Co1-xCdxFe2O4

106

1 2 3 4 5 6 7

-50000

0

50000

100000

150000

200000

250000

Die

lectr

ic C

onsta

nt (

')

log (f)

x = 1.0

x = 0.8

x = 0.6

x = 0.4

x = 0.2

x = 0.0

0.0 0.2 0.4 0.6 0.8 1.0

0

50

100

150

200

250

300

D

iele

ctr

ic C

on

sta

nt (

')

Concentration (X)

Freq 100k

Freq 500k

Freq 1.2 M

Freq 3 M

Figure 6.14 Effects of Cd concentration at different frequencies of the applied electric

field on dielectric constants for Co1-xCdxFe2O4

At low frequency region, the dielectric constant has high values. In the high-

frequency region, values of () decrease to very small values. The decline in

Figure 6.13 Effects of Cd concentration and frequency of the applied

electric field on dielectric constants for Co1-xCdxFe2O4

107

dielectric constant is fast at low frequencies and turn into slow decline at higher

frequencies approaching to frequency independent behavior. When an alternating

electric field is applied on dielectric material, dipoles orientation changes as the

direction of the applied field. For lower frequencies, the orientation of dipoles can

easily follow the applied field. At comparatively higher frequencies, the direction of

dipoles begins lagging the applied field caused by inertial effects and the dielectric

constant turns to a complex quantity. Grain boundaries are found to be more effective

for lower frequencies whereas the relatively low resistive grains are more effective on

higher frequencies [123]. In ferrites the exchange of electrons between Fe2+ and Fe3+

determines polarization. The high value of dielectric constant at lower frequency is

because of a large amount of species such as Fe2+ ions, grain surface defects and

oxygen vacancies [113]. Polarization phenomenon in ferrite material occurs by a

mechanism similar to conduction process. Considering composition the dielectric

constant increases up to x = 0.4 and then decreases for further increase in Cd+2

content. It has been reported that Cd ions have preference for tetrahedral A-sites

occupy [107] while Fe and Co ions occupy both of sites tetrahedral (A) and octahedral

(B) sites partially [112]. According to this behavior addition of Cd ion at the expense

of Co ion causes the migration of Fe3+ from tetrahedral (A) sites to octahedral (B)

sites. With increasing in Fe3+ at B-sites the hopping rate between Fe3+and Fe2+

increases from one B-site to another neighboring B-site. This hopping mechanism

leads to increase dielectric dispersion. Because the hopping length increases with an

increase in Cd2+ contents. So with further addition of Cd2+ ions, the increase in jump

length between two nearest B-sites can go up to such extent that may cause the

decrease in hopping phenomenon. Therefore, dielectric constant decreases with

further increase in Cd2+ ions.

6.3.2.2 Dielectric losses (tan δ) and dielectric loss factor (ε'')

The change in tan δ and loss factor as a function of frequency for all compositions

at room temperature is shown in Figures 6.15 and 6.16 respectively. In plots for tan δ

the peaks emerge when hopping frequency of localized electron matches with the

externally applied alternating electric field. The shifting of these peaks towards higher

108

frequency region is attributed to the increase of the hopping rate of the charge

carriers.

Figure 6.15 Effects of Cd concentration (x) and frequency of the applied electric field

on dielectric loss (tanδ) for Co1-xCdxFe2O4

The peak behavior of tan δ can be clearly explained according to Rezlescuu model

[38]. According to this model

= 1 (6.1)

Where =2fmax and is the relaxation time in hopping process. The relaxation

time of hopping process is inversely proportional to the jumping probability per unit

time (P). According to this relation

= 1/P (6.2)

It is obvious from relations (6.1) and (6.2) that ( fmax ) and (P) are proportional to

each other. The peaks for loss tangent are found to shift towards the higher

frequencies up to x = 0.4 and then with further increase in Cd concentration peaks

shift towards low frequency region. This behavior is in accordance with dielectric

constant. In crystal system for Co1-xCdxFe2O4 the existence of Co3+ and Co2+ ions

furnishes p-type carriers. The local dislocation of p-type carriers along the externally

1 2 3 4 5 6 7

0

2

4

6

tan

log (f)

x = 0.0

x = 0.2

x = 0.4

x = 0.6

x = 0.8

x = 1.0

109

applied electric field also contributes for net polarization along with n-type carriers.

