Posiandon (x =

CobHepta

itron d Phot

nan= 0.0 –

Co++ O-

heating

balt Sulphateahydtrate (1.

Cobalt Oxi (1.0)

CHann

tocatnocry– 2.0)

+

O- C

C

+

C

e .0)

ide

Cobalt

pH

HAnihilaalyticystall) spin

O-

Cr+++ O-

Cr+++

+

CoCrxFe2-x

ChromHepx

t oxide (x)

H = 3.5

PTation c degine

nel fer

O-

O- Fe+++

Fe+++

O4 (Final p

mium Sulphaxahydtrate (x

Ferric oxi

TERspe

gradaCoC

rrites

+

Heati O-

product)

ate x)

de (2-x)

R - ectrostion sCrxFes

ing and strin

Ferrous SuHeptahydtr

pH = 1OxidatOH- -

4 scopystudye2-xO4

ng at 60 °C

Na+ OH-

ulphate rate (2-x)

10.5 tion by OH-

y y 4

Chapter 4

4.1

 

4.1 X – ray powder diffractometry, Thermo Gravimetric (TG) and

Differential Scanning Calorimetric (DSC) analysis of

CoCrxFe2-xO4 spinel ferrite precursors

4.1(A) X – ray powder diffraction patterns analysis

Shown in Figure 4.1.1 are the room temperature (300 K) X – ray powder diffraction

patterns for the two end members, x = 0.0, CoFe2O4 and x = 2.0, CoCr2O4 precursors

of CoCrxFe2-xO4 spinel ferrite system.

Figure 4.1.1: X – ray powder diffractograms for Co – Cr ferrite precursors at

300 K.

Chapter 4

4.2

 

The spectra for the undigested particles have broad peaks without any

structure. The background noise and broadness of the peaks are characteristic of

particles with nano meter dimensions [1]. This happens because in nano size particles

there are insufficient diffraction centers that cause the line broadening [2].

4.1 (B) Thermo Gravimetric Analysis (TGA) and Differential Scanning

Calorimetric (DSC) study

The observed water content was determined from the weight difference measured at

30 – 700 °C from thermogravimetric data. The thermogravimetric analysis (TGA)

measurement will help to determine whether the change is physical or chemical in

nature. The reaction is chemical in nature if a mass change is associated with it and

physical if no mass change occurs.

The representative thermogravimetric (TG) traces for spinel ferrite precursor

with x = 0.0, 0.3, 1.1, 1.5 and 2.0 compositions of the system CoCrxFe2-xO4 are shown

in the Figure 4.1.2. DSC profiles, where the endothermic heat flow is measured versus

temperature are shown in Figure 4.1.3. The TGA traces exhibit three distinct weight

loss steps. The TG curves show total weight loss of 12 to 29% for x = 0.0 to 2.0

compositions in the temperature range 30 – 700 °C. In the temperature range

30 – 150 °C, 8 – 18 % of subsequent weight loss is observed, which is accompanied

by an endothermic broad hump around 95 ± 10 °C in the differential scanning

calorimetry (DSC) curves. This event is attributed to the dehydration process wherein

significant amount of adsorb water is released from the wet chemically synthesized

ferrite compounds.

Such peak is associated with nucleation of ferrite crystal from amorphous

precursor powder (Figure 4.1.1). The observed endothermic peak represents

decomposition of the metal hydroxides to oxides [4]. In the graphs it is mentioned that

sample started to degrade (decomposed) from 200 °C (Figure 4.1.3). The fact that

there are no such peaks above 105 °C shows that the formation of ferrite is almost

complete at this temperature and annealing above 200 °C is likely to give only grain

growth. This is consistent with the X-ray diffraction results described in the next

section. The further 2 – 8 % weight loss during 150 – 250 °C may be correlated with

Chapter 4

4.3

 

the removal of dispersant anion and hydroxyl group as reflected in EDAX spectra of

the samples heated at 200 °C.

The observed increase in total weight loss from 12 % for x = 0.0 composition

to 29 % for x = 2.0 composition, suggests that Cr3+ ions have tendency to adsorb

water in the lattice structure during chemical reaction of ferritization. The samples

heated at 300 °C had almost constant weight upon heating to higher temperature,

revealing the fact that Co – Cr ferrites are free from dispersant anions and hydroxyl

groups [3].

The powdered samples were dried at 473 K under vacuum for 8 h after

confirmation from TGA analysis, and the same samples were used for further

characterizations.

The various parameters determined from TGA and DSC measurements such

as total percentage weight loss, peak area, peak height and endothermic enthalpy are

given in Table 4.1.1.

Figure 4.1.2: TGA traces for cobalt – ferri – chromite precursors.

Chapter 4

4.4

 

x= 1.1

x= 0.0

Chapter 4

4.5

 

Figure 4.1.3: Differential scaning calorimetry curves for x = 0.0, 1.1, 1.5 and 2.0

compositions of CoCrxFe2-xO4 system.

x= 1.5

x= 2.0

Chapter 4

4.6

 

Table 4.1.1: Paramters determined from TGA and DSC analysis.

Cr3+

content (x)

Weight loss

(in %) at 700 °C

Peak area

(mJ)

Peak

height

(mW)

Peak

temperature

(°C)

Enthalpy

(J/g)

0.0 12 221.06 0.93 85.38 86.52

0.3 20 − − − −

1.1 26 257.17 1.03 104.6 102.01

1.5 27 261.31 1.12 103.8 102.72

2.0 29 309.91 1.23 104.08 116.29

Conclusions

(i) The undigested particles of ferrite precursors shows amorphous like strucutre.

(ii) The weight loss increases with increase of Cr3+ - substitutuion in CoFe2O4,

suggest, Cr3+ ions have tendency to adsorb water during chemical reaction.

(iii) The ferritization or crystallization is taking place for temperature of 200 °C.

Chapter 4

4.7

 

References

1. J. P. Chen, C. M. Sorensen, K. J. Klabunde, G. C. Hadjipanayis, E. Devlin,

A. Kostikas, ‘Size-dependent magnetic properties of MnFe2O4 fine particles

synthesized by co-precipitation’, Phys. Rev. B, 54 (13) (1996) 9288 – 9296.

2. M. K. Rangolia, M. C. Chhantbar, A. R. Tanna, K. B. Modi, G. J. Baldha,

H. H. Joshi, ‘Magnetic behaviour of nano – sized and coarse powders of Cd –

Ni ferrites synthesized by wet – chemical route’, Ind. J. Pure Appl. Phys. 46

(2008) 60 – 64.

3. M. Rajendran, S. Deka, P. A. Joy, A. K. Bhattacharya, ‘Size-dependent

magnetic properties of nanocrystalline yttrium iron garnet powders’, J. Magn.

Magn.Mater. 301 (1) (2006) 212 – 219.

4. M. Javed Iqbal, M. Rukh Siddiquah,’Structural, electrical and magnetic

properties of Zr–Mg cobalt ferrite’, J. Magn. Magn. Mater., 320 (6) (2008)

845 – 850.

Chapter 4

4.8

 

4.2 Elemental analysis by Energy Dispersive Analysis of X-rays

(EDAX)

The wet-chemical methods or soft chemical routes like co-precipitation [1], sol-gel

[2], combustion [3], reverse micelle technique [4], hydrothermal route [5] for growing

nano-crystallites (bottom-up techniques) of ferrite materials are prone to

contamination and loss of compositional stoichiometry. Moreover, it is difficult to

obtain consistency in the physical properties of electro-ceramics synthesized by

chemical routes, particularly for the system containing more than three cations,

percentage substitution is very small or tetravalent – pentavalent cationic substituted

systems. The resultant off – stoichiometric composition shows unusual or unexpected

behaviour that is not possible to explain on the basis of normal stoichiometric

composition. Thus, to ratify the purity and surety of the chemical composition, energy

dispersive analysis of X-rays (EDAX) measurement was carried out at 300 K.

The EDAX spectra for the typical compositions of the system: CoCrxFe2-xO4,

with x = 0.0 (CoFe2O4), x = 1.1 (CoCr1.1Fe0.9O4) and x = 2.0 (CoCr2O4) are shown in

Figure 4.2.1.

The energy of the K, L and M series X-rays increase with increasing atomic

number (Z). For the normal energy range, typical of most spectrometers, 15 – 20 kV,

light elements will emit X-rays of the L series or K and L series. Intermediate

elements will emit X-rays of the L series or K and L series. On the other hand, heavy

elements will emit X-rays of the M series or L and M series [6]. Thus, it is possible to

record a wide range of elements simultaneously during a given scan.

In the spinel ferrite system under investigation, CoCrxFe2-xO4 light elements

like oxygen (O) (Z = 8), intermediate elements such as Cobalt (Co) (Z = 27),

Chromium (Cr) (Z = 24) and Iron (Fe) (Z = 26) are present while heavy elements are

not present.

The EDAX patterns shown in Figure 4.2.1, clearly show three characteristic

X-ray lines located between 5.3 keV to 7.7 keV energies for un-substituted cobalt

ferrite, CoFe2O4 (x = 0.0). For all the Cr3+ - substituted cobalt ferrite compositions

(x = 0.1 – 1.9), five peaks and for cobalt chromite, CoCr2O4 (x = 2.0), four peaks are

observed between said energy range.

Chapter 4

4.9

 

Figure 4.2.1: Room temperature (300 K) EDAX spectra for CoCrxFe2-xO4 spinel

ferrite system (x = 0.0, 1.1, 2.0).

x = 0.0

x= 1.1

x = 2.0

Chapter 4

4.10

 

The maximum observed at ~ 6.4 keV for all spectra is assign to FeKα line.

Here, it is important to note that CoKα (6.93 keV) line interferes (overlaps) with FeKβ

(7.06 keV) line in the spectra. The small peak at extreme right of the spectra centered

at ~ 7.65 keV clearly comes from CoKβ1. The intense peak located at 5.405 keV and

small peak appeared at ~ 5.95 keV are corresponding to CrKα1 and CrKβ1 respectively.

The maximum observed in the left part of the spectra at ~ 0.5 keV clearly comes from

O Kα while for peak that is located at ~ 0.75 keV comes from FeLα1 characteristic

line. It is important to note that if a Kα line is identified in a spectrum then a Kβ line

should exist having approximately a tenth of counts of the Kαline (Kα : Kβ= 10 : 1)

[6]. We have observed same relationship for CoKα and CoKβ1, CrKα1 and CrKβ1 lines

in the spectra. The low intensity peak at extreme left of the spectra centered at ~ 0.25

keV connected with carbon (CKα) characteristic line of impurity. The hardly visible

peaks at ~ 1.06 keV, ~ 2.4 keV and ~ 3.7 keV belong to impurity peak of sodium,

NaKα1/Naβ1, sulphur, SKα1/SKβ1 and calcium, CaKβ1 characteristic line respectively.

The origin of sulphur and sodium impurities is well understood from the chemical

used for the synthesis of spinel ferrite system, CoCrxFe2-xO4, i.e. CoSO4.7H2O,

FeSO4.7H2O and Cr2(SO4)3.6H2O as well as NaOH used as a precipitant. On the other

hand, impurity like carbon was likely to have been introduced from carbon-coated

shields to the pole pieces and special EDAX specimen holder made of carbon,

beryllium and aluminium used in the EDAX spectrometer [7]. The origin or source of

calcium impurity is not clearly understood. Further, the incorporation of Cr3+ in the

place of Fe3+ was indicated by the intensities of the respective peaks in the EDAX

patterns.

The atomic percentage (at %) and weight percentage (wt %) of constituent

elements, (Co, Cr, Fe and O) are calculated theoretically from the intermediate

chemical composition CoCr1.1Fe0.9O4 (x = 1.1) and that obtain from EDAX elemental

analysis are shown in Table 4.2.1. It can be seen that the stoichiometry is very close to

the anticipated values with small deficiencies of Cr3+ ions.

The EDAX results suggested that the precursors had fully undergone the

chemical reaction to form ferrite material of the expected composition.

Chapter 4

4.11

 

The peak to background (P/B) ratio for different elements is found to be large,

so background did not introduce much error. The (P/B) ratio for constituent elements

is also included in Table 4.2.1.

Table 4.2.1: Estimated stoichiometry for CoCr1.1Fe0.9O4 ferrite composition

from EDAX analysis.

Element

Weight percentage (wt %)

Atomic percentage (at %)

(P/B) ratio

Expected EDAX

analysis Expected

EDAX

analysis

O 27.78 27.56 57.14 56.90 197

Fe 21.82 21.36 12.86 12.66 78

Co 25.58 25.28 14.28 14.20 33

Cr 24.82 24.30 15.72 15.31 53

Total 100.00 − 100.00 − −

Conclusions

(i) The analysis of EDAX spectra have confirmed expected stoichiometry

without the loss of any ingredient.

(ii) All the peaks are well assigned in accordance with the standard positions.

(iii) The source of impurity peaks correspond to C, Na and S is well understood

but the origin Ca impurity in the spectra is not clear.

Chapter 4

4.12

 

Illustrative calculations for atomic percentage and weight percentage

Atomic percentage (at%) for CoCr1.1Fe0.9O4

(i) Molecular weight : (1) (58.93 amu) + (1.1) (52.0 amu) + (0.9)(55.85 amu)

+ (4) (16 amu)

= 58.93 + 57.2 + 50.265 + 64

= 230.395 amu

In the spinel ferrite material with general chemical formula, A+21B+3

2O-24, there are

total 7 atoms per formula unit.

(ii) Atomic contribution

Total no. of atoms is 7, corresponding molecular weight is 230.395 amu

∴ for 4 Oxygen atoms corresponding molecular weight is how much?

