CHAPTER 4 INFLUENCE OF Eu SUBSTITUTION ON STRUCTURE ...

77

CHAPTER 4

INFLUENCE OF Eu SUBSTITUTION ON STRUCTURE,

MAGNETIC, OPTICAL AND DIELECTRIC PROPERTIES OF

BiFeO3 MULTIFERROIC CERAMICS

4.1 INTRODUCTION

Magnetoelectric multiferroics, combining ferroelectric and magnetic properties, show

considerable promise for applications in several devices such as electric-field-controlled

ferromagnetic resonance devices, transducers with magnetically modulated piezoelectricity, and

multiple-state memory elements [157-158]. The magnetoelectric coupling between magnetic and

electrical orders in these multiferroics has enabled us to control magnetic polarization by

applying electric field and vice versa [159]. Amongst naturally occurring multiferroics,

BiFeO3 has been considered as a unique material for the development of multifunctional devices

because of its characteristic ferroelectric, magnetic, and optical properties. However, synthesis of

pure phase BFO is complicated due to the high probability of impurity phase formation and

strong dependence of the physical properties on the oxygen stoichiometry and crystal perfection

[160]. Therefore, special attention has been paid to the improvement of the ferroelectric

properties and the destruction of the cycloidal antiferromagnetic order in BFO.

In order to improve multiferroic properties of BFO several attempts have been made. In

particular, rare earth (RE) ions (Sm, La, Gd) substitution at Bi3+ site has been proven effective in

modifying the structure of BFO and thus its multiferroic properties [161-163]. Recently,

enhanced magnetic properties have been observed in bulks, nanoparticles and thin films of

BiFeO3 by doping magnetically active Eu3+ ions at the Bi site [90, 164-165]. The choice of Eu as

doping ion has several advantages: (i) since Eu3+ has a smaller ionic radius compared to other

reported rare-earth ions, it is expected to induce larger structural distortions in BFO; (ii) as

Eu3+ is magnetically active ion, therefore expected to couple with Fe3+ ions to produce

ferromagnetic ordering which has a giant impact on magnetization of BFO even at very low

doping concentration. Therefore, in this chapter, we have discussed detailed structural, magnetic,

Chapter-4

78

optical and dielectric properties of Eu doped BFO (i.e. Bi1-xEuxFeO3 with x = 0.0, 0.03, 0.05,

0.07, 0.10, 0.12 and 0.15) ceramics prepared by solid state reaction method.

4.2 EXPERIMENTAL DETAILS

The high purity Bi2O3, Fe2O3 and Eu2O3 oxides (99.99% pure, Sigma Aldrich) were used

to prepare Bi1-xEuxFeO3 ceramics with x = 0.0, 0.03, 0.05, 0.07, 0.10, 0.12, and 0.15. These

powders were weighed in stoichiometric proportions and grounded using a mortar pestle in

acetone medium for 4 h. The homogeneous mixture of powders was calcined at 700°C for 2 h.

The fine calcined powders were used to make circular pellets of 10 mm diameter and 1-2 mm

thickness. All the pellets were sintered at 820°C for 2 h for densification. Phase analysis of the

samples was done by X-ray diffraction (XRD) patterns recorded by XRD-6000 Shimadzu

diffractometer with CuKα radiation ( λ = 1.5406 Å). The XRD data were analyzed by a Rietveld

refinement technique using FULLPROF package. Raman spectra were recorded on Renishaw

Raman spectrometer by using 514.5 nm Ar ion laser. Scanning electron microscope (SEM) was

used to study surface morphology. The magnetic properties of the samples were measured with a

vibrating sample magnetometer (VSM, LakeShore 7410). The electron spin resonance (ESR)

spectra were measured at 300 K with X-band (9.8 GHz) Brucker EMX system. UV-Visible

diffuse reflectance spectra (UV-vis DRS mode) of the samples were measured by Ocean optics

UV-Visible 4000. The Fourier transformed infrared (FTIR) spectra were recorded by Perkin

Elmer Spectrum BX-II. The dielectric behavior of the samples was studied using PSM 1735.