But this contribution of p-type carriers’ remains smaller as compared to the electron

hopping between Fe3+ and Fe2+.


on dielectric loss factor () for Co1-xCdxFe2O4

In the present crystal system the conduction mechanism is attributed to the hopping of

electrons between Fe3+ and Fe2+ and hopping of holes among Co3+ and Co2+. Initially

the addition of Cd2+ contents at the cost of cobalt pushes some of the Fe3+ ions from

A-sites to B-sites. The increase in Fe3+ ions at octahedral sites raises the electron

hopping. With further increase in cadmium the increase of jumping length makes

hopping of charge carriers difficult. Consequently the hopping phenomenon begins

lagging the applied field.

() also shows dispersion as a function of frequency exactly in the similar way as

that of (). The values of () decreases faster than () and becomes closer in the

high-frequency region.

In Figure 6.17 the variation in tangent loss at different frequencies for Co1-xCdxFe2O4

is shown. Plots in the figure indicates that tangent loss is higher for x = 0.6. It initially

increases with increasing Cd contents and then decreases.

1 2 3 4 5 6 7

-50000

0

50000

100000

150000

200000

250000

300000

350000

400000

Die

lectr

ic L

oss F

acto

r (

'')

log (f)

0.0

0.2

0.4

0.6

0.8

1.0

110

0.0 0.2 0.4 0.6 0.8 1.00.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

tan

Concentration (x)

100 kHz

500 kHz

1.2 MHz

3 MHz

Figure 6.17 Effects of Cd concentration (x), at different frequency of the applied

electric field on dielectric loss (tanδ) for Co1-xCdxFe2O4

6.3.2.3 AC conductivity (ac)

As explained earlier in the cubic close packed crystal lattice of spinel ferrites the most

favorable electron hopping process at B-B sites. Jump length calculations shown in a

previous section demonstrate that the hopping length between two metal ions on (B)

sites is less than the hopping length between two metals ions at (A) site. Hence the

hopping probably between A to A site is very small. Also the hopping conduction

among A-A sites is not possible because preferred site occupancy for iron ions is B-

site [110]. Effects of Cd concentration (x) and frequency of the applied electric field

on ac conductivity (ac) for Co1-xCdxFe2O4 are shown in Figure 6.18.

111


on ac conductivity (ac) for Co1-xCdxFe2O4

It has been reported in literature (ac) is an increases with frequency in case of

electron hopping mechanism and decreases with increasing frequency for band

conduction. The increase in (ac) with externally applied electric field frequency is

due to the pumping force of the externally applied frequency which helps in pushing

the charge carriers in between localized sites and also liberating the electrons trapped

in various trapping centers. Plots in Figure 6.18 show that ac approximately remains

constant at lower frequencies and then starts increasing for higher frequencies.

Alternating current conductivity (ac ) increases with increase in Cd contents up to

x=0.4 and then decrease for further addition of Cd concentration. It is the same trend

as explained in the case of () and (tan δ).

1 2 3 4 5 6 7

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

ac c

on

du

ctivity (

-m)-1

log (f)

x = 0.0

x = 0.2

x = 0.4

x = 0.6

x = 0.8

x = 1.0

112

In Figure 6.19, variation in ac conductivity at different frequencies for Co1-xCdxFe2O4 is

given. AC conductivity is higher for higher frequencies of applied electric field.

0.0 0.2 0.4 0.6 0.8 1.0

-0.002

0.000

0.002

0.004

0.006

0.008

0.010

0.012

ac (

-cm

)

Concentration x

1 kHz

10 kHz

100 kHz

1 MHz

Figure 6.19 Effects of Cd concentration (x) at different frequencies of the applied

electric field on ac conductivity (ac) for Co1-xCdxFe2O4

The graphs for change in ac conductivity with respect to the Jonscher’s power law

[113] are shown in Figure 6.20. The particular values of ‘n’ between ‘0’ and ‘1’show

the hopping mechanism of ac conduction.

113

0.0 0.5 1.0

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

Po

we

r n

Concentration x


concentration of composition Co1-xCdxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0)

For all the compositions Nyquist plots are shown in Figure 6.21. The Nyquist plots

provide very useful information about the reactive and real components of the

material. To differentiate the contribution of grains and grain boundaries the complex

plane plots has been drawn in the frequency range of 100Hz to 3MHz at room

temperature. Nyquist plots were attained by plotting the () against (). Two

semicircles trend, which are attributed to the resistance of the grains and grain


on two parallel R–C elements in series. The plots shown in Figure 6.21 indicate that

the resistance of the grain boundaries is higher than that of entire grains.