= 131.65 amu

(iii) Contribution in percentage

For the total molecular weight

of 230.395 amu, contributions from oxygen atoms is 131.65 amu

∴ for 100 amu contributions from oxygen atoms is how much ?

= 57.14 % (Expected)

= 56.90 % (Observed)

Wight percentage (wt %) for CoCr1.1Fe0.9O4

For the total molecular weight

of 230.395 amu, contribution from Fe ion is 50.265 amu

for 100 amu contribution from Fe ion is how much?

= 21.82 % (Expected)

= 21.16 % (Observed)

Chapter 4

4.13

 

References

1. K. B. Modi, N. H. Vasoya, V. K. Lakhani, T. K. Pathak, P. M. G. Nambissan,

‘Crystal defects and cation redistribution study on nanocrystalline cobalt –

ferri-chromites by positron annihilation spectroscopy’, Int. J. Spectro., 2013

(2013) 1 – 11.

2. I. Ahmad, T. Abbas, M. U. Islam, A. Maqsood, ‘Study of cation distribution

for Cu–Co nano ferrites synthesized by the sol–gel method’, Ceram. Int. 39 (6)

(2013) 6735 – 6741.

3. S. S. Manoharan, K. C. Patil, ‘Combustion Synthesis of Metal Chromite

Powders’, J. Am. Ceram. Soc., 75 (4) (1992) 1012 – 1015.

4. P. Pulisova, J. Covac, A. Voigt, P. Raschman, ‘Structure and magnetic

properties of Co and Ni nano-ferrites prepared by a two step direct

microemulsions synthesis’, J. Mag. Mag. Mater, 341 (2013) 93 – 99.

5. S. Phumying, S. Labuayai , E. Swatsitang, V. Amornkitbamrung, S. Maensiri,

‘Nanocrystalline spinel ferrite (MFe2O4, M = Ni, Co, Mn, Mg, Zn) powders

prepared by a simple aloevera plant-extracted solution hydrothermal route’,

Mater. Res. Bul., 48 (6) (2013) 2060 – 2065.

6. http://www.charfac.umm.edu.

7. http://nanoanalysis.materials.ox.ac.uk.

   

Chapter 4

4.14

 

4.3 Particle size distribution study

Size of particles influences many properties of particulate material. Furthermore, it is

a valuable indicator of quality and performance of material. In many applications,

particle size plays critical role, for example, it determines (a) appearance and gloss of

paint (b) flavor of coco powder (c) reflectivity of highway paint (d) absorption rate of

pharmaceuticals (e) appearance of cosmetics and (f) hydration rate and strength of

cement. Of course, this is more applicable to nano regime.

Particle size growth may be monitored during operations such as granulation

or crystallization. Determining the particle size of powders requiring mixing is

common since materials with similar and narrower distributions are less prone to

segregation [1].

Particle size distribution data can be presented in tabular format i.e.

numerically or graphically. In graphical form data are presented in differential and

cumulative distribution curves. Both the forms are interrelated, if one differentiated

the cumulative distribution curve, the differential distribution is obtained. On the

other hand, if one integrated the differential distribution curve, the cumulative

distribution is obtained [2]. The differential distribution shows the relative amount at

each particle size. From the different size distribution, measures of central tendency

such as the modal and mean diameters are determined. The diameter at the peak of

the differential distribution is the modal diameter while the mean diameter is the

average diameter.

The corresponding cumulative distribution curve demonstrates the relative

amount at or below a particular size. The median diameter is another measure of

central tendency. It is the diameter at the 50th percentile, designed d50. Quartile

diameters include d75, d50 and d25. There are several measures of absolute width one

can derive given the cumulative distribution. One common measure is the span, d90-

d10. A dimensionless measure of width is the relative span defined as span/d50. Other

relative measures of width include percentile ratios such as d90/d10 and d75/d25.

 

Chapter 4

4.15

 

 

 

Figure 4.3.1: Illustrative differential size distribution and cumulative

undersize distribution curves.

 

Figur

x= 2

x= 1

x =

re 4.3.2: Pa

com

2.0

1.1

0.0

article size d

mpositions

Chapter 4

4.16

distribution

of CoCrxFe

curves for x

e2-xO4 spinel

x = 0.0, 1.1 a

l ferrite syst

and 2.0

tem.

Chapter 4

4.17

 

Typical particle size distribution patterns, differential size distribution and

cumulative undersize distribution, for x = 0.0 and 1.1 and 2.0 compositions are shown

in Figure 4.3.2. The distribution is bimodal (double peaked) as well as not mono

disperse (all one size). Earlier, a bimodal size distribution was observed for the as

prepared sample and the sample annealed at 300 °C with an average particle sizes of

(∼ 2 nm and ∼ 8 nm) and (∼ 4 nm and ∼ 8 nm) respectively, for the two samples of

nickel ferrite synthesized using the sol-gel process [3]. On the other hand, the

existence of a broad (bimodal) distribution in the crystallite size has also been

reported for the high energy mechanically milled NiFe2O4, MgFe2O4, Fe3O4

nanoparticles and other ultrafine mechanically alloyed materials [4]. The origin of the

bimodality lies in the growth process by which the particles are formed. In a chemical

growth process such as co-precipitation which is used here, growth occurs by initial

nucleation and growth via a ‘seed and grow’ mechanism, followed by Ostwald

ripening. For smaller particle systems, where the growth has been restricted, some

original seeds remain in the colloid. For the larger particle systems a greater

percentage of the seeds have been observed in the ripening process leading to a more

uniform particle size distribution [5].

A careful examination of Figure 4.3.2 shows that for x = 0.0 composition two

well separated peaks are observed. The peak on the left hand side is with less

intensity and asymmetric one i.e. the curve tails to the left more than to the right, that

means the skew is negative. The peak on the right hand side with more intensity

having symmetric differential distribution i.e. has zero skew. The skew is positive if

curve tails to the right more than to the left. The reference point for tailing is with

respect to the modal diameter. On increasing Cr3+ - substitution (x) in the system,

CoCrxFe2-xO4, for x = 1.1 composition (CoCr1.1Fe1.9O4) both the peaks get merged and

also have same intensity. On the other hand, for cobalt chromite (x = 2.0), once again

two well resolved peaks are observed. The left hand side peak is with more intensity

and asymmetric in nature while the peak on the right hand side is with less intensity

but symmetric one.

There are several measures of width. One measure of width is FWHM, the full

width at half maximum. It is obtained by drawing a horizontal line at 50% of the

maximum and taking the difference between the two places it intersects the

Chapter 4

4.18

 

distribution. HWHM, the half width at half maximum, is another measure of width. It

is defined as FWHM/2. A relative fractional measure of width is obtained by dividing

HWHM by the measure of central tendency from which it was derived, the modal

diameter (HWHM/modal diameter).

The size of the particles shown in Figure 4.3.2 is in micrometer (μm) order.

These particles were agglomerates which were found to break further and further to

submicron level with more and more powerful ultrasonic de-agglomeration

techniques.

The important parameters such as: modal diameter, mean/average diameter,

full width half maximum (FWHM), span (d90-d10), relative span (span/d50), quartile

ratio (d75/d25) and median diameter (d50), for CoCr2O4 (x = 2.0) is determined and

tabulated in Table 4.3.1

Table 4.3.1: Measures of central tendency and width for CoCr2O4 (x = 2.0)

composition.

Parameter LHS Peak RHS Peak

Modal diameter 85 μm 300 μm

Average diameter 95 μm 350 μm

FWHM 177.5 μm 200 μm

Span 246.5 μm −

Relative span 4.93 −

Quartile ratio 8.12 −

Median diameter 55 μm −

Relative percent measure of width 104.4 % −

d10 = 4.5 μm, d25 = 15.4 μm, d50 = 55 μm, d75 = 125 μm and d90 = 250 μm.

Chapter 4

4.19

 

Conclusions

(i) Particle size distribution curves analysis suggests that the distribution is

bimodal as well as not mono disperse and having negative skew.

(ii) The size of the particles is in micrometer order and ultrasonic de-

agglomeration is required before further analysis.

(iii) Various important parameters can be determined from the differential size

distribution and cumulative undersize distribution curves.

(iv) The reason for bimodality lies in the growth mechanism and the relative

proportion of particles formed by nucleation and Ostwald ripening.

Chapter 4

4.20

 

References

1. http://www.Horiba.com.

2. http://www.Brookhaveninstruments.com.

3. R. Malik, S. Annapoorni, S. Lamba, V. R. Reddy, Ajay Gupta, P. Sharma,A.

Inoue. ‘Mossbauer and magnetic studies of nickel ferrite nanoparticles:

Effect of size distribution’, J. Magn. Magn. Mater., 322 (2010) 3742 – 3747.

4. V. Sepelak, D. Baabe, D. Mienert, D. Schultze, F. Krumeich, F. J. Litterst,

K. D. Becker, ‘Evolution of structure and magnetic properties with annealing

temperature in nanoscale high-energy-milled nickel ferrite’, J. Magn. Magn.

Mater., 257 (2003) 377 – 386.

5. M. Blanco – Mantecon, K. O’Grady, ‘Grain size and blocking distributions in

fine particle iron oxide nanoparticles’ J. Magn. Magn. Mater., 203 (1999)

50 – 53.

 

   

Chapter 4

4.21

 

4.4 Transmission Electron Microscopic (TEM) analysis

The morphology of Co – Cr ferrites nanoparticles prepared by the co-precipitation

route was examined by transmission electron microscopy (TEM). Figure 4.4.1(a – e)

shows bright field TEM pictures for x = 0.0, 0.3, 1.1, 1.5 and 2.0 compositions of

spinel ferrite system, CoCrxFe2-xO4. TEM micrographs show highly agglomerated

particles of nanoscale nature for all the compositions. This is because of the fact that

they experience permanent magnetic moment proportional to their volume. Hence,

each particle is permanently magnetized and gets agglomerated. The system under

investigation possesses cations which are highly magnetic in nature, Co2+ (3 µB),

Cr3+ (3 µB), Fe3+ (5 µB), such clustering of nanoparticles is expected. It can be seen

that for cobalt ferrite, CoFe2O4 (x = 0.0), clusters of irregularly shaped nanoparticles

with very few needle-shape nanoparticles, while for all other compositions no such

needle shaped particles are observed. In the present case, from TEM images it is not

possible to estimate particle size accurately, but in broad sense we can see that

particle size decreases with increasing Cr3+ - content (x) in the system. Earlier, we

have observe very systematic transformation of spherically shaped particles to

lamellar to thick needle shape particle morphology and decrease in particle size with

increase in Mg2+ - concentration (x) for Mn2+ in the system, MgxMn1-xFe2O4 (x = 0.0 –

1.0), [1]. The observed reduction in particle size and presence of needle shape

particles can be explain on the similar line of argument.

The growth of crystals in the solution is governed by different parameters.

Among all such parameters the most important being the molecular concentration of

the material approaching the tiny crystal surface during the growth process. Because

of the liberation of the latent heat at the surface, the local temperature is normally

higher than the solution temperature. The surface temperature affects the local

molecular concentration at the crystal surface and hence the crystal growth [2]. It

seems that the formation of cobalt chromite (x = 2.0) is more exothermic as compared

to the formation of cobalt ferrite, CoFe2O4 (x = 0.0). According to Jain et al [3], the

enthalpy of formation for CoFe2O4 is 11.28 eV/f.u. and 14.84 eV/ f.u. for CoCr2O4.

Thus, it is quite expected that when Cr3+ is substituted for Fe3+ in the system,

CoCrxFe2-xO4, more amount of heat will be liberated, which will increase the

temperature of the growing crystal surface, resulting in decrease of the molecular

Chapter 4

4.22

 

concentration approaching at the crystal surface and hence, hindering the crystal

growth. i.e. reduction in particle size.

The selected area electron diffraction (SAED) patterns for x = 0.0, 0.3, 1.1, 1.5

and 2.0 compositions are also shown in the Figure 4.4.1 (a) – (e). The patterns show

the Debye-Scherrer rings indicating the presence of structurally disordered regions in

the wet chemically synthesized Co-Cr ferrites. The rings are consistent with the cubic

spinel structure with an intense ring patterns form (hkl) planes. No secondary phases

are found. The diffused SAED patterns indicate nano crystalline nature of the

particles [4].

(a) (a)

(a) (a)

Chapter 4

4.23

 

(b) (b)

(b) (b)

(c) (c)

(c) (c)

(2 2 0)

(3 1 1)

(4 0 0)

(5 1 1)

(4 4 0)

Chapter 4

4.24

 

Figure 4.4.1: TEM and SAED images for CoCrxFe2-xO4 system

(a) CoFe2O4 (x = 0.0) (b) CoCr0.3Fe1.7O4 (x = 0.3) (c) CoCr1.1Fe0.9O4 (x = 1.1)

(d) CoCr1.5Fe0.5O4 (x = 1.5) (e) CoCr2O4 (x = 2.0) compositions.

(d) (d)

(d) (d)

(e) (e)

(e) (e)

(2 2 0)

(3 1 1)

(4 0 0)

(5 1 1)

(4 4 0)

Chapter 4

4.25

 

Conclusions

(i) Highly agglomerated particles for all the compositions are due to the magnetic

nature of the constituent cations.

(ii) The reduction in particle size with Cr3+ - substitution is mainly due to the more

enthalpy of formation needed for CoCr2O4 as compared to that for CoFe2O4.

(iii) SAED patterns confirm nano crystalline nature of wet- chemically synthesized

ferrite particles.

Chapter 4

4.26

 

References

1. Kunal B. Modi, Nimish H. Vasoya, Vinay K. Lakhani, Tushar K. Pathak,

‘Spherical to Needle Shaped Particles Transformation Study on

Nanocrystalline Mg–Mn Ferrites’, J. Adv. Micro. Res., 7 (2012) 40 – 43.