4.3 RESULTS AND DISCUSSION

4.3.1 STRUCTURAL STUDIES

4.3.1.1 X-RAY DIFFRACTION STUDIES

The phase formation and structural modifications due to Eu substitution in BFO ceramics

have been investigated by X-ray diffraction technique. The XRD pattern for Bi1-xEuxFeO3

ceramics with x = 0.0, 0.03, 0.05, 0.07, 0.10, 0.12 and 0.15 are shown in Figure 4.1. The XRD

pattern of the pure BFO sample reveals the formation of Rhombohedral R3c structure with minor

impurity phases such as Bi2Fe4O9 and Bi24Fe2O39. However, the introduction of Eu in BFO

suppressed the formation of these impurity phases for x > 0.03 (Figure 4.1).

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79

Figure 4.1: XRD patterns of Bi1-xEuxFeO3 ceramics with x = 0.0-0.15.

Moreover, on increasing Eu3+ concentration, small amount of orthoferrite EuFeO3 phase

appears for x = 0.15 sample. The diffraction patterns for x≤0.10 samples indicate a

rhombohedrally distorted perovskite structure. However, a compositional phase transition from

rhombohedral (R3c) to orthorhombic (Pn21a) phase has been noticed for x>0.10 samples. The

magnified view of XRD pattern is displayed in Figure 4.2 for clear structural transformation

beyond x = 0.10 sample. The structural transformation is evident by the splitting of (012) peak

around 22°, the appearance of (111) peak around 25° and the splitting of double peaks at 32° into

four peaks [24]. Such type of structural phase transition may be ascribed to induce distortion in

the structure due to smaller ionic radius of Eu3+ (1.07 Å) than that of Bi3+ (1.17 Å).

It is well known that BFO has rhombohedral structure with space group R3c, and EuFeO3

has an orthorhombic structure with space group Pnma. The space group Pn21a is a subgroup of

centrosymmetric orthorhombic Pnma space group with a breaking of the symmetric center. To

explore detailed structural parameters, XRD pattern were refined by Rietveld analysis. We

refined x = 0.0-0.10 samples according to the rhombohedral structure, while x = 0.12 and 0.15

Chapter-4

80

samples were refined by considering a combination of the rhombohedral (R3c) and the

orthorhombic (Pn21a) phases as shown in Figure 4.3. Rietveld refinement has been carried out

using wyckoff positions (6a, 6a, 18b) corresponding to (Bi/Eu, Fe, O) for R3c space group and

wyckoff positions (4c, 4b, 4c, 4d) corresponding to (Bi/Eu, Fe, O1, O2) for Pn21a space group,

respectively. The background was fitted with sixth order polynomial, while the peak shapes were

described by pseudo-voigt profiles. During the refinement process, various factors such as scale

factor, zero correction, background, half width parameters, lattice parameters (a, b, c), positional

coordinates (x, y, z) and thermal parameters were varied. Occupancy parameters of the ions were

kept fix during refinement and hence obtained refined structural parameters are listed in Table

4.1. We have also determined the ratio of the two phases from Rietveld analysis as shown in

Table 4.1. It is found that x = 0.12 sample contains 84.4% R3c phase and 15.6% Pn21a phase,

while x = 0.15 sample contains 61.9% R3c phase and 38.1% Pn21a phase. This confirms that the

structure gradually changed from R3c to Pn21a phase with increasing Eu3+ concentration. Several

recent reports have also shown coexistence of orthorhombic phase of Pnma/Pn21a/Pbnm space

group along with R3c space group [132, 166-167].

Figure 4.2: Enlarged view of the diffraction patterns around 220 (a), 250 (b) and 320 (c) angle for x = 0.10-0.15

samples. Here symbol O indicates the orthorhombic phase.

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81

Figure 4.3: Rietveld refined XRD patterns for Bi1-xEuxFeO3 ceramics with x = 0.0-0.15.

The evolution of orthorhombic phase in the samples with increasing Eu3+ concentration

can be explained on the basis of tolerance factor (t) which is associated with the cationic size

mismatch between Bi3+ and Eu3+ ions. Goldschmidt proposed a factor to quantify size mismatch

between A and B cations to the cubic perovskite (ABO3) topology [168]. The unit value of t

corresponds to an ideal perovskite with undistorted structure, while t<1 has a distorted perovskite

system indicating tilt or rotation of the FeO6 octahedra. The values of t were calculated by using

equation (3.1). The calculated values of t were found to be 0.8909, 0.8899, 0.8892, 0.8885,

0.8875, 0.8868, and 0.8858 for x = 0.0-0.15 samples, in order.