114

Figure 6.21 Nyquist plots for Co1-xCdxFe2O4

0 2000 4000 6000 8000 10000

0

2000

4000

6000

8000

10000

12000

Die

lectr

ic L

oss F

acto

r (

'')


x= 0.8

0 5000 10000 15000 20000 25000

-5000

0

5000

10000

15000

20000

25000

30000

35000

40000

Die

lectr

ic L

oss F

acto

r (

'')


x = 0.6

0 50000 100000 150000 200000

-50000

0

50000

100000

150000

200000

250000

300000

350000

400000

Die

lectr

ic L

oss F

acto

r (

'')


x = 0.4

-500 0 500 1000 1500 2000 2500 3000 3500

0

1000

2000

3000

4000

5000

Die

lectr

ic L

oss F

acto

r (

'')


x = 1.0

0 50 100 150 200 250 300 350 400

0

100

200

300

400

500

600

Die

lectr

ic L

oss F

acto

r (

'')


x = 0.0

0 20000 40000 60000 80000 100000 120000

-20000

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

200000

Die

lectr

ic L

oss F

acto

r (

'')


x = 0.2

115

Table 6.1 Crystallite size d(311), lattice constant(a), Cell volume (V), hopping lengths

LA(Å) and LB(Å) X-ray density (ρx), measured density (ρm), porosity (P) for different

values of ‘x’ in Co1-xCdxFe2O4


d(3 11)(nm) 35(4) 18(2) 16(2) 18(2) 18(2) 19(3)

a(Å) 8.362(1) 8.401(1) 8.443(3) 8.501(2) 8.553(1) 8.602(2)

V(Å3) 584 593 601 614 625 636

LA(Å) 3.619(1) 3.638(1) 3.654(3) 3.680(2) 3.702(1) 3.723(2)

LB(Å) 2.955(1) 2.970(1) 2.982(3) 3.003(2) 3.021(1) 3.041(2)

ρx(g/cm3) 5.336 5.365 5.376 5.367 5.263 5.405

ρm(g/cm3) 3.570 2.517 2.537 2.989 2.340 2,819

P 0.331 0.473 0.463 0.467 0.439 0.427

116

Summary

Co1-xCdxFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) nanocrystalline particles were

synthesized by a wet-chemical co-precipitation technique. Specific thermal transition

temperatures were analyzed by differential scanning calorimetry (DSC) and

thermogravimetric analysis (TGA) to estimate the sintering temperature. After

analysis of DSC and TGA plots the 600°C temperature was chosen for sintering. We

obtained pure phase spinel crystal structure. FCC spinel crystal structure was

confirmed by X-ray diffraction analysis. The crystallite sizes of the samples were in

the range from 162 nm to 354 nm. Lattice constant calculations show that lattice

constant increases with increase in Cd concentration gradually. Cation distribution

was found by XAFS analysis which is a powerful technique for determination of local

crystal structure with elemental specification. XANES spectra shows that an

increasing fraction of Fe goes to tetrahedral sites as a function of Co and Co occupies

the octahedral B sites. Pure Cd ferrites have normal spinel structure. Crystal structure

transforms gradually from normal spinel to inverse spinel with increasing contents of

Co. The dc electrical resistivity for all compositions was measured by two probe

method from 313 K to 700 K temperature range. It was observed that dc electrical

resistivity increases significantly with increase in Cd concentration. Activation

energies calculated by linear fitting of resistivity measurements for all the samples

confirms that the conduction in Cd doped Co nanoferrites is due to polaron hopping.

The drift mobility of charge carriers increased with increase in temperature but

decreases with increase in Cd concentration. The dielectric dispersion and loss tangent

exhibits normal behavior in the frequency range 100Hz-3MHz at room temperature. It

was found that the replacement of Co with Cd enhances the value of , ε'', tan δ and

ac up to x=0.4 and then decreases gradually with further increase in Cd contents. The

ac electrical conductivity ( σac ) increases with increase in applied frequency of the

electric field. AC conductivity is inversely proportional to hopping length, but is

directly proportional to the hopping rate of electrons. With increase in ferrous ions at

octahedral sites, the conductivity increases but as the jumping length increases, it

leads to the decrease in conduction mechanism. The Nyquist plots indicate that the

resistive and capacitive properties are mainly endorsed as a result of the processes

associated to the grains and grain boundaries.

117

Chapter 7

7 Comparison of doping effects

118

We have observed the change in structural and electrical properties of the CoFe2O4

nano crystalline ferrite particles when the Co element was replaced gradually by Zn,

Mn and Cd. A comparison of these changes is presented here.