2. R. F Strickland-Constable, ‘Kinetics and Mechanism of Crystallization’,

Academic Press, New York (1968).

3. A. Jain, G. Hautier, S. P. Ong, C. J. Moore, C. C. Fischer, K. A. Persson,

G. Ceder, ‘Formation of enthalpies by mixing GGA and GGA+U

calculations’, Phys. Rev. B, 84 (4) (2011) 045115 – 045124.

4. S. Verma, P. A. Joy, ‘Low temperature synthesis of nanocrystalline lithium

ferrite by a modified citrate gel precursor method’, Mater. Res. Bul., 43 (12)

(2008) 3447 – 3456.

   

Chapter 4

4.27

 

4.5 X-ray powder diffraction patterns analysis and structural

parameters determination

The polycrystalline samples of CoCrxFe2-xO4 spinel ferrite system were characterized

by X-ray powder diffractometry (XRD) to ascertain the mono-phase structure

formation, to deduce lattice parameter and cation distribution and particle size

verification. Typical XRD patterns of CoCrxFe2-xO4 samples with x = 0.0, 0.7, 1.1,

1.3, 1.7 and 2.0 are shown in Figure 4.5.1. The background noise and the broadness of

the peaks are characteristic of particles with nanometer dimensions since there is not

sufficient number of crystallographic planes to result in sharp diffraction lines. The

XRD patterns also showed that all the samples have the single phase spinel structure.

No extra lines corresponding to any other phase or non-reacted ingredients were

detected. The diffraction patterns could be indexed for a face centered cubic (fcc)

structure [1]. The cell edge parameter for each composition was determined by using

the ‘Powder-X’ software [2]. The concentration dependence of the lattice constant

(aexp) determined from the XRD pattern analysis is presented in Table 4.5.1.

The lattice constant remains more or less constant initially but rapidly decreases for

higher concentrations of Cr3+. Usually in a solid solution of ferrites within the

miscibility range, a linear change in lattice constant with concentration of the

components is observed. The observed change in lattice constant value with Cr3+-

content (x) is attributed to the small difference in the ionic radii of the constituent

cations, Fe3+ (0.640Å) and Cr3+ (0.630Å), and change in the distribution of cations

among the available A- and B-sites of the spinel lattice.

The various physical properties of ferrites are sensitive to the nature, the

valence state and distribution of cation over the tetrahedral (A-) and octahedral (B-)

sites of the spinel lattice. Therefore, the knowledge of cation distribution is essential

to understand the various physical properties of spinel ferrites.

Cation arrangements are not unique in spinel ferrites. Each spinel compound

possesses at least three degree of freedom, which it uses in its search for an

equilibrium structure: oxygen positional parameter (u), lattice constant (a) and cation

inversion parameter. The parameter ‘u’ varies, primarily, in accordance with the

radius ratio between the A – and B – sites cations, rA/rB (or rB/rA). This is to say, the

A – and B – sites bond lengths adjust themselves by variation in ‘u’ until the A- and

Chapter 4

4.28

 

B- sites volumes “best – fit” the cations. The parameter ‘a’ varies in accordance with

the average of the A – and B- sites cationic radii (i.e. with 0.33 rA + 0.67 rB). The

entire frame work of the unit cell swells or contracts to accommodate the size of the

cations. The cation inversion parameter varies based on a much more complex set of

factors. Some of the principal factors that influence cation inversion include [3]:

(i) temperature (ii) the electrostatic contribution to the lattice energy (iii) cationic radii

(iv) cationic charge and (v) crystal – field effects.

In order to determine the cation distribution, the XRD line intensity calculations

were made using the formula suggested by Buerger [4].

.LPFI m2

hklhkl = (1)

Here, Ihkl is the relative integrated intensity, Fhkl is the structure factor, Pm is the

multiplicity factor and L = (1+cos22θ)/(sin2θ cosθ) is the Lorentz polarization factor.

According to Ohnishi and Teranishi [5], the intensity ratios of planes I220/I440 and

I400/I422 are considered to be sensitive to the cation distribution. There exists distinct

contrast in the atomic scattering factors of Cr3+ or Fe3+ and Co2+ cations present in the

system. This makes the determination of the cation distribution quite reliable. Any

alteration in the distribution of cations causes a significant change in the XRD

intensity ratios. Therefore, in the process of arriving at the final cation distribution,

the site occupancy of all the cations was varied for many combinations and those that

agreed with the experimental intensity ratios are shown in Table 4.5.2. The final

cation distributions were deduced simultaneously by considering the Bragg plane

ratios, the fitting of the magnetization data at 80 K and the ion distribution parameters

of Fe3+among the A – and B – sites of spinel lattice derived from Mossbauer spectral

analysis [6].

 

F

S

T

an

(T

Figure 4.5.1:

tructural p

The X-ray de

nd Wijn [7]

Table 4.5.1)

: Room te

nanocryst

arameters d

ensity (ρx) o

, ρx = 8M/N

, NA the Av

emperature

talline CoCr

determinati

f the sample

NAa3, where

vogadro’s nu

Chapter 4

4.29

X-ray p

rxFe2-xO4 sp

on

es was deter

e, M is the m

umber (6.02

powder dif

pinel ferrite

rmined using

molecular w

2 x 1023 mol

ffraction p

system.

g relation gi

eight of the

le-1) and ‘a’

patterns fo

ven by Smit

composition

is the lattic

or

th

n,

ce

Chapter 4

4.30

 

constant. As there are 8 formula unit in the unit cell so 8 is included in the formula.

The ρx is inversely proportional to the lattice constant, which decreases with

increasing Cr3+- concentration; ρx is expected to increase with increasing (x). The

X-ray density (ρx) decreases with Cr3+- ion concentration, because the decrease in

molecular weight overtakes the decrease in volume of the unit cell. Bulk density (ρ)

of the samples was determined by employing the Archimedes principle using xylene

(ρ = 0.87 gcm-3) as the buoyant to obtain fairly good results. It is observed that ρx of

each sample is greater than the corresponding sintered density (ρ). This may be due to

the existence of pores in the samples. Pore fraction (f) was calculated using the

relation f = (1 – ρ/ρx) and percentage porosity was calculated using the relation

P = f * 100%. The variation of porosity (P) with Cr3+ content (x) is a result of the

interplay between ρ and ρx.

The average particle size (D) for the different compositions has been

calculated from the broadening of the respective high intensity (311) peak using the

Debye – Scherrer formula D = Kλ/B.cosθ, Here,λ   is the wavelength of the CuKα

radiation (= 1.54059 Å), shape factor, K = 0.9, related both to the crystalline shape

and the way in which B and D are defined. B is the contribution to the XRD peak

width, full width at half maximum (FWHM) due to the small size of crystallites in

radians. The contribution must separate out from the measured line width, BM, which

includes instrumental broadening Bins., which is always present irrespective of the

particle size. For this, one can record X-ray powder diffraction pattern of a well

crystallized, bulk standard material such as silicon powder under identical geometrical

conditions and measure the peak width Bins. Usually, Bins. of a conventional X-ray

powder diffractometer is 0.1° (= 0.001744 radian), the broadening parameter B is

obtained from the relation: B = (BM2 – Bins.2)1/2. Here, the particle size is calculated

considering the B (FWHM) obtained by Gaussian fitting of most intense peak. The

particle size reduces from 72 nm for cobalt ferrite, CoFe2O4 (x = 0.0) to 17 nm for

cobalt chromite, CoCr2O4 (x = 2.0) (Table 4.5.1).

The particle size estimated from TEM analysis is found to be greater than the

particle size estimated from X-ray diffraction pattern analysis by Debye – Scherrer’s

formula. This is because of the fact that X-ray diffraction gives the information of

crystalline region only and the contribution from the amorphous grain surface does

Chapter 4

4.31

 

not considered. On the other hand, TEM gives the overall picture of the nanoparticles.

By analyzing TEM and XRD, one can have almost complete picture of the particle

size, their distribution and morphology.

An attempt has been made to calculate effective surface area (S) for the

selected ferrite compositions. Assuming spherically shaped particles, the specific

surface area (S, cm2/g) is given by [8]: S = 6000/D.ρ, where, D is the particle diameter

in nm and ρ is the bulk density of the particle is g/cm3. The specific surface area

(Table 4.5.1) of the particle increases as the particle size decreases as expected.

Table 4.5.1: Molecular weight, lattice constant (a), X-ray density (ρx), bulk

density (ρ), pore fraction (f), porosity (P), particle size (D) and

effective surface area (S) for CoCrxFe2-xO4 spinel ferrite system.

Besides using experimentally found value of lattice constant and oxygen

positional parameter (u), it is possible to calculate the value of the mean ionic radius

per molecule of the tetrahedral and octahedral sites, rA and rB, respectively, based on

the cation distribution for each composition using the relation [9]:

Cr3+ –

Content

(x)

Mw x 10-3

(kg/mole)

aexp(Å)

±0.002Å

ρx ρ

(g/cm3) f

P

(%)

D(nm)

±1 nm

S x 104

(cm2/g)

0.0 234.63 8.363 5.329 4.887 0.083 8.3 72 17.05

0.1 234.24 8.363 5.319 4.835 0.091 9.1 − −

0.3 233.48 8.362 5.303 4.762 0.102 10.2 65 19.38

0.7 231.94 8.358 5.276 4.690 0.111 11.1 60 21.32

1.1 230.40 8.352 5.253 4.560 0.132 13.2 39 33.74

1.3 229.63 8.348 5.243 4.383 0.164 16.4 33 41.48

1.5 228.85 8.344 5.232 4.264 0.185 18.5 28 50.25

1.7 228.08 8.342 5.219 4.154 0.204 20.4 21 68.78

1.8 227.69 8.340 5.213 4.123 0.209 20.9 − −

1.9 227.31 8.339 5.205 4.107 0.211 21.1 20 73.05

2.0 226.92 8.337 5.201 3.994 0.232 23.2 17 88.37

Chapter 4

4.32

 

[ ])r(Fe)(Fef)r(Cr)(Crf)r(Co)(Cofr 33c

33c

22cA

++++++ ⋅+⋅+⋅= (2)

[ ])r(Fe)(Fef)r(Cr)(Crf)r(Co)(Cof21r 33

c33

c22

cB++++++ ⋅+⋅+⋅=

(3)

where, fc and r, are the fractional concentration and ionic radius of respective cation

on the respective site. The ionic radius of Co2+ (0.720 Å), Cr3+ (0.630 Å) and Fe3+

(0.640 Å) ions are taken with reference to coordination 6. Using these formulae, the

mean ionic radius of the tetrahedral (A-) sites (rA) and of the octahedral (B-) sites (rB)

have been calculated and are listed in Table 4.5.2.

Table 4.5.2: The cationic distribution in the samples at different Cr3+

concentrations (x).

Cr3+ -

content

(x)

Cation distribution

A – Site B – Site

0.0 (Fe3+0.9Co2+

0.1)A [Co2+0.9Fe3+

1.1]B O42-

0.1 (Fe3+0.8Co2+

0.2)A [Co2+0.8Cr3+

0.1Fe3+1.1]B O4

2-

0.3 (Fe3+0.6Co2+

0.4)A [Co2+0.6Cr3+

0.3Fe3+1.1]B O4

2-

0.7 (Fe3+0.3Co2+

0.7)A [Co2+0.3Cr3+

0.7Fe3+1.0]B O4

2-

1.1 (Fe3+0.3Co2+

0.7)A [Co2+0.3Cr3+

1.1Fe3+0.6]B O4

2-

1.3 (Fe3+0.3Co2+

0.7)A [Co2+0.3Cr3+

1.3Fe3+0.4]B O4

2-

1.5 (Fe3+0.3Co2+

0.7)A [Co2+0.3Cr3+

1.5Fe3+0.2]B O4

2-

1.7 (Fe3+0.3Co2+

0.7)A [Co2+0.3Cr3+

1.7]B O42-

1.8 (Co2+1.0)A [Cr3+

1.8Fe3+0.2]B O4

2-

1.9 (Co2+1.0)A [Cr3+

1.9Fe3+0.1]B O4

2-

2.0 (Co2+1.0)A [Cr3+

2.0]B O42-

It can be seen that rB decreases slowly while rA increases rapidly with

increasing Cr3+ - content (x) in the system, which in turn causes the lattice constant

‘a’, to decrease with Cr3+ substitution (x). It can be concluded that the octahedral sites

substitution plays a dominant role in influencing the variation of ‘a’ with

concentration (x).

Chapter 4

4.33

 

According to Steinfink et al [10,11] the tolerance factor, T, for the spinel

structured materials is defined as:

⎟⎟⎠

⎞⎜⎜⎝

⎛+

+⎟⎟⎠

⎞⎜⎜⎝

⎛++

=OB

O

OB

OA

RrR

21

RrRr

31T (4)

where, notations have their usual meaning. For an ideal spinel structure T value is

close to unity. It is found that for all the synthesized ferrites, value of T is close to

unity suggesting defect free formation of spinel structure and the value increases with

increase in Cr3+ - concentration (x) in the system (Table 4.5.3).

It is known that there is a correlation between the ionic radius and the lattice

constant. The radii of the tetrahedral and octahedral sites in a spinel ferrite can also be

calculated using the formulae given by [12]:

0A Ra41u3r −⎟⎠⎞

⎜⎝⎛ −= (5)

0B Rau85r −⎟

⎠⎞

⎜⎝⎛ −= (6)

where R0 represents the radius of the oxygen ion (taken as 1.32Å) and u is the oxygen

positional parameter. These relationships are further used to calculated the lattice

parameter theoretically ‘ath’ [13].