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82

Table 4.1: Rietveld refined structural parameters for Bi1-xEuxFeO3 ceramics

Sample Crystal Structure Lattice parameters Atomic positions R-factors (%)

Rhombohedral a = 5.5786 Å Bi/Eu (0, 0, 0) Rp = 6.54, Rwp = 9.16

x = 0.0 (100%) c = 13.8667 Å Fe (0, 0, 0.2151) RBragg = 14.0, Rf = 9.86

V = 373.73 Å3 O (0.4230, 0.0484, 0.9598)


x = 0.03 (100%) c = 13.8406 Å Fe (0, 0, 0.2244) RBragg = 2.70, Rf = 2.69

V = 372.12 Å3 O (0.1015, -0.6907, 0.3888)


x = 0.05 (100%) c = 13.8223 Å Fe (0, 0, 0.2253) RBragg = 2.56, Rf = 2.30

V = 371.07 Å3 O (0.1130, -0.684, 0.3869)


x = 0.07 (100%) c = 13.8198 Å Fe (0, 0, 0.2253) RBragg = 2.36, Rf = 2.43

V = 371.07 Å3 O (0.0945, -0.6911, 0.3888)


x = 0.10 (100%) c = 13.7741 Å Fe (0, 0, 0.2266) RBragg = 2.36, Rf = 2.43

V = 368.61 Å3 O (0.0951, -0.6979, 0.3844)


(84.4%) c = 13.8049 Å Fe (0, 0, 0.2293) RBragg1= 3.29, Rf1 = 2.60

x = 0.12 V = 370.58 Å3 O (0.0953, -0.6975, 0.3705)

Orthorhombic a = 5.5989 Å Bi/Eu (0.0472, 0.2832, 0.9835) RBragg2 = 2.76, Rf2 = 3.69

(15.6%) b = 7.8103 Å Fe (0.0089, 0.0, 0.4774)

c = 5.4407 Å O1 (0.3737, 0.2415, 0.0232)

V = 237.92 Å3 O2 (0.3259, 0.2644, 0.0669)

O3 (0.2185, 0.2446, 0.5361)


(61.9%) c = 13.7822 Å Fe (0, 0, 0.2276) RBragg1 = 3.54, Rf1 = 3.01

x = 0.15 V = 369.17 Å3 O (0.1096, -0.6786, 0.3567)

Orthorhombic a = 5.6158 Å Bi/Eu (0.0413, 0.2677, 0.9859) RBragg2 = 6.84, Rf2 = 6.58

(38.1%) b = 7.8154 Å Fe (-0.0347, 0.0, 0.48136)

c = 5.4294 Å O1 (0.2215, 0.1102, -0.3206)

V= 238.29 Å3 O2 (0.3052, 0.2819, 0.0628)

O3 (0.1782, 0.2033, 0.5422)

The decline in the value of t with increasing doping indicates the substitution of Eu3+ ions

in BFO introduces a change in the octahedral tilt system with reduced unit cell volume because of

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83

the difference in ionic radii of Eu3+ and Bi3+ ions. The average crystallite size was estimated by

using Debye-Scherer formula θβλ cosKD = , where λ is the wavelength of X-ray, β is full

width at half maximum (FWHM), θ is the diffraction angle and D is crystallite size. The

calculated crystallite sizes were found to be 72, 59, 52, 51, 47, 44 and 38 nm for x = 0.0-0.15

samples, respectively.

4.3.1.2 RAMAN SPECTROSCOPY STUDIES

Raman spectra are very sensitive to atomic displacement and the elements present in the

compound. To further study the structural changes demonstrated by XRD patterns, Raman

analysis of these samples has been carried out. According to the group theory, Raman active

modes of BFO with rhombohedral R3c symmetry can be summarized using the irreducible

representation EAcRRaman

94 13, +=Γ . Here, character A represents the symmetric transformation

of a molecule under the rotation about the principle axis of symmetry and E corresponds to a

twofold degenerate vibration. Raman spectra for Bi1-xEuxFeO3 samples measured at room

temperature are shown in Figure 4.4.