7.1 Thermal Studies

Thermal studies for particular transition temperatures were done by differential

scanning calorimetry (DSC) and Thermogravimetric Analysis (TGA) for M Fe2O4

(M= Co, Zn, Mn, Cd). The analysis was done for as synthesized powder before any

further heat treatment for all compositions in temperature range from room

temperature to 1000° and graphs are shown in Figure 7.1 and 7.2.

Figure 7.1 The differential scanning calorimetric (DSC) thermogram of A Fe2O4

(A= Co, Zn, Mn, Cd)

0 200 400 600 800 1000

-0.025

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

0.020

0.025

Exo Up Temprature (°C)

MnFe2O4

ZnFe2O4CoFe2O

4

CdFe2O4

Heat

Flo

w (

W/g

)

119

Figure 7.2 The thermogravimetric analysis (TGA) thermogram of AFe2O4 (A = Co,

Zn, Mn, Cd)

For all compositions in the DSC graphs a wide exothermic peak without any sharp

peak for any kind of transition indicates the temperature range for crystal recovery

temperature. TGA graphs for all synthesized compositions shows that almost all

major weight losing phenomena have been completed before 600°C whereas in case

of MnFe2O4 the weight loss was completed at about 800°C. At initial stage from room

temperature to about 100°C the weight loss can be associated to dehydration of the

material then up to about 300°C weight loss is attributed to the decomposition of

nitrates and at this stage the crystallization of sample materials started. In the next

stage the weight loss is due to the removal of structural water. Above 600°C the major

weight loss processes were completed for CoFe2O4, ZnFe2O4 and CdFe2O4 but the

weight loss for MnFe2O4 was completed up to about 950°C. Also from the combined

analyses of DSC and TGA the estimated temperature of 600°C as sintering

0 200 400 600 800 1000

60

65

70

75

80

85

90

95

100

Temprature (°C)

ZnFe2O

4

CdFe2O

4

CoFe2O

4

MnFe2O

4

We

igh

t (

)

120

temperature was selected to have maximum phase with same sintering conditions so

that their properties could be compared. Sintering at 600°C for 3 hours gives cubic

spinel phase for all compositions.

7.2 Structural Changes

For all compositions Co1-xMxFe2O4 (M= Zn, Mn, Cd & x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)

the structural analyses was done by XRD and XAFS techniques. The variation in all

structural parameters are given as under.

7.2.1 Crystal Structure

The XRD patterns confirmed FCC spinel structure with space group fd3m for all

synthesized nanocrystalline particles with compositions Co1-x Mx Fe2O4 (M= Zn, Mn,

Cd & x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0). Crystallite sizes of all sample materials were in

range the range from 10nm to 36nm.

7.2.2 Lattice Constant

For all compositions lattice constant was found to increase with increasing contents of

the dopant elements which were Zn, Mn and Cd at the expense of Co concentration

This behavior was expected because of the replacement of Co with atoms ( Zn, Mn

and Cd ) having larger atomic radii as compared to that of Co. Atoms with larger

atomic radii created expansion in the lattice structure. Hence lattice constants were

increased. This increase in lattice constant also influenced the hopping lengths and

electrical properties of the prepared materials. This is due to the fact that the ionic

radii of the Co2+, Zn2+, Mn2+ and Cd2+ are in the order Co2+ < Zn2+ < Mn2+ < Cd2+.

Therefore when we replace Co2+ by an element of larger ionic radius gradually the

lattice increases. Here the expansions occur linearly with every dopant element

without upsetting the crystal structure in accordance with Vegard’s law.

The lattice constant values for all compositions are given here in the table 7.1.

121

Table 7.1 Lattice constants “a” in Angstroms (Å) for all compositions Co 1-xMxFe2O4

(M= Zn, Mn, Cd & x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)

Composition x = 0.0 x = 0.2 x = 0.4 x = 0.6 x = 0.8 x =1.0

Co1-x Znx Fe2O4 8.362(1) 8.368 (5) 8.393(1) 8.411(3) 8.425(1) 8.443(1)

Co1-x MnxFe2O4 8.362(1) 8.3641) 8.373(2) 8.384(8) 8.465(8) 8.408(1)

Co1-x Cdx Fe2O4 8.362(1) 8.401(1) 8.443(3) 8.501(2) 8.553(1) 8.602(2)

7.2.3 Hopping Lengths

The distances between cations at tetrahedral A sites (LA) and between octahedral B-

sites (LB) were calculated for all the sample compositions and are given here in the

table 7.2. Hopping lengths play an important role in the conduction mechanism

because the charge carriers have to hop between ions of the same element existing in

different valence states at equivalent lattice sites for conduction in ferrites. The

change in the hopping lengths LA and LB are shown in the table 7.2. As explained in

the above section this is due to the fact that the ionic radii of the Co2+, Zn2+, Mn2+

and Cd2+ are in the order Co2+ < Zn2+ < Mn2+ < Cd2+ . Hence it was expected that

with replacement of Co2+ by elements of larger ionic radii the increase in lattice

constant will increase the hopping lengths. Hopping lengths increases gradually with

increase in dopant concentrations as the increase in dopant fractions causes more

lattice expansion.