From equation (6), one can write,

aRr

85u 0B +−= (7)

By substituting the value of ‘u’ from equation (7) in to equation (5) and rearranging

the terms,

( )⎥⎦

⎤⎢⎣

⎡−⎟

⎠

⎞⎜⎝

⎛ +−

+=

41

aRr

853

Rra

0B

0A

( )2a8R8r5a3)R(r 8a

0B

0A

−−−

+=

)R8(r3aRr

8aa3

0B

0A

+−+

=

)Rr 8()R(r38a33 0A0B +=+−

[ ])R(r3)Rr (33

8a 0B0Ath +++= (8)

Chapter 4

4.34

 

It has been observed that the theoretically calculated values of lattice constant

follow the same trend as that obtained experimentally, although the values are

generally smaller than the experimental ones [13–15]. Theoretical calculations

presume an ideal close packed structure and valence state of the cations and thus

corresponding values of ionic radii have to be taken into consideration.

The oxygen positional parameter or anion parameter (u) for each composition was

calculated using the formulae available in the literature [3].

22R181R

4811

32R

41

u 2

21

22

m3

−

⎟⎠⎞

⎜⎝⎛ −+−

= (9)

22R181R

4811

1211R

21

u 2

21

22

3m4

−

⎟⎠⎞

⎜⎝⎛ −+−

= (10)

41

3aRr

u 0A3m4 ++

= (11)

0.07054

A

B3m4

rr

0.3876u−

⎟⎟⎠

⎞⎜⎜⎝

⎛=

(12)

where, R=(B-O) / (A-O). The bond lengths, B – 0 and A – O are average bond lengths

calculated based on the cation distribution listed in Table 4.5.2, where, 0BrO-B R+=

and 0A RrO-A += ,equation (9) gives ‘u’ assuming centre of symmetry at (1/4, 1/4,

1/4) for which uidea = 0.250 (origin at the B-site), while equations (10) – (12) give ‘u’

assuming centre of symmetry at (3/8, 3/8, 3/8) for which uidea = 0.375 (origin at the A-

site). The values of 3m4u determined from different formulae are in agreement with

each other (Table 4.5.3). To convert origin from A-site to B-site, 1/8uu m33m4 += [3],

relationship has been used. In an ideal fcc structure 0.250.u0.375;u m33m4 ==

Chapter 4

4.35

 

Table 4.5.3: Ionic radius (r), lattice constant (a), oxygen positional parameter

(u) and tolerance factor (T) for CoCrxFe2-xO4 spinel ferrite system.

Although, most ferrites generally have ‘u’ greater than this ideal value

[14,16], it is slightly larger in the present series (Table 4.5.3), implying that the

oxygen ions are displaced in such a way that in the A-B interaction, the distance

between A and O ions and between B - O ions decreased. This leads to an increase in

the A – A and B – B interactions. As ‘u’ increases from its ideal value, anions move

away from the tetrahedrally coordinated A - site cations along the ‹111› directions,

which increase the volume of each A - site interstice while the octahedral B –sites

become correspondingly smaller. The Fe2+ - ion (0.740 Å) is one of the largest

divalent ions found in spinels, whereas the Al3+ ion is the smallest of the trivalent

spinel cation: thus the r(Fe2+) / r(Cr3+) ratio is large, which favors large u values [3].

Using the experimental values of ‘a’ and anion parameter (u) of each

composition in equations [3, 14, 16, 17] interatomic distances has been calculated.

Cr3+ -

content(x)

rA

(Å)

rB

(Å)

ath

(Å)

m3u

(1/4,1/4,1/4)

3m4u (3/8, 3/8, 3/8)

Eq.(9) Eq.(10) Eq.(11) T

0.0 0.648 0.676 8.352 0.2609 0.3874 0.3858 0.3864 1.044

0.1 0.656 0.672 8.352 0.2604 0.3875 0.3864 0.3870 1.045

0.3 0.672 0.663 8.353 0.2632 0.3842 0.3876 0.3880 1.049

0.7 0.696 0.649 8.353 0.2645 0.3887 0.3892 0.3895 1.054

1.1 0.696 0.646 8.346 0.2643 0.3895 0.3893 0.3896 1.055

1.3 0.696 0.646 8.345 0.2671 0.3918 0.3894 0.3897 1.055

1.5 0.696 0.645 8.342 0.2649 0.3888 0.3895 0.3897 1.056

1.7 0.696 0.644 8.340 0.2646 0.3891 0.3895 0.3898 1.056

1.8 0.720 0.631 8.343 0.2664 0.3911 0.3912 0.3912 1.061

1.9 0.720 0.631 8.341 0.2663 0.3910 0.3912 0.3912 1.062

2.0 0.720 0.630 8.341 0.2671 0.3910 0.3913 0.3912 1.062

Chapter 4

4.36

 

( ) ⎟⎠⎞

⎜⎝⎛ −=

212u2ad 3m41/2

AE Shared tetrahedral edge

( ) ( )3m41/2BE 2u12ad −= Shared octahedral edge

( )1/2

3m423m4BEu 16

113uu4ad ⎥⎦⎤

⎢⎣⎡ +−= Unshared octahedral edge (13)

⎟⎠⎞

⎜⎝⎛ −=

41u3ad 3m4

AL Tetrahedral bond length

( )1/2

m3423m4BL 64

43u4

11u3ad ⎥⎦⎤

⎢⎣⎡ +−= Octahedral bond length

Table 4.5.4: Edge length and bond length for Co – Fe – Cr – O system.

The calculated values of edge lengths and bond lengths are given in

Table 4.5.4. It is found that with an increase in Cr3+ - concentration, shared octahedral

edge length and octahedral bond length decrease. These may be due to the

replacement of larger Fe3+ -ions by smaller Cr3+ ions on the octahedral site. On the

other hand, shared tetrahedral edge length and tetrahedral bond length increase while

unshared octahedral edge length remains uninfluenced by Cr3+ - substitution.

Cr3+-content (x) dAE (Å) dBE (Å) dBEu (Å) dAL (Å) dBL (Å)

0.0 3.2496 2.6631 2.9639 2.1481 1.9923

0.1 3.2520 2.6607 2.9639 2.1497 1.9931

0.3 3.1735 2.7384 2.9600 2.0978 2.0164

0.7 3.2784 2.6307 2.9644 2.1672 1.9826

1.1 3.2947 2.6098 2.9621 2.1779 1.9740

1.3 3.3477 2.5545 2.9645 2.2130 1.9560

1.5 3.2752 2.6240 2.9595 2.1651 1.9781

1.7 3.2813 2.6161 2.9586 2.1691 1.9757

1.8 3.3280 2.5685 2.9604 2.1999 1.9613

1.9 3.3254 2.5707 2.9603 2.1983 1.9594

2.0 3.3244 2.5700 2.9594 2.1976 1.9588

Chapter 4

4.37

 

Figure 4.5.2: Configuration of ion pairs in spinel ferrites with favorable

distances and angles for effective magnetic interactions.

The configuration of ion pairs in spinel ferrites with favorable distances and

angles for magnetic interactions are shown in Figure 4.5.2. The interionic distances

between the cations (Me-Me) (b, c, d, e, and f) and between the cation and anion

(Me-O) (p, q, r and s) were calculated using the experimental values of lattice

constant (aexp) and oxygen positional parameters (u3m) (Tables 4.5.1 and 4.5.3) by the

relations [18, 19]:

Me – O Me - Me

⎟⎠⎞

⎜⎝⎛ −= 3mu

21ap

1/22

4ab ⎟⎠⎞

⎜⎝⎛=

1/23m 381uaq ⎟⎠⎞

⎜⎝⎛ −=

1/211

8ac ⎟⎠⎞

⎜⎝⎛=

1/23m 1181uar ⎟⎠⎞

⎜⎝⎛ −=

1/23

4ad ⎟⎠⎞

⎜⎝⎛=

(14)

1/23m 321u

3as ⎟

⎠⎞

⎜⎝⎛ +=

1/23

83ae ⎟

⎠⎞

⎜⎝⎛=

1/264af ⎟⎠⎞

⎜⎝⎛=

The overall strength of the magnetic interactions (A−B, B−B and A−A)

depends upon the bond length and bond angles between the cations and cation - anion.

The strength is directly proportional to bond angle but inversely proportional to bond

length. It is seen from Table 4.5.5 that both interatomic distances between the cations

Chapter 4

4.38

 

(b, c, d, e and f) decrease with increasing Cr3+- concentration (x). These results are

accordance with decrease in unit cell volume. The bond angles (θ1, θ2, θ3, θ4 and θ5)

(Figure 4.5.2) were calculated by simple trigonometric principle using the interionic

distances with the help of following formulae:

⎥⎦

⎤⎢⎣

⎡ −+= −

2pqcqpcosθ

2221

1

⎥⎦

⎤⎢⎣

⎡ −+= −

2prerpcosθ

2221

2

(15)

⎥⎦

⎤⎢⎣

⎡ −+= −

2psfspcosθ

2221

4

⎥⎦

⎤⎢⎣

⎡ −+= −

2rqdqrcosθ

2221

5

It is found that angles θ1, θ2 and θ5 decrease while θ3 and θ4 increase with

increase in Cr3+- content (x). The observed decrease in θ1, θ2 and θ5 suggest

weakening of the A–B and A–A interactions while increase in θ3 and θ4 indicative of

strengthening of the B−B interaction on Cr3+-substitution in the system.

⎥⎦

⎤⎢⎣

⎡ −= −

2

221

3 2pb2pcosθ

Chapter 4

4.39

 

Table 4.5.5: Interionic distances (b, c, d, e, f and p, q, r, s) and bond angles (θ) for

Co – Fe – Cr – O system.

(Distances in Å and angles in degrees)

Cr3+-

content

(x)

0.0 0.1 0.3 0.7 1.1 1.3 1.5 1.7 1.8 1.9 2.0

b 2.9563 2.9568 2.9564 2.9550 2.9527 2.9515 2.9500 2.9492 2.9487 2.9485 2.9476

c 3.4671 3.4671 3.4667 3.4650 3.4623 3.4609 3.4592 3.4582 3.4576 3.4575 3.4564

d 3.6213 3.6213 3.6208 3.6191 3.6163 3.6149 3.6131 3.6120 3.6114 3.6112 3.6101

e 5.4319 5.4319 5.4313 5.4287 5.4245 5.4223 5.4196 5.4180 5.4171 5.4168 5.4152

f 5.1212 5.1212 5.1207 5.1182 5.1142 5.1122 5.1096 5.1081 5.1072 5.1070 5.1055

p 1.9995 2.0038 1.9801 1.9683 1.9684 1.9442 1.9617 1.9636 1.9483 1.9490 1.9417

q 1.9662 1.9590 1.9992 2.0170 2.0126 2.0522 2.0194 2.0145 2.0401 2.0386 2.0495

r 3.7687 3.7548 3.8321 3.8683 3.8577 3.9377 3.8709 3.8614 3.9106 3.9076 3.9285

s 3.6696 3.6672 3.6802 3.6847 3.6808 3.6929 3.6804 3.6779 3.6859 3.6853 3.6881

θ1 121.92 122.06 121.19 120.78 120.84 119.97 120.66 120.75 120.18 120.22 119.98

θ2 138.60 139.20 135.87 134.29 134.62 131.56 133.94 134.29 132.30 132.25 131.56

θ3 95.34 95.08 96.58 97.29 97.18 98.76 97.51 97.35 98.35 98.29 98.74

θ4 126.66 126.60 126.93 126.03 127.06 127.37 127.12 127.09 127.29 127.28 127.37

θ5 70.46 70.86 68.68 68.56 67.85 65.80 67.40 67.62 66.30 66.38 65.80

Chapter 4

4.40

 

Conclusions

(i) We have successfully synthesized single phase (fcc), defect free

nanocrystalline spinel structured ferrite materials of CoCrxFe2-xO4

(x = 0.0 – 2.0) system by the coprecipitation route.

(ii) It is found that Cr3+ - ions have strong preference for the octahedral B– site,

Fe3+ - ions distributed among the A – and B – sites almost equally, while

Co2+ - ions initially have preference for the B – site (x = 0.0 – 0.6) and for

higher concentration of x, (x ≥ 0.7), it preferably occupy the A– site. The

system gradually transfers from inverse spinel to mixed spinel to normal

spinel structure on increasing Cr3+ - concentration.

(iii) The particle size rapidly decreases while all other structural parameter slowly

decreases with increasing Cr3+ - substitution (x) in the system.

(iv) Various structural parameters can be determined from X-ray powder

diffraction pattern analysis and are found useful to explain other physical

properties.

(v) The strength of the B– B interaction increases while A–B interaction decreases

with Cr3+ - substitution for Fe3+ in the system.

Chapter 4

4.41

 

References 1. B. D. Cullity, ‘Elements of X-ray diffraction’, Addision Wesley, (1978).

2. C. Dong, ‘Powder X: Windows-95-based program for powder X-ray

diffraction data processing’, J. Appl. Cryst., 32 (4) (1999) 838 – 847.

3. K. E. Siokafus, J. M. Wills, N. W. Grimes, ‘Structure of Spinel’,

J. Am. Ceram. Soc., 82 (12) (1999) 3279-3291.

4. M. J. Buerger, ‘Crystal Structure Analysis’, Wiley, (1980).

5. H. Ohnish, T. Teranishi, ‘Crystal distortion in copper ferrite-chromite series’,

J. Phys. Soc. Jpn., 16 (1961) 35 – 43.

6. V. T. Thanki, ‘Study on magnetic properties of oxide materials’, Ph.D. Thesis,

Saurashtra University, Rajkot, (1996).

7. J. Smith, H. P. J. Wijn, ‘Ferrites’, Wiley, (1959).

8. C. G. Whinfrey, D. W. Eckart, A. Tauber, ‘Preparation and X-Ray Diffraction

Data1 for Some Rare Earth Stannates’, J. Am. Chem. Soc., 82 (1960) 2695 –

2699.

9. M. George, A. M. John, S. S. Nair, P. A. Joy, M. R. Anantharaman, ‘Finite

size effects on the structural and magnetic properties of sol–gel synthesized

NiFe2O4 powders’, J. Magn. Magn. Mater. 302 (1) (2006) 190 – 195.

10. K. Kugimiya, H. Steinfink, ‘Influence of crystal radii and electro negativities

on the crystallization of AB2X4 stoichiometries’, J. Inorg. Chem., 7 (9) (1968)

1762 – 1770.

11. R. Sharma, S. Singhal, ‘Structural, magnetic and electrical properties of zinc

doped nickel ferrite and their application in photocatalytic degradation of

methylene blue’, Physica B, 414 (2013) 83 – 90.

12. K. J. Standly, ‘Oxide Magnetic Materials’, Clarandom Press, (1972).

13. S. A. Mazen, M. H. Abdallah, B. A. Sabrah, H. A. M. Hasham, ‘The Effect of

Titanium on Some Physical Properties of CuFe2O4’, Phys. Status. Solidi

A, 134 (1992) 263-271.

Chapter 4

4.42

 

14. R. K. Sharma, Varkey Sebastain, N. Lakshmi, K. Venugopalan, V. R. Reddy,

Ajay Gupta, ‘Variation of structural and hyperfine parameters in nanoparticles

of Cr-substituted Co-Zn ferrites’, Phys. Rev. B, 75 (2007) 144419 – 144424.

15. H. Bhargava, N. Lakshmi, V. Sebastina, V. R. Reddy, K. Venugopalan, Ajay

Gupta, ‘Investigation of the large magnetic moment in nano-sized

Cu0.25Co0.25Zn0.5Fe2O4’, J. Phys D: Appl. Phys. 42(2009)245003-245010.

16. V. K. Lakhani, T. K. Pathak, N. H. Vasoya, K. B. Modi, ‘Structural

parameters and X-ray Debye temperature determination study on copper-

ferrite-aluminates’, Solid State Sci., 13 (2011) 539 – 547.

17. T. Abbas, Y. Khan, M. Ahmad, S. Anwar, ‘X-ray diffraction study of the

cation distribution in the Mn-Zn-ferrites’, Solid State Commun., 82 (9) (1992)

701-703.

18. J. B. Goodenough, ‘An interpretation of the magnetic properties of the

perovskite-type mixed crystals La1−xSrxCoO3−λ’, J. Phys. Chem. Solids,

6 (1958) 287-297.

19. J. Kanamori, ‘Super exchange interaction and symmetry properties of electron

orbitals’, J. Phys. Chem. Solids. 10 (2-3) (1959) 87-98.

 

Chapter 4

4.43

 

4.6 Crystal defects and cation redistribution study on

nanocrystalline cobalt – ferri – chromites by positron

annihilation spectroscopy

Crystalline materials with the characteristic spinel structure, comprising of well

designated tetrahedral (A-) and octahedral (B-) sites, constitute a very interesting class

of condensed matter systems evoking interest even from the very fundamental science

viewpoint [1, 2]. Cobalt ferrite (CoFe2O4), Cobalt chromite (CoCr2O4) and their solid

solutions with a typical crystalline structure AB2O4 are candidate materials in this

class where, A is normally a divalent and B a trivalent ion. These materials have

attracted a large number of chemists, physicists and metallurgists to study their

different aspects, including the structure and properties, using both theoretical

modeling and wide variety of experimental tools [3-5]. Ferrites composed of

nanometer-sized particles elevates this interest to new dimensions as the very large

network of interfaces will then play a decisive role in controlling the atomic transport

and spatial rearrangement of atoms within the structure. So far as the tools for

investigation into such details are concerned, conventional experimental methods such

as X-ray diffraction (XRD) and transmission electron microscopy (TEM) have helped

to obtain substantial information on a macroscopic scale. But, defect-specific probes

such as positron annihilation spectroscopy are needed to pinpoint their role in the

post-synthesis and characterization treatments such materials have to undergo. We

present in this section the results of our investigation carried out on CoFe2O4, both on

the nascent mother sample and samples in which the Fe3+ ions are replaced by Cr+3

ions, i.e. CoCrxFe2-xO4 , x = 0.0 – 2.0. The purpose of the work is double-sided. In this

section the emphasis is given to demonstrate the ability of positron annihilation

techniques to sense such changes. The latter is of importance since such works are

scarcely available in literature so far and secondly it offers a viable investigative

probe for such studies which are highly essential in the current scenario of novel

materials and arising challenges in their understanding.

In the previous sections (4.2 – 4.5) the changes occurring at the different

stages of Cr3+ - concentration (x) have been obtained from more conventional

experimental tools such as XRD, TEM, EDAX etc.

Chapter 4

4.44

 

According to the existing literature, cobalt ferrite (CoFe2O4) is an inverse

spinel and taken to be collinear ferrimagnet [6], while cobalt chromite, CoCr2O4, is a

normal spinel with a canted ferromagnetic structure and its Curie temperature is 97 K

[7]. Previous studies of magnetic properties and Mossbauer spectroscopy on mixed

cobalt-ferri-chromites,CoCrxFe2-xO4 of coarse-grain composition indicated that

canting of magnetic structure is observed when Co2+ is present at the tetrahedral (A-)

site [8,9]. While magnetization measurements on the same system could be explained

by Neel’s model as far as the series remained inverse spinel, they deviated

significantly when they began to have normal spinel structure [10]. Recently,

structural and magnetic properties of nanocrystalline CoCrxFe2-xO4 (0 ≤ x ≤ 1.0)

system prepared by the sol-gel auto combustion route have been studied by Jadhav et

al [11].

The redistribution of cations when one species is replaced by ions of

neighboring elements in the periodic table has been of tremendous significance in

modifying the properties as well as in giving rise to new phenomena and processes

[6,8,9]. The transformation of a nearly-complete inverse spinel ferrite, CoFe2O4, to a

normal spinel chromite, CoCr2O4, through successive replacement of the Fe3+ ions by

Cr3+ ions has been found to generate drastic changes in the cationic distribution in the

structure, owing to lattice contraction as well as the likely presence of vacancy-type

structural defects. Although substitution effects generally prompt such redistributions,

concomitant lattice contraction or expansion can also influence it and quite often it

will result in the generation of structural defects in the form of unoccupied lattice

sites. The latter are potential sites for investigation by positron annihilation

experiments. Such a defect-sensitive spectroscopic probe will be of immense benefit

as it can pinpoint the origin and evolution of such defects and their dominating role

over the redistribution of ions in the lattice. As is popularly known, the positron

lifetimes and Doppler broadening of the electron-positron annihilation gamma ray

spectrum are directly related to the electron density and momentum distribution in a

material and hence the information carried by the signals of annihilation can unravel

the material properties in the atomic scale [12]. The details regarding the positron

annihilation studies of the powdered samples and the methods of data analysis to

extract the relevant information can be found from [13, 14].

Chapter 4

4.45

 

4.6 (A) Positron lifetimes in the un-substituted sample, CoFe2O4 (x = 0.0)

The positron lifetime spectra were analyzed using the PALSfit computer program

developed by the Risoe group [15]. The spectra of all the samples were fitted to obtain

variances of fit within satisfactory limits (1.07±0.12). The fits yielded three distinct

lifetimes τ1, τ2 and τ3 in all the cases and their magnitudes, as discussed below, were

characteristic of positron trapping in specific sites within the spinel structure or

positronium formation within the grain boundaries. In the CoFe2O4 (x = 0.0) sample,

the intermediate lifetime τ2 was found as high as 356 ps with relative intensity I2 =

48.2 %. The normal interpretation for observation of such a well-resolved longer

lifetime with appreciable intensity is the presence of vacancy-type crystalline defects

within the material since positrons get trapped in such lower-than-average-electron-

density sites. This is a reasonable assumption since it is nearly impossible to

synthesize ferrites with fully occupied crystalline structure. Besides, those positrons

managing to diffuse out to the vacancies on the nanocrystalline grain interfaces may

also contribute to this component. The reason is that the thermal diffusion lengths of

positrons in oxide materials are typically about 50-60 nm [16, 17]. Hence, a small

fraction of positrons would inevitably diffuse and migrate to the surfaces of the

nanocrystals (which are of sizes about 60-65 nm) before their annihilation. Despite

prolonged heating, the grain dimensions could not be increased to more than the

above limit. On the other hand, the diffusion lengths in the present case could be

shortened due to the trapping of positrons by vacancy clusters if present within the

nanoparticles. The positron lifetime in the perfect crystalline sample (τf, for which no

theoretical value is available) can be calculated using the trapping model equation

[18]: ,τI

τI

τI

τ1

3

3

2

2

1

1

f

++= Substituting the experimental values of the positron lifetimes

and their intensities of the CoFe2O4 sample in the above equation, we obtain τf = 199

ps. The shorter lifetime component τ1 is obviously less than this value in all the cases

due to admixing with the Bloch state residence time of trapped positrons [12]. A small

contribution coming to τ1 from parapositronium atoms of lifetime 125 ps is ignored as

the intensity of this component, one-third that of the orthopositronium intensity I3, is

negligibly small.

Chapter 4

4.46

 

The annihilation characteristics of positrons diffusing out to the grain surfaces

are also reflected in the variation of the longest lifetime τ3 and its intensity I3. The

magnitude of this lifetime (1.8 – 2.1 ns) is typical of the “pick-off” annihilation of

orthopositronium atoms formed at the interfacial regions of the grains [12]. Although

positronium formation is not significant enough to alter the interpretations in metallic

oxides, it has been found still relevant enough to force a three-component analysis of

the positron lifetime spectra of nanocrystalline materials [19-21], and the intensity I3,

despite being relatively small (0.8 – 1.4 %), indicates the presence of large free

volume regions in the intergranular regions of materials when composed by

nanometer-sized particles or grains.

Figure 4.6.1: The lattice constant (a) and the radii of the tetrahedral (rA) and

octahedral (rB) sites versus Cr3+ - concentration (x).

4.6 (B) Results of Cr3+ substitution

Figures 4.6.2 (a) and (b) describe the changes occurring in the positron annihilation

parameters as a result of Cr3+ substitution for Fe3+ in CoFe2O4. A close look into the

trends of variation helps to identify three distinct stages of defects evolution and/or

structural variations. In the first stage spreading over the concentration x = 0.1 to 0.7,

the two positron lifetimes τ1 and τ2 show remarkable increase in the initial stage and

Chapter 4

4.47

 

attain saturation. The longer lifetime τ3 and its intensity I3 show characteristic

decrease that will be discussed later. The second stage of variation is marked for x>

0.7 till 1.7 during which the lifetimes decrease and the intensity sharply rises. All

these trends are just reversed once again in the last stage from x = 1.8 to 2.0.

The variation of the different positron annihilation parameters with Cr3+-

substitution is thus highly complex in nature, as the potential trapping centers might

have changed during the different stages of substitution, due to not only the arrival of

a new element but also the relative displacement they may cause in the positioning of

the other ions already present in the crystalline structure. Certain information in this

direction is available from the results of CDBS measurements shown in Figure

4.6.3(a).

Figure 4.6.2: (a): The positron lifetimes τ1 and τ2 and intensity I2 versus Cr3+

concentration (x) (b): The orthopositronium lifetime τ3 and intensity I3 versus

versus Cr3+ concentration (x).

Chapter 4

4.48

 

The data has been analyzed using the usual quotient spectral method in which

the projected one-dimensional spectra on the ((E1−E2)/2) axis of the counts in the

window ((E1+E2)/2) = 511 ± 1.2 keV are peak-normalized and divided by that of a

pure reference sample (Si single crystals) [14, 22]. The choice of Si to serve as a

reference is not unjustified since it is not a constituent of the material at any stage in

this investigation and the purpose is to magnify the differences in shapes of the

momentum distribution curves for easy understanding and interpretation. In Figure

4.6.3(a)(i),the ratio curves of the samples with the two extreme compositions,

CoFe2O4(x = 0.0) and CoCr2O4 (x = 2.0), are shown together with the identical curves

obtained for the constituent elemental samples. Figure 4.6.3(a) (ii) illustrates the

curves obtained similarly for the Cr3+- substituted samples of a few representative

concentrations. The ratio curves of the samples are found having a characteristic

peaks at pL= 10.3×10-3 m0c (where m0 is the electron mass and c is the velocity of

light). The peaks of the ratio curves of the three constituent metals, i.e., Co, Fe and

Cr, appear at 15.0×10-3 m0c, 12.2×10-3 m0c and 11.3×10-3 m0c respectively but with

decreasing amplitudes. This observation is consistent with the decreasing number of d

-electrons and decreasing radius of the 3d-shell. That the peak of the ratio curve of

either the pure or any of the Cr3+- substituted samples does not coincide with those of

the elemental curves is ample proof to suggest that positrons are not trapped in

oxygen vacancies, a fact even otherwise vindicated by their positive charge that will

repel positrons. On the other hand, trapping take place in the cationic vacancies and

the peak at pL = 10.3×10-3 m0c common to all the samples and irrespective of the Cr3+

- concentration (x) indicate the encirclement of the defects by oxygen ions. In several

of our recent studies on nanocrystalline oxide semiconductors, we have similarly

obtained the peak due to annihilation with oxygen electrons at pL = 10.3×10-3 m0c

[23-25].