122

Table 7.2 Hopping lengths between cations at tetrahedral A sites (LA) in Angstroms

(Å) and between octahedral B sites (LB) in Angstroms (Å) for all sample compositions

Co1-xAx Fe2O4 (A= Zn, Mn, Cd & x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)

Composition


Co 1-x Znx Fe2O4

LA (Å) 3.619(1) 3.624(5) 3.632(1) 3.641(1) 3.645(1) 3.654(1)

Co 1-x Znx Fe2O4

LB(Å) 2.955(1) 2.959(5) 2.966(1) 2.973(1) 2.976(1) 2.983(1)

Co1-xMnxFe2O4

LA(Å) 3.619(1) 3.619(1) 3.624(1) 3.629(8) 3.663(8) 3.637(1)

Co1-xMnxFe2O4

LB(Å) 2.955(1) 2.955(1) 2.959(1) 2.963(8) 2.991(8) 2.970(1)

Co1-xCdxFe2O4

LA(Å) 3.619(1) 3.638(1) 3.654(3) 3.680(1) 3.702(1) 3.723(1)

Co 1-xCdxFe2O4

LB(Å) 2.955(1) 2.970(1) 2.982(3) 3.003(1) 3.021(1) 3.041(1)

7.3 Occupancy of interstitial sites

As we have discussed that spinel ferrites are classified as normal, inverse and

intermediate spinel ferrites depending upon the allocation of divalent and trivalent

metal cations at Tetrahedral (A) and octahedral (B) interstitial sites. In the

composition Co Fe2O4 when the Co is replaced gradually by Zn the crystal structure

transforms itself from inverse spinel to normal spinel gradually depending upon the

amount of Zn contents at the expense of Co. Similarly when Co is replaced by Mn

and Cd, the crystal structure transforms gradually. It is a very important result

revealed from our study that the cation distribution is compositional dependent even

when the crystallite sizes are ranging from 10nm to 35 nm in all compositions. These

results evidently suggest that we can change the degree of inversion sequentially by

changing the stoichiometry. Hence, we can control the material properties as per need

by choosing the appropriate composition.

123

7.4 DC electrical properties

7.4.1 DC electrical resistivity

The dc electrical resistivity for all the sample compositions was measured by the two-

probe method in the temperature range of 313 K to 700 K. It was observed that the dc

electrical resistivity increased significantly with increase in hoping lengths for charge

carriers. The variation in the electrical resistivity depends upon the location of the

cations in the spinel structure and also by the microstructures of the material. When

Co was replaced by Zn, Mn and Cd the dc electrical resistivity increased significantly

with increase in dopant contents gradually. The composition with Cd as a dopant

element showed higher electrical resistivity as compared to the other two

compositions with Zn and Mn as a dopant element. Cd has higher ionic radius as

compared to Zn and Mn, hence it increases hopping lengths for charge carriers by

creating larger expansion in lattice. Hence another important result is revealed here

that considering the variation generated in hopping lengths and site occupancy we can

modify the electrical conduction as per required for a particular application. Variation

in dc resistivity with composition at 423K, 473K, 573K and 673K as a function of

concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd) are given in Figures 7.3 to 7.6

respectively.

124

0.0 0.2 0.4 0.6 0.8 1.0

0

1x105

2x105

3x105

4x105

5x105

6x105

7x105

-cm

)

-cm

)

Concentration x

Zn Doped

Mn Doped

423K

-2.0x109

0.0

2.0x109

4.0x109

6.0x109

8.0x109

1.0x1010

1.2x1010

1.4x1010

1.6x1010

Cd Doped

Figure 7.3 Variation in dc resistivity with composition at 423K as a function of

concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd).