The identical elemental environment around the positron trapping sites at all

concentrations of Cr3+ substitution is further verified from the S versus W plot shown

in Figure 4.6.3 (b) that is normally used to identify the changes in the predominant

type of positron trapping defects at different stages of variation of the experimental

parameters. The S and W parameters have been derived from the CDB spectra as the

counts falling under segments respectively from 0 to 3.75×10-3 m0c and from

Chapter 4

4.49

 

7.5×10-3 m0c to 12.25×10-3 m0c normalized by the total counts accumulated under 0 to

37.5×10-3 m0c. The S - W plot is linear and all the points lie essentially on a straight

line. This indicates that positrons essentially encounter similar elemental

environments irrespective of the cationic redistribution. This is further credence to the

argument that the defects which trap positrons are surrounded by oxygen ions and

therefore the traps are none other than the cationic vacancies. But there are variations

in the intensity of annihilation with the oxygen electrons, as indicated by the

individual variations of the S and W parameters with Cr3+- concentration (x), shown

in Figures 4.6.4(a) and (b). (For the sake of clarity, the curves of not all the samples

are shown in Figure 4.6.3(a) (i) or (ii) but the peak coordinates of all the curves are

shown in Figure 4.6.3 (b). From Figures 4.6.4(a) and (b) also, we can distinguish from

one another basically three regions, the demarcation being identical to that mentioned

in the case of the positron lifetime results. The first two regions (x = 0.0 – 0.7 and

x = 0.7 – 1.7) are characterized by a fall and rise of the peaks of the curves and the last

stage is marked by again a fall. It can be argued that, although the annihilation

environment of positrons essentially remain identical, they are trapped at different

stages of Cr3+ substitution by defects situated at different sites in the lattice structure.

These points are further discussed in detail afterwards.

The insensitivity of the CDB spectra to the oxygen vacancies can be explained

on the basis of the results of positron lifetime measurements as well. The difference

between the lifetime characteristic of defects (i.e., τ2) and the bulk lifetime τf is

normally considered as an indication of the size of the defect. The enhancement in

positron lifetime due to trapping in monovacancies is ~ 40-80 ps in typical metals and

alloys [26]. Assuming that τ2 = 356 ps is an upper limit of the positron lifetime in

vacancy clusters in the un-substituted sample, τ2 – τf = 356 – 199 = 157 ps will

correspond to defects much larger than monovacancies. Theoretical estimations in Fe,

which is normally bcc in structure but a constituent of the present samples, have

shown the enhancement of positron lifetime in a neutral vacancy cluster composed of

4-5 neighboring monovacancies as 152 ps [27]. Considering these facts, the positron

trapping site in the undoped alloy can be conceived to be a vacancy cluster composed

of the monovacancy created by the absence of a doubly ionized cation and four of its

coordinated oxygen ions.

Chapter 4

4.50

 

Figure 4.6.3: (a) The ratio curves generated from the coincidence Doppler

broadening spectra of the different samples - (i) elements Co, Fe and Cr besides

CoCrxFe2-xO4 of x = 0.0 and 2.0; (ii) CoCrxFe2-xO4 of x = 0.0, 0.3, 0.7, 1.7 and 1.9.

All the spectra had been peak-normalized and then divided by that of a pure

reference Si sample in order to generate the ratio curves. (b) The S – W plot of

the Cr3+- substituted samples.

Based on X-ray diffraction, magnetization and Mossbauer results, Mohan et al

[10] have shown that the CoFe2O4 (x = 0.0) is a nearly-complete inverse spinel and

the ionic distribution in it is of the form (Fe3+0.9Co2+

0.1)A [Co2+0.9Fe3+

1.1]B O42-. The

absence of a Co2+ ion with four oxygen neighbor ions will give rise to a neutral penta-

vacancy cluster in which positrons can be trapped and annihilated. The other

possibility of the absence of a trivalent cation with four neighboring oxygen ions

cannot be ruled out as it would have enhanced positron trapping due to surplus

negative charge and hence it is necessary to point out whether the said vacancy cluster

is centered at the A-site or at the B-site. This can be answered by looking at the

effects of Cr3+ substitution on the cationic redistribution. A schematic diagram

Chapter 4

4.51

 

showing the two neighboring octants of a normal spinel structure is shown in Figure

4.6.5. At the very onset of substitution, a drastic drop in the intensity of the peak in

the CDB spectra (represented by the W parameter) is observed (Figures 4.6.3(a)(ii)

and 4.6.4(b)) and it indicates the diminishing positron annihilation probability with

oxygen electrons. The ionic distributions obtained from the X-ray diffraction peak

intensity analysis for samples with the different concentrations of the Cr3+ ions are

given in Table 4.6.1. Thus, for example, the distribution is (Fe3+0.8Co2+

0.2)A

[Co2+0.8Cr3+

0.1Fe3+1.1]B O4

2- for x = 0.1. This implies that the substituted Cr3+ ions

initially replace equal number of Fe3+ ions from the B-sites but simultaneously equal

number of Fe3+ ions from the A-sites move over to the B-sites in exchange of Co2+

ions from the B-sites to the A-sites. In effect, an inversion of the spinel structure is

prompted as a result of the substitution process. Hence, as shown in Table 4.6.1, the

number of Fe3+ ions at the A-sites decrease whereas that at the B-sites remains

unaltered till x = 0.7 compositions. Since, CDB spectra indicate diminishing

annihilation with oxygen electrons and the positron lifetime τ2 increase from 356 ps to

374 ps, it is reasonable to argue that the defects in the sample with x = 0.1 is larger in

size and increasingly deficient in oxygen ions than those in the un-substituted(x = 0.0)

sample. In other words, the defects were centered at the A-sites in the un-substituted

sample and at the B-sites in the substituted samples. We attribute the second positron

lifetime component τ2 to such large vacancy clusters.

As already stated, the substitution or doping resulted in sharp rises in the two

lifetimes, τ1 and τ2. The intensity I2 however did not show any change. It is therefore a

local effect in which the vacancy cluster has undergone an increase in size. From

EDAX studies (section 4.2), we have estimated the actual concentration of the Cr3+

ions effectively substituted in the crystallites. It has been found that the B-sites

suffered from non-stoichiometric deficiencies of Cr3+ ions and hence the Co2+ ions

transferred to the A-sites is also less in number than that predicted by the

formula(Fe3+0.9-xCo2+

0.1+x)A [Co2+0.9-xCr3+

xFe3+1.1]BO4

2-. The result is that the vacancies

so created will add to the existing vacancy clusters, resulting in further increase in

their size and thereby enhancing the positron lifetimes. However, the deficiency

decreases on subsequent doping and therefore the lifetime τ2 and intensity I2 remain

rather unchanged in the range of concentration from 0.1 to 0.7.

Chapter 4

4.52

 

Table 4.6.1: The cationic distribution in the samples at different Cr3+-

concentration (x).

Figure 4.6.4: The S and W parameters versus Cr3+ - concentration (x).

Cr3+ -

content

(x)

Cation distribution

A – Site B – Site

0.0 (Fe3+0.9Co2+

0.1)A [Co2+0.9Fe3+

1.1]B O42-

0.1 (Fe3+0.8Co2+

0.2)A [Co2+0.8Cr3+

0.1Fe3+1.1]B O4

2-

0.3 (Fe3+0.6Co2+

0.4)A [Co2+0.6Cr3+

0.3Fe3+1.1]B O4

2-

0.7 (Fe3+0.3Co2+

0.7)A [Co2+0.3Cr3+

0.7Fe3+1.0]B O4

2-

1.1 (Fe3+0.3Co2+

0.7)A [Co2+0.3Cr3+

1.1Fe3+0.6]B O4

2-

1.3 (Fe3+0.3Co2+

0.7)A [Co2+0.3Cr3+

1.3Fe3+0.4]B O4

2-

1.5 (Fe3+0.3Co2+

0.7)A [Co2+0.3Cr3+

1.5Fe3+0.2]B O4

2-

1.7 (Fe3+0.3Co2+

0.7)A [Co2+0.3Cr3+

1.7]B O42-

1.8 (Co2+1.0)A [Cr3+

1.8Fe3+0.2]B O4

2-

1.9 (Co2+1.0)A [Cr3+

1.9Fe3+0.1]B O4

2-

2.0 (Co2+1.0)A [Cr3+

2.0]B O42-

 

F

ch

in

C

in

st

ap

co

in

ar

C

(F

io

ex

p

si

rB

ta

[1

ar

th

tr

Figure 4.6.5:

On th

hange despi

nversion pro

Cr3+ ions occ

ndicate furth

tarts decreas

ppears the

oncentration

ncreases (Fig

re smaller

CoCrxFe2-xO4

Figure 4.6.1

onic radius

xperimentall

arameter (u)

ites, rA and

B = (⅝ - u)a

aken as = 0.3

1,2]. The site

re also show

he vacancies

rapped in red

: Schematic

normal sp

The smal

tetrahedr

he other han

ite the subs

ocess in whic

cupy the B-

her inversion

sing (Table

sharpest a

n, indicating

gure 4.6.2 (a

in size can

4 samples st

). The contr

of Cr3+ (0

ly found va

) [1], it is po

rB respectiv

– R0, Here,

379 conside

e radii for th

wn in Figure

s at the octa

ducing num

c represent

pinel struct

ll solid and

al and octah

nd, the conc

titution by

ch the Co2+

-sites. The

n and hence

4.6.1). Whe

and the po

a reduction

a)). The fact

n be unders

teeply reduc

raction of th

.630 Å) co

alues of the

ossible to ca

vely, using t

R0 is the rad

ring that Co

he different c

e 4.6.1. Incre

ahedral sites

mber in the C

Chapter 4

4.53

tation of th

ture. The la

d open cir

hedral sites

centration o

Cr3+ ions ti

ions migrate

Cr3+ substit

the effectiv

ereas at x =

ositron lifet

n in size of t

that positron

stood as fo

es in sampl

he lattice can

ompared to

lattice con

alculate the r

the relations

dius of the o

oCrxFe2-xO4is

composition

easing conce

and, owing

Cr3+-vacancy

he two nei

arge circles

rcles repres

respectivel

of Fe3+ ions

ill x = 0.7

e to the A-si

tution (x) ab

ve number o

0.7, the pe

times start

the vacancy

ns are now a

ollows. The

es with incr

n be attribut

that of Fe3

nstant (a) an

radii of the t

s [1], rA =

oxygen ion (

s fully inver

ns estimated

entration of

to the posit

y complexes

ighboring o

represent

sent the ca

ly.

at the B-s

due to the

ites [10]. Th

bove this va

of Fe3+ ions

eak in the C

decreasing

clusters. Th

annihilating

lattice con

reased Cr3+

ted to the sli3+ (0.640 Å

nd the oxyg

tetrahedral a

(3)1/2(u - ¼

(taken as 1.3

rse at x = 0.0

from the abo

Cr3+ will re

tive charge,

so formed.

octants of

oxygen ion

ations at th

sites does no

simultaneou

his means, th

alue does no

at the B-site

CDB spectrum

g above th

he intensity

at sites whic

nstant of th

concentratio

ightly smalle

Å). Using th

gen position

and octahedr

¼)a – R0 an

32 Å) and u

0 compositio

ove equation

esult in fillin

positrons ar

On the othe

a

ns.

he

ot

us

he

ot

es

m

his

I2

ch

he

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er

he

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nd

is

on

ns

ng

re

er

Chapter 4

4.54

 

hand, while the octahedral sites were large enough to accommodate the Co2+ ions, the

radii of the tetrahedral sites are magnitude-wise smaller than its ionic radius (0.740 Å)

and hence it is likely that a fraction of the tetrahedral sites are unoccupied during the

inversion of the spinel structure at concentrations x< 0.7. As a result, positrons now

that they are no more trapped at the octahedral vacancies will move over to the

vacancies at the tetrahedral sites and get trapped there. The fast decreasing positron

lifetimes support this argument since the tetrahedral vacancy clusters are smaller than

the octahedral ones. The availability of additional trapping sites expectedly increases

the intensity I2 as well. Considering that positrons are now getting trapped in the

vacancy clusters present at the tetrahedral sites, the lattice contraction would directly

influence their lifetime and the observations in the range mentioned above are in

accordance with the same. A careful study of the ionic distribution shown in

Table 4.6.1 makes us realize that from x = 0.7 onwards till x = 1.7, the occupancies of

Fe3+ and Co2+ at the A-sites remain unchanged whereas the Cr3+ ions directly replace

the Fe3+ ions at the B-sites. At x = 1.7, all the Fe3+ ions at the B-sites are replaced by

Cr3+ ions (Table 4.6.1).

The last stage in the variation of the positron annihilation parameters versus

the Cr3+ concentration (x) is observed between x = 1.8 and 2.0. During x = 0.7 to 1.7,

the cation distribution at the A-sites remained unaltered as Fe3+0.3Co2+

0.7 while Cr3+

ions monotonically replaced the Fe3+ ions at the B-sites with the Co2+ concentration at

the B-sites remaining unaltered as 0.3. As the lattice contraction continues, the

transformation of the spinel structure from the inverse to the normal configuration that

had started during x = 0.1 - 0.7 and discontinued during x = 0.7 - 1.7 gets completed.

From octahedral site stabilization energy considerations, it is known that cobalt-

chromite (CoCr2O4) is a normal spinel [10]. In earlier positron annihilation studies of

nanocrystalline ZnFe2O4 [19] and NiFe2O4 [20,28], the positron lifetimes had been

observed to decrease when a normal spinel ferrite transforms to an inverse spinel and

conversely they increase when the transformation is just the opposite. These

observations had been verified through Mossbauer spectroscopic studies too [20,28].