0.0 0.2 0.4 0.6 0.8 1.0

0.0

2.0x104

4.0x104

6.0x104

8.0x104

1.0x105

1.2x105

1.4x105

1.6x105

1.8x105

-cm

)

-cm

)

Concentration x

Zn Doped

Mn Doped

473K

0.0

5.0x108

1.0x109

1.5x109

2.0x109

Cd Doped



125

0.0 0.2 0.4 0.6 0.8 1.0

2.0x103

4.0x103

6.0x103

8.0x103

1.0x104

1.2x104

1.4x104

-cm

)

-cm

)

Concentration x

Zn Doped

Mn Doped

0.0

5.0x106

1.0x107

1.5x107

2.0x107

2.5x107

Cd Doped

573K



0.0 0.2 0.4 0.6 0.8 1.0

0.0

5.0x102

1.0x103

1.5x103

2.0x103

2.5x103

-cm

)

-cm

)

Concentration x

Zn Doped

Mn Doped

-2.0x105

0.0

2.0x105

4.0x105

6.0x105

8.0x105

1.0x106

Cd Doped

673K



126

7.3.2 Drift mobility

Another important parameter is drift mobility which depends upon activation

energies. Drift mobility increases with temperature in all sample compositions.

Therefore conduction increases with rise in temperature as the conduction

measurements were done as a function of temperature. The Cd doped Co ferrites

nanoparticles shows lowest values of drift mobility as compared to that of the Co

ferrites with Zn and Mn as a dopant element.

7.4.2 Activation energies

The values of activation energies (E) were evaluated from the slopes of the linear

plots of dc electrical resistivity. In comparison the activation energies was higher

when Co was replaced by Cd in cobalt ferrites. The hopping depends upon the

activation energy, which is associated with the electrical energy barrier experienced

by the electrons during hopping. Variation in activation energy with composition as a

function of concentration x in Co1-xAxFe2O4 (A= Zn, Mn, Cd) is shown in Figure 7.7.

0.0 0.2 0.4 0.6 0.8 1.0

0.5

0.6

0.7

0.8

0.9

1.0

E

(e

V)

Concentration x

Zn Doped

Mn Doped

Cd Doped

Figure 7.7 Variation in activation energy with composition as a function of

concentration x in Co1-xAxFe2O4 (A= Zn, Mn, Cd).

127

7.5 AC electrical properties

7.5.1 Dielectric constant

The dielectric constant for Zn, Mn and Co substituted nanocrystalline cobalt ferrites

was measured at room temperature in the frequency range from 100 Hz to 3 MHz.

The characteristic features of the dielectric properties are known to arise from the

distribution and type of cations at the tetrahedral A sites and octahedral B sites of the

spinel structure. Dielectric properties are associated with charge polarization. The

dielectric constant increases gradually by increasing contents of Zn and Mn , but in

case of Cd doping it increases up to x = 0.4 and then decreases for further increase in

Cd contents in Co1-xMx Fe2O4 (M= Zn, Mn, Cd & x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0).

Here too the hopping length is doing effect. For increased hopping lengths due to Cd

doping the polarization process becomes slow which dimness the dielectric constant.

Dielectric constant is higher for Mn Fe2O4. For all composition the dielectric constant

decreases with increase in frequency. Variation in dielectric constant with

composition at 100 kHz, 1.2 MHz and 3 MHz frequency as a function of

concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd) is shown in Figures 7.8 to 7.10

respectively.

0.0 0.2 0.4 0.6 0.8 1.0

-300

0

300

600

900

1200

1500

1800

Die

lectr

ic C

on

sta

nt

(')

Concentration x

Zn Doped

Mn Doped

Cd Doped

100kHz

Figure 7.8 Variation in dielectric constant with composition at 100 kHz frequency as a

function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd).

128

0.0 0.2 0.4 0.6 0.8 1.0

0

100

200

300

400

500

Die

lectr

ic C

on

sta

nt

(')

Concentration x

Zn Doped

Mn Doped

Cd Doped

1.2MHz

Figure 7.9 Variation in dielectric constant with composition at 1.2MHz frequency as

a function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd).

0.0 0.2 0.4 0.6 0.8 1.0

-100

-50

0

50

100

150

200

250

300

Die

lectr

ic C

on

sta

nt

(')

Concentration x

Zn Doped

Mn Doped

Cd Doped

3MHz

Figure 7.10 Variation in dielectric constant with composition at 3MHz frequency as a


129

7.5.2 Dielectric loss (tan δ)

The change in dielectric loss (tan δ) as a function of frequency for all compositions at

room temperature was measured. In graphs for tan δ the peaks rises when hopping

frequency of localized electron matches with the externally applied alternating electric

field. Apparently the non-monotonic behavior is due to the dependence of dielectric

losses on particular resonance frequency. The peaks arise at different frequencies

depending upon the polarization mechanism. The peak’s positions changes with

frequency and composition because for each composition the resonance frequencies

will be different. The shifting of these peaks towards higher frequency region is

attributed to the increase of the hopping rate of the charge carriers as discussed in the

previous chapters. Dielectric losses also depend upon the entire microstructures and

density of the material. This type of behavior has already been reported for a family of

ferrites [124]. It was observed that in our prepared composition Co0.6Cd0.4Fe2O4 has

lowest dielectric losses values also less than the reported values for the nearest similar

composition [117]. Variation in dielectric loss with composition at 100 kHz, 1.2 MHz

and 3 MHz frequency as a function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn,

Cd) is shown in Figures 7.11 to 7.13 respectively.

0.0 0.2 0.4 0.6 0.8 1.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Die

lectr

ic L

oss (

tan)

Concentration x

Zn Doped

Mn Doped

Cd Doped

100kHz

Figure 7.11 Variation in dielectric loss with composition at 100kHz frequency as a


130

0.0 0.2 0.4 0.6 0.8 1.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Die

lectr

ic L

oss (

tan)

Concentration x

Zn Doped

Mn Doped

Cd Doped

1.2MHz

Figure 7.12 Variation in dielectric loss with composition at 1.2MHz frequency as a


0.0 0.2 0.4 0.6 0.8 1.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Die

lectr

ic L

oss (

tan)

Concentration x

Zn Doped

Mn Doped

Cd Doped

3MHz

Figure 7.13 Variation in dielectric loss with composition at 3MHz frequency as a


131

7.5.3 AC conductivity (ac)

AC conductivity studies for all synthesized compositions show that ac approximately

remains constant at lower frequencies and then starts increasing for higher

frequencies. In case of Cd doped cobalt ferrites the alternating current conductivity

(ac ) increase with increase in Cd contents upto x = 0.4 and then decrease for further

addition of Cd concentration. Where as in case of Zn doped cobalt ferrites ac

conductivity decreases with increase in Zn concentration. When Mn was used to

replace Co from Cobalt ferrite again ac decreases with increase in Mn concentration.

Variation in ac conductivity with composition at 100 kHz, 1.2 MHz and 3 MHz

frequency as a function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd) is

shown in Figures 7.14 to 7.16 respectively.

0.0 0.2 0.4 0.6 0.8 1.0

0.000

0.001

0.002

0.003

0.004

0.005

0.006

a

c (

S-c

m-1)

Concentration x

Zn Doped

Mn Doped

Cd Doped

100kHz

Figure 7.14 Variation in ac conductivity with composition at 100 kHz frequency as a


132

0.0 0.2 0.4 0.6 0.8 1.0

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

a

c (

S-c

m-1)

Concentration x

Zn Doped

Mn Doped

Cd Doped

1.2MHz

Figure 7.15 Variation in ac conductivity with composition at 1.2 MHz frequency as a


0.0 0.2 0.4 0.6 0.8 1.0

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

a

c (

S-c

m-1)

Concentration x

Zn Doped

Mn Doped

Cd Doped

3MHz

Figure 7.16 Variation in ac conductivity with composition at 3 MHz frequency as a


133

The graphs for change in ac conductivity with respect to the Jonscher’s power law are

shown in Figure7.17. The particular values of ‘n’ between ‘0’ and ‘1’show the

hopping mechanism of ac conduction.

0.0 0.2 0.4 0.6 0.8 1.0

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

Po

we

r n

Concentration x

Zn doped

Mn doped

Cd doped


concentration of composition Co1-xAxFe2O4 (A=Zn, Mn, Cd) (x=0.0, 0.2, 0.4, 0.6 0.8,

1.0)

In present work, the dielectric constant appeared to decrease with increase in

frequency. Maxwell Wagner model is applicable in this case since the interfacial

polarization was observed and hence the relaxation peaks can also be seen in the

figures. It was also observed that by varying the doping content x, the magnitude of

the dielectric constant varies. This refers to the fact that by increasing the dopant x the

energy stored by the dielectric material was increased. The polarization is also vitally

affected by the number of Fe ions on octahederal sites since they play significant role

in electron exchange interactions Fe2+ Fe3+. The ac conductivity remains

constant at initial frequency but at higher frequency the ac conductivity increased due

to increased rate of electron hopping mechanism. The constancy of the ac

conductivity at lower frequency was due to low rate of Fe ions hopping. AC

conductivity is directly dependent on the dielectric constant, dielectric loss and

frequency and in the figures above this phenomenon is showing its clear dependence

on composition as well.