Since, the two positron lifetimes, τ1 and τ2, drastically increase during x = 1.8 and 2.0

(Figure 4.6.2 (a)), this stage is attributed to the total transformation of the partly

inverse CoCrxFe2-xO4 to the fully normal CoCr2O4. Note further that, unlike during

Chapter 4

4.55

 

x = 0.1 – 0.7 when the intensity I2 did not show any change, it decreases in the final

stage of inversion indicating the full occupancy of the A-sites by Co2+ ions. The fact

remains that spinel structures normally suffer from non-stoichiometric disorders and

therefore vacancy clusters are inherently in-built in the structure. The large value of τ2

with still an appreciable intensity I2 supports this argument.

As has been already pointed out, the longest lifetime τ3 and its intensity I3 are

due to the nanocrystalline dimensions of the samples and they result from positronium

atoms annihilating at the intergranular region. Hence they need not necessarily reflect

the effects of any change in the vacancy cluster dynamics within the grains. Yet, τ3

shows a sudden decrease during the initial stage x = 0.1 to 0.7 and then remain

constant (Figure 4.6.2(b)). The intensity I3 gradually falls during this stage but shows

a characteristic rise during the second stage x = 0.7 to 1.7 and then remain constant

(Figure 4.6.2(b)). The initial fall can be attributed to small traces of Cr3+ ions,

unsuccessful in being incorporated into the spinel structure and hence left to remain in

trace amounts within the intergranular region. EDAX analysis also indicated

frustration even within the lattice due to the failure in complete substitution of Fe3+ by

the Cr3+ ions in the system. In the latter stage (i.e., x > 0.7), the lattice contraction has

expectedly resulted in a decrease by 0.3% in the grain size and thereby the number of

positrons reaching out on the grain surfaces has slightly increased.

Although it is known that the magnetic properties of the samples undergo

rapid and interesting changes during the Cr3+ - substitution, correlating such changes

to the behavior of positron annihilation parameters is never straightforward and is not

attempted here. The structural properties and their changes, as depicted by the

positron annihilation parameters and their variations, may influence the magnetic

properties, which need to be investigated by appropriate experimental methods.

Chapter 4

4.56

 

Conclusions

In understanding the effects of Cr3+-substitution in place of Fe3+ in CoFe2O4 studied

by positron annihilation spectroscopy, we have offered the physical interpretation of

results in terms of three distinct stages of defect evolution and interaction. First, the

un-substituted ferrite (CoFe2O4, x = 0.0) itself was found to contain large vacancy

clusters. These clusters are identified as being present at the A-sites with the divalent

Co2+ ion and four of its coordinated oxygen ions making way for such very strong

trapping centers for positrons. At the onset of Cr3+- substitution (x), the positron

lifetimes increased due to the transfer of positron trapping into defects to the B-sites.

In the second stage from x = 0.7 to x = 1.7, a concomitant lattice contraction

influenced the positron annihilation characteristics. This contraction is attributed to

the slightly smaller ionic radius of Cr3+ than that of Fe3+. There is also a change in the

positron trapping sites from the vacancy clusters at the B-sites back to those at the A-

sites. The last stage is marked by a full inversion of the structure to that of a normal

spinel chromite and the positron annihilation parameters depicted this stage with a

characteristic reversal of the trend of variation with the Cr3+-concentration.

Finally, we conclude that positron lifetime measurements complemented by

results from coincidence Doppler broadening spectroscopy can be a viable alternative

experimental tool to monitor the generation and evolution of structural disorders in

AB2O4 (where A and B are divalent and trivalent metals respectively) systems during

physical treatments like doping and grain size reduction. Positron annihilation

parameters are seen to sense, directly or indirectly, physical phenomena of different

kinds and implications like the redistribution of cations, lattice contraction or

expansion and structural transformations in certain cases [19-21].

Chapter 4

4.57

 

References

1. J. Smit, H. P. J. Wijn, ‘Ferrites—Physical Properties of Ferromagnetic

Oxides in Relation to Their Technical Applications’, N.V. Philips

Gloeilampenfabrieken, Eindhoven, The Netherlands, (1959).

2. F. Scordari, ‘Fundamentals of Crystallography’, edited by C. Giacovazzo,

Oxford University Press, New York, USA, (1992).

3. W. B. Cross, L. Affleck, M. V. Kuznetsov, I. P. Parkin, ‘Self propagating

high-temperature synthesis of ferrites MFe2O4 (M =Mg, Ba, Co, Ni, Cu, Zn);

reactions in an external magnetic field’, J. Mater. Chem., 9 (10) (1999)

2545 – 2552.

4. K. Tomiyasu, J. Fukunaga, H. Suzuki, ‘Magnetic short range order and

reentrant-spin-glass-like behavior in CoCr2O4and MnCr2O4by means of

neutron scattering and magnetization measurements’, Phys. Rev. B, 70 (21)

(2004) 214434 – 214445.

5. V. T. Thanki, N. N. Jani, U. V. Chhaya, H. H. Joshi, R. G. Kulkarni,

‘Magnetic properties of CoFe2− Cr O4 synthesized by co-precipitation

method’, Asian Jour. Phys., 6 (1-2) (1997) 222 – 226.

6. G. A. Sawatzky, F. van der Woude, A. H.Morrish, ‘Cation distributions in

octahedral and tetrahedral sites of the ferromagnetic spinel CoFe2O4’

J. Appl. Phys., 39, (2) (1968) 1204–1205.

7. G. Lawes, B. Melot, K. Page et al., ‘Dielectric anomalies and spiral magnetic

order in CoCr2O4’, Phys. Rev. B, 74 (2) (2006) 024413 – 024419.

8. N. Menyuk, K. Dwight, A. Wold, ‘Ferrimagnetic spiral configurations in

cobalt chromite’, Journal de Physique, 25 (1964) 528 –5 36.

9. A. Hauet, J. Teillet, B. Hannoyer, M. Lenglet, ‘Mossbauer study of Co and Ni

ferrichromites’, Phys. Stat. Solidi (A), 103 (1) (1987) 257 – 261.

10. H. Mohan, I. A. Shaikh, R. G. Kulkarni, ‘Magnetic properties of the mixed

spinel CoFe2− CrxO4’, Phys. B, 217 (3-4) (1996) 292 – 298.

11. B. G. Toksha, S. E. Shrisath, M. L. Mane, S. M. Patange, S. S. Jadhav,

K. M. Jadhav, ‘Autocombustion high-temperature synthesis, structural, and

magnetic properties of CoCr Fe2− O4 (0 ≤ ≤ 1.0)’, The Jour. Phys. Chem. C,

115 (43) (2011) 20905 – 20912.

Chapter 4

4.58

 

12. R. W. Siegel, ‘Positron annihilation spectroscopy’, Annual Review of

Materials Science, 10 (1980) 393 – 425.

13. S. Biswas, S. Kar, S. Chaudhuri, P. M. G. Nambissan, ‘Mn2+-induced

substitutional structural changes in ZnS nanoparticles as observed from

positron annihilation studies’, J. Phys.: Cond. Matter, 20 (23) (2008) 235226

– 1 – 10.

14. P. Asoka-Kumar, M. Alatalo, V. J. Ghosh, A. C. Kruseman, B. Nielsen,

K. G. Lynn, ‘Increased elemental specificity of positron annihilation spectra’,

Phys. Rev. Lett., 77 (10) (1996) 2097 – 2100.

15. J. V. Olsen, P. Kirkegaard, N. J. Pedersen, M. Eldrup, ‘PALSfit: a new

program for the evaluation of positron lifetime spectra’ Phys. Status Solidi

(C), 4, (10) (2007) 4004 – 4006.

16. T. Koida, S. F. Chichibu, A. Uedono et al., ‘Correlation between the

photoluminescence lifetime and defect density in bulk and epitaxial ZnO’,

Appl. Phys. Lett., 82 (4) (2003) 532 – 534.

17. A. Zubiaga, F. Tuomisto, F. Plazaola et al., ‘Zinc vacancies in the

heteroepitaxy of ZnO on sapphire: influence of the substrate orientation and

layer thickness’, Appl. Phys. Lett., 86 (4) (2005) 042103 – 042103 – 3.

18. P. Hautojarvi, C. Corbel, ‘For a detailed discussion ondifferent cases of

positron trapping in solids’, in Positron Spectroscopy of Solids, 491–532, IOS

Press, Amsterdam, The Netherlands, 1995.

19. P. M. G. Nambissan, C. Upadhyay, H. C. Verma, ‘Positron life time

spectroscopic studies of nanocrystalline ZnFe2O4’, J. Appl. Phys., 93 (10)

(2003) 6320 – 6326.

20. S. Chakraverty, S. Mitra, K. Mandal, P. M. G. Nambissan, S. Chattopadhyay,

‘Positron annihilation studies of some anomalous features of NiFe2O4

nanocrystals grown in SiO2’, Phys. Rev. B, 71 (2005) 024115 – 024121.

21. S. Chakrabarti, S. Chaudhuri, P. M. G. Nambissan, ‘Positron annihilation

lifetime changes across the structural phase transition in nanocrystalline

Fe2O3’, Phys. Rev. B, 71 (6) (2005) 064105 – 064110.

Chapter 4

4.59

 

22. Y. Nagai, T. Nonaka, M. Hasegawa et al.’ Direct evidence of positron trapping

at polar groups in a polymer-blend system’, Phys. Rev. B, 60 (17) (1999)

11863 – 11866.

23. T. Ghoshal, S. Biswas, S. Kar, S. Chaudhuri, P. M. G. Nambissan, ‘Positron

annihilation spectroscopic studies of solvothermally synthesized ZnO

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074702, Virtual Journal of Nanoscale Science and Technology, 17 (8) 2008.

24. T. Ghoshal, S. Kar, S. Biswas, S. K. De, P.M. G. Nambissan, ‘Vacancy-type

defects and their evolution under Mn substitution in single crystalline ZnO

nano cones studied by positron annihilation’, J. Phys. Chem. C, 113 (9) (2009)

3419–3425.

25. B. Roy, B. Karmakar, P. M. G. Nambissan, M. Pal, ‘Mn substitution effects

and associated defects in ZnO nano particles studied by positron annihilation’,

Nano, 6 (2) (2011) 173 – 183.

26. I. K. MacKenzie, ‘Positron Solid State Physics’, edited by W. Brandt and

A. Dupasquier, North Holland, Amsterdam, The Netherlands, (1983).

27. M. J. Puska, R. M. Nieminen, ‘Defect spectroscopy with positrons: a general

calculational method’, J. Phys. F, 13 (2) (1983) 333 – 346.

28. S. Mitra, K. Mandal, S. Sinha, P. M. G. Nambissan, S. Kumar,

‘Size and temperature dependent cationic redistribution in NiFe2O4 (SiO2)

nanocomposites: positron annihilation and Mossbauer studies’, J. Phys. D, 39

(19) (2006) 4228 – 4235.

 

Chapter 4

4.60

 

4.7 Photodegradation of 2, 4 – Dichlorophenoxyacetic Acid by Cr3+

substituted CoFe2O4 nanoparticulates During the last few decades, the use of pesticides and herbicides in agriculture

becomes worldwide. These herbicides and pesticides are now abundantly found in

water and food. Among all such agrochemicals, the herbicide,

2, 4–dichlorophenoxyacetic acid (2, 4-D) has been widely applied to control broad –

leaved weeds. 2, 4–dichlorophenoxyacetic acid, 2, 4−D, is a type of phenoxy acid

herbicide. The salts and esters of 2, 4–D are efficient, highly selective herbicides and

plant growth regulators [1]. This herbicide was registered for the first time in 1947

and till today it is one of the most used herbicide in the world [2]. It is the second

pesticide more used in Brazil and has the advantage of being non – volatile [3]. In

1982, the world health organization (WHO) considered 2, 4-D as moderately toxic

(class II) and recommended a maximum concentration of 0.1 part per million (ppm) in

drinking water. However, it may enter water bodies after usage in farmland [4], or for

improper disposal [5], resulting in its broad residues in environment [6,7]. The

herbicide 2, 4−D is also known to persist in the solid and contaminate surface and

ground water. For the health of not only human beings but also for animals, exposure

to this chemical has been proven to be harmful [8–10]. The intentional removal of this

chemical from water is necessary because the degradation of 2, 4–D is very slow in

water, with a half life of about 6 days to over 170 days in different situations [11–13].

Hence, the search and development of an effective process of degradation for such

herbicides has become a necessary task. Among the various removal methods

employed e.g. adsorption [14,15], biodegradation [16], photocatalytic degradation

[17] etc, photocatalytic degradation found most suitable [18]. This is due to the fact

that this technique is capable to distract entire chemical structure of 2, 4-D, further it

is cheap, simple in operating conditions and techniques. The molecular structure and

chemical properties of 2, 4-D have been summarized in Table 4.7.1.

Over the past few years’ studies on oxidic solid solutions as catalysts have

been steadily developed and today for commercial applications many industrial

establishments have shown interest in such compounds. Out of various models

explaining heterogeneous catalysis one which propose a relationship between

 

el

ph

ca

du

o

S

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so

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The p

atalysts, CoC

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Table

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olution with

measured us

pectrophotom

lotting the c

was shown in

Figure 4.7

Figure

resence of n

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Dich

ructure of ca

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purpose of

CrxFe2-xO4 (x

he decompo

and microstr

e 4.7.1: The

on of 2, 4-D

, 4-D in 100

h different

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oncentration

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7.1: Calibr

e 4.7.2 sho

nanoparticles

2.0 composit

2,4-

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atalysts and

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x = 0.0 – 2.0

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n of 2, 4-D s

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Chapter 4

4.61

d catalytic ac

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Chemical

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the effect o

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100 ppm wa

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4–D degrad

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White

p

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stalline ferri

different tim

ed in the ligh

s of 2, 4-D.

by dissolvin

the 2, 4-

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dation in th

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e to Yellow

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lk

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ht

ng

D

as

00

by

es

he

7,

Chapter 4

4.62

 

Chapter 4

4.63

 

Chapter 4

4.64

 

Figure 4.7.2: UV-visible spectra of 2, 4-D degradation in the presence of

nanocrystalline CoCrxFe2-xO4 photocatalysts.

Chapter 4

4.65

 

Figure 4.7.3 shows the percentage degradation of 2, 4–D herbicide on the

nanoparticles of CoCrxFe2-xO4 spinel ferrite system with different Cr3+ - content (x)

under ultraviolet light irradiation at different time interval. Blank experiments indicate

that the direct photolysis of 2, 4-D is negligible when illuminated with ultraviolet light

in the absence of ferrite catalysts. The adsorption-desorption equilibrium of 2, 4-D

herbicide and CoCrxFe2-xO4 were used as starting solutions (t = 0 min.). The variation

of percentage degradation as a function of Cr3+ - concentration (x) in the system for

different irradiation time is not systematic. In general percentage degradation of 2, 4-

D herbicide enhances with ultraviolet irradiation time. If we concentrate on

percentage degradation of 2, 4-D as a function of Cr3+ - content (x) for irradiation time

of 60 minutes duration, it can be seen that the variation can be divided into three

regions (Figure 4.7.4). During the initial stage, x = 0.0 – 0.7, percentage degradation

shows a sudden decrease from 36.8 % to 19.9 %, but shows characteristic rise during

the second stage, x = 0.7 – 1.7 (19.9% to 37.2%), and then gradually falls during the

last stage, x = 1.7 – 2.0 (37.2% to 21.2%).

Figure 4.7.3: Cr3+ - concentration (x) dependence of 2, 4-D photocatalytic

degradation with ultraviolet light irradiation time.

Chapter 4

4.66

 

The observed change in percentage degradation as a function of

Cr3+- concentration (x) may be explain by considering various structural and

microstructural parameters that affect photocatalytic activity of ferrite nanoparticles.

The governing factors are: (i) particle size, shape of particles, effective surface area

and surface to volume ratio (ii) optical energy band gap (iii) intensity of incident

radiation (iv) electronic configuration of transition metal ions (v) enthalpy of

formation (vi) crystal structure, degree of crystallinity, porosity etc. (vii) pH of the

solution [20] (viii) solvents micellar environments and interfering substances

(ix) other parameters such as glass made cuvetti as a reaction vessel, absence of

oxygen in the reaction medium, use of a UV light of same intensity but as a single

source, solar light as a substitution of UV light, the irradiation done at a stretch and

step by step etc [20].

The understanding of the role played by each of these parameters is rather

difficult and required more detailed work on each factor. Further, when the range of

the compositions under study is very wide, as in the present case, it is quite expected

that at different stages different factors play important or governing role. We have

attempted to explain the variation of percentage degradation of 2, 4–D as a function of

Cr3+ - concentration (x) in the system, CoCrxFe2-xO4, in best possible manner.

According to literature, the direct optical band gap for epitaxial thin film of

CoFe2O4 (x = 0.0) is 2.7 eV [21] on the other hand for CoCr2O4 (x = 2.0) thin film

band gap is reported to 3.1 eV [22]. Because of the higher band gap, the number of

electrons reaching the conduction band is relatively low, consequently, number of

holes in the valence band decrease with increase in Cr3+ - concentration (x) in the

system. Both these electrons and holes interact with surface bound H2O or OH⎯ to

produce ●OH radicals. According to Viswanath et al [23] these radicals are main

active species in the photocatalytic degradation process. Earlier, it has been reported

that some of the ternary semiconductors, InAsxP1-x and nanocomposites of Ni – Zn

ferrites with copolymer matrix of aniline and formaldehyde [24], disclosed that band

gap value varies inversely with change in the lattice constants value, as function of

concentration (x) in the system. Thus, the increase in optical band gap value from 2.7

eV for CoFe2O4to 3.1 eV for CoCr2O4 is consistent with the decrease in lattice

constant value with increasing Cr3+-concentration (x) in the system (Table 4.5.1).

Chapter 4

4.67

 

Based on the above facts, now it is possible to explain variation of percentage

degradation of 2, 4–D herbicide as a function of Cr3+ - concentration (x) for the

nanocrystalline spinel ferrite system, CoCrxFe2-xO4.

Figure 4.7.4: Percentage degradation of 2, 4-D as a function of Cr3+ - content (x)

in the system, CoCrxFe2-xO4 at ultraviolet light illumination time of

60 minutes.

The numbers of electrons in d – orbit of transition metal ions play an

important role in determining the catalytic behaviour. It is known that the cations with

a completely filled outermost orbit are more stable as compared to cations with a half

filled outer most orbit (e.g. Zn2+, Al3+, Fe3+, Mn2+). On the other hand, transition

metal ions with partially filled d – orbit (e.g. Cr3+, Ni2+ and Co2+) are more reactive.

In the spinel ferrite system under study, CoCrxFe2-xO4, highly magnetic Fe3+ (5 μB)

ions having a half filled (3d5) outer most orbit is replaced by less magnetic Cr3+ (3 μB)

ions with a partially filled outermost orbit (3d3), which actively contribute to the

reaction process.

As discussed, reduction in grain size results in increase in the effective surface

area and surface to volume ratio. These lead to higher catalytic activity as compared

with larger crystals of the same mass present on the active surface. In general, the

surface area is related to the mass and heat transfer between the particle and their

Chapter 4

4.68

 

surrounding (i.e. enthalpy of formation). Particles that are too small will not catalyze a

reaction (not enough electrons). In addition to size of particle, the shape of nano

particles can also provide a sensitive knob for tuning its catalytic activity and

selectivity.

On the other hand, higher porosity provides a larger amount of surface sites

for organic pollutant and the interface for reaction of the contaminant molecules and

oxygen species occur more easily and rapidly. In the case of nanocrystalline

photocatalysts high porosity is more favorable, because the organic contaminant can

penetrate or get absorbed on the catalyst structure more easily thereby providing a

larger reaction surface for catalytic activity. This leads to conclude that the

appropriate choice of substitution results in particle size reduction that in turn increase

porosity, which provides higher adsorption centers that lead to the enhancement of the

photocatalytic activity.

Spinel ferrite materials are shown to be effective photocatalysts by utilizing

light energy to create electron (e⎯)/hole (h+) pairs on the photocatalytic surface [25].

These e⎯/h+ pairs can be utilized for oxidation and reduction processes, which can

further involve in the formation of reactive oxygen species, such as O2⎯ and ●OH

respectively. The radicals thus formed were further aid in the decomposition of

contaminants. Further, to enhance the formation of reactive oxygen species, H2O2

oxidant is added to the reaction mixture, as in the present synthesis procedure.

According to the following reactions, iron cations, ferrous (Fe2+) and ferric (Fe3+)

ions, react with H2O2 results in formation of highly reactive ●OH radicals.

Fe2+ + H2O2 →Fe3+ + OH− + OH+

Fe3+ + H2O2 →Fe2+ + ●OOH + H+

H2O2 + OH→ H2O + ●OOH

H2O2 + e- → OH− + ●OH

H2O + h+ →H+ + ●OH

Based on the above facts, now it is possible to explain variation of percentage

degradation of 2, 4-D herbicide as a function of Cr3+ - concentration (x) for the

nanocrystalline spinel ferrite system, CoCrxFe2-xO4.

The initial decrease in percentage degradation of 2, 4 – D herbicide with Cr3+ -

concentration (x) from x = 0.0 – 0.7 is mainly due to the increase in optical energy

Chapter 4

4.69

 

band gap. The particle size is found to decrease and as a result effective surface area

is expected to increase (Table 4.5.1), leading to higher catalytic activity with Cr3+ -

substitution but this is not the case. Furthermore, porosity is found to increase from

11.9 % to 14.3 % for x = 0.0 – 0.7 compositions, as well as less effective Fe3+ ions are

replaced by effective Cr3+ ions in the system, thus, one can expect enhancement in

photocatalytic activity but this is also not the case. The observed sudden increase in

degradation of 2, 4-D, with Cr3+ - substitution for x = 0.7 to x = 1.7, may be correlated

with decrease in particle size, increase in porosity and replacement of Fe3+ ions by

Cr3+ ions in the system. On the other hand, observed decrease in photocatalytic

activity for x = 1.7 to 2.0 compositions is due to the almost no change in particle size

(19 ± 2 nm), and porosity (20 ± 1 %), that is dominated by increase in optical band

gap value. Here it is important to note that reduction in particle size in the substitution

range x = 0.7 to 1.7 is much larger than the x = 0.0 – 0.7 and x = 1.7 – 2.0 ranges

(Table 4.5.1). Small particle provide a tremendous driving force for diffusion

especially at elevated temperature [26]. Similarly relative change in porosity for x =

0.7 – 1.7 compositions is much higher than the x = 0.0 – 0.7 and x = 1.7 – 2.0

substitution ranges. The observed increase in porosity provides large surface area. The

larger the effective surface area available, the larger will be the number of surface

sites available for adsorption process of oxygen, giving higher catalytic response [27].

Very recently we have carried out crystal defects and cation redistribution

study on nanocrystalline cobalt – ferri – chromites, CoCrxFe2-xO4, (x = 0.0 – 2.0) by

positron annihilation spectroscopy [28]. Interestingly, the variation of the

orthopositronium life times and intensities is consistent with the compositional

dependence of percentage degradation of 2, 4-D (Figure 4.7.4) with three distinct

stages. This suggests that photocatalytic degradation is also influenced by crystal

defects like vacancy clusters and redistribution of cations as a function of Cr3+ -

substitution (x). Following the cation distribution formulae which indicated partial

inversion of the inverse spinel ferrite (x = 0.0 – 0.7), subsequent stabilization over a

range of substitution (x = 0.7 – 1.7) and finally the full inversion to the normal spinel

chromite (CoCr2O4, x = 2.0). According to this study, in the intermediate range of

substitution (x = 0.7 – 1.7), lattice contraction prevented a fraction of Co2+ ions

released from the B – sites from entering the tetrahedral sites and these vacancies at

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the A – sites trapped positrons. The observed increase in percentage photocatalytic

degradation for this range of the compositions may be responsible to those defects

evolutions or large vacancy clusters formation. Further, investigation is in progress.

Earlier, a correlation study has been carried out between bulk physical

properties and catalytic performance over decomposition of alcohols by ferrites, ferro

chromites and chromites [19]. The results on three spinel ferrite systems,

CuCrxFe2-xO4, MgCrxFe2-xO4 and CoCrxFe2-xO4 (x = 0.0, 1.0 and 2.0) have shown that

percentage degradation of alcohol decreases with decrease in lattice constant value

and increase in electronic activation energy.

For the system under investigation, lattice constant value is found to decrease

with increase in Cr3+ - concentration (x). Thus, decrease in percentage degradation for

x = 0.0 – 0.7 and x = 1.7 – 2.0 is consistent with previous results, but observed

increase in percentage degradation for the intermediate range, x = 0.7 – 1.7, is rather

unexpected. On the other hand, with replacement of highly magnetic Fe3+ - ions with

magnetic moment of 5 μB by less magnetic Cr3+ ions (3 μB), one can expect increase

in resistivity [29] and activation energy [30] values. As the catalysis involves transfer

of electrons/holes from the surface of the catalyst to substrate molecule and the

process is reversible, that is, greater the activation energy, the greater will be the

energy required for electronic transition resulting in decreased activity of

photocatalysts. Accordingly, percentage degradation of 2, 4–D herbicide is expected

to decrease as observed for initial (x = 0.0 – 0.7) and final (x = 1.7 – 2.0) stages.

The maximum photocatalytic activity observed for pure CoFe2O4 (x = 0.0)

(∼36.8%) among the whole range of the compositions, besides having many

unfavorable parameters (large value of lattice constant, low value of porosity, large

grain size etc) as compared to other nano ferrite compositions, may be due to the

presence of few nanoparticles with needle like shape as observed from TEM analysis

(Figure 4.4.1 (a)). According to Qu et al [31] needle shaped particles have greater

surface area than lamellar or rod shaped particles and thus enhanced photo catalytic

activity. Earlier, Darshane et al [32] have reported that percentage decomposition of

1 - Octanol by the spinel system Ga1-xFexCuMnO4 increased with increase in lattice

constant, Curie temperature and concentration of Fe(III) irons at the A – site. In the

present study based on the cation distribution formulae it is found that the

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concentration of ferric (Fe3+) ions at the tetrahedral (A-) site decreases with increase

in Cr3+ - concentration (x), for x = 0.0 – 0.7 compositions. For x = 0.7 – 1.7

compositions the concentration of Fe3+ remain constant i.e. 0.3, while for higher

concentration of x, 1.7 < x ≤ 2.0, Fe3+ ions at the A-site are completely absent. Thus,

the variation of percentage degradation of 2, 4–D as a function of Cr3+ - substitution

may be correlated with concentration of Fe3+ ions at the A – site as suggested in [32].

Chapter 4

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Conclusions

(i) Ferrite nanoparticles of CoCrxFe2-xO4 system with variable compositions

synthesized by the co-precipitation technique can be used as an effective

photocatalytic material for the degradation of 2, 4−D herbicide under

irradiation of UV light.

(ii) The variation of percentage degradation of 2, 4−D with Cr3+-substitution is

divided into three distinct stages, each one dominated by one or more

structural and microstructural parameters.

Chapter 4

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