134

Chapter 8

Conclusions

135

8. Summary/Conclusions

In the present study nanocrystalline Cobalt ferrites, Cobalt-Zinc ferrites, Cobalt-

Manganese ferrites and Cobalt-Cadmium ferrites particles were synthesized and

investigated very precisely for site occupancy and cation distribution. The nominal

compositions were Co1-xAxFe2O4 (A=Zn, Mn, Cd and x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0).

Initially nanocrystalline Co Fe2O4 particles were prepared and then Co was replaced

gradually by Zn, Mn and Cd. Co-precipitation method was successfully used as a

synthesis technique for all the sample materials so that by equal synthesis conditions

their final properties could be compared. Characterization techniques like X-ray

diffraction, differential scanning calorimetry, thermogravimetric analysis, X-ray

absorption fine structure spectroscopy (XAFS), ac electrical properties (dielectric

constant, dielectric loss tangent tan δ, ac conductivity) as a function of frequency and

dc electrical properties as a function of temperature were used. Thermal analysis

provides useful information about specific critical temperatures. Thermal analyses

showed that after completion of almost all weight losing processes at temperature

about 600°C the synthesized material (CoFe2O4, ZnFe2O4 and CdFe2O4) get stability.

While in case of MnFe2O4 weight losing processes continue till 950°C. FCC spinel

crystal structure was achieved by sintering at 600°C of the samples prepared by Co-

precipitation method at reaction temperature of 70°C. FCC spinel crystal structure

with the space group Fd3m was confirmed by XRD. Crystallite sizes obtained though

Scherrer formula were in the nano regime. A clear increase in lattice constants was

observed with the gradual replacement of Co ions with the substituted ions of Zn, Mn

and Cd as Co, Zn, Mn and Cd has ionic radius in an increasing order respectively.

Cation distribution was found by XAFS technique for determination of local crystal

structure with elemental specification. It was found that although crystallite sizes were

in nano regime the shift from inverse spinel (CoFe2O4) to normal spinel structure

(ZnFe2O4) strongly obeys a regular shift pattern depending upon the concentration of

dopant element Zn in cobalt ferrite crystal system. XANES studies showed that

ZnFe2O4 has a normal spinel structure while CoFe2O4 has an inverse spinel structure.

The shift from inverse to normal spinel structure depends upon the site occupancy in

tetrahedral and octahedral sites. Similar pattern was observed in case of replacement

of Co with Mn and Cd. Hopping lengths for charge carriers increased gradually with

136

increase in lattice constants depending upon the dopant type and dopant

concentration. Increase in electrical resistivity was observed with increase of hopping

lengths. DC electrical resistivity decreased with increase in temperature. Activation

energy calculations showed that the conduction mechanism was polaran hopping. The

dielectric constant decreased with increase of frequency and also it was observed

composition dependent. AC conductivity was frequency dependent and increased with

increase in applied frequency. Microstructures and material density also do effect in

electrical properties The Nyquist plots indicated that the resistive and capacitive

properties are mainly endorsed as a result of the processes associated to the grain and

grain boundaries. Grains and grain boundaries have different resistance levels. It

revealed that we can control the crystal structural properties and conduction

mechanism by selecting the proper chemical composition in Co based nanocrystalline

ferrite particles. The lattice parameters in the crystal structure. These studies

addressed effectively the basic question that how we can change the ultimate material

properties by choosing a suitable composition in Co based nanoferrites because spinel

crystal structure has ability to receive a range of dopant elements.

8.2 Future work and recommendations

The applications of spinel ferrites prepared at nano length scale encompass an

inspiring range. These applications of ferrites are originated from the fundamental

characteristics of ferrites such as considerable saturation magnetization, a high value

of electrical resistivity, low down electrical losses, and inherited fine level of

chemical stability. Based on the present studies, we propose to prepare the gas sensors

by using the ferrite compositions like substituted cobalt–zinc spinel ferrite and doped

cobalt ferrites for humidity sensors and for sensing the most polluting gases such as

CO2, O3, NO2, SO2, and CO. Study of magnetic properties of the prepared samples

and their correlation to the structural and electrical properties will be an interesting

study.

137

Chapter 9

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138

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Synthesis of Cobalt Based Nanoferrites and Study of Their ... · Dr. Muhammad Maqsood, Director,...

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