Figure 4.4: Room temperature Raman spectra of Bi1-xEuxFeO3 ceramics with x = 0.0-0.15.

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In order to clarify the Raman active modes in the samples, the measured spectra were fitted and

decomposed into individual Lorentzian components. The deconvoluted Raman spectra with

assigned modes are shown in Figure 4.5.

Figure 4.5: Deconvoluted Raman spectra of Bi1-xEuxFeO3 ceramics with x = 0.0-0.15.

In the present study, we observed 12 Raman active modes (4A1 and 8E) for x = 0.0-0.10

samples and 11 Raman active modes (4A1 and 7E) for x = 0.12 and 0.15 samples. The positions

of all Raman active modes are listed in Table 4.2. The observed mode positions are compared

with reported Raman modes for bulk BFO [131]. The changes in the low frequency modes are

associated with the change in the occupation level of Bi atoms of perovskite unit, whereas the

high frequency modes are induced by internal vibration of Fe atoms in the FeO6 octahedra.

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85

Among these modes, the first three A1 modes (A1-1, A1-2, and A1-3) and the E mode at 261 cm-1

are governed by the Bi-O covalent bonds, which in turn, are the origin of ferroelectric distortion.

It can be noticed in Figure 4.5 that on increasing x from 0.0 to 0.15, the A1-1, A1-2, and A1-3

modes continuously shift towards higher wavenumber values due to lower atomic mass of Eu

than that of Bi. This shifting indicates that the Eu atom is getting substituted at Bi-site in the BFO

lattice. We observed that the intensity of A1-1 mode remains stronger than A1-2 mode for all

samples. This implies that that the Bi-O covalent bonds and non-centrosymmetric ferroelectric

distortion remains stable for all samples, although the percentage of Bi-O bond decreases owing

to the Eu substitution in BFO. In addition, disappearance of E mode at 260 cm-1 and appearance

of a broad band around 280 cm-1 suggests structural changes with increasing Eu concentration

[126, 169]. Furthermore, Raman spectrum of x = 0.15 sample shows relatively different features

compared with pure BFO. The three strong and wide bands can be observed at 470, 520 and 620

cm-1 for x = 0.15 sample and these bands are indication of distortion in the FeO6 octahedra [170].

Table 4.2: Reported Raman data on bulk BFO [131] and deconvoluted Raman modes for Bi1- xEuxFeO3 ceramics

Raman

modes

(cm-1

)

Yang et

al. [131]

(cm-1

)

x = 0.0

(cm-1

)

x = 0.03

(cm-1

)

x = 0.05

(cm-1

)

x = 0.07

(cm-1

)

x = 0.10

(cm-1

)

x = 0.12

(cm-1

)

x = 0.15

(cm-1

)

A1-1 139 139.1 139.2 141.8 143.6 145.1 144.8 145.5

A1-2 171 172.1 173.7 174.2 174.6 175.5 175.8 176.5

A1-3 217 220.6 232.4 235.9 235 235 236 235.2

E 260 259.6 260.6 261.6 261.7 259.6 ---- ----

E 274 278.3 279.1 276.9 276.9 277.9 279.4 277.4

E 306 304.3 302.6 301.5 293.9 303.5 306.5 308.1

E 344 344.6 339.7 344.1 329.1 332.4 335.3 337.4

E 368 370.7 369.6 373.4 372.8 375.4 375.1 377.7

A1-4 430 434.9 442.5 439.1 434.8 437.4 435.7 421.4

E 468 469.2 476.2 474.1 475.6 477.3 476.9 475.1

E 520 529.2 528.5 527.8 532.2 529.4 529 528.7

E 611 616.2 620.6 617.9 625.3 622.6 619.4 619.9

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4.3.1.3 MORPHOLOGICAL STUDIES

The surface microstructures of Bi1-xEuxFeO3 ceramics are shown in Figure 4.6.

Figure 4.6: SEM images for Bi1-xEuxFeO3 ceramics with (a) x = 0.0 (b) x = 0.03, (c) x = 0.05, (d) x = 0.07

(e) x = 0.10 (f) x = 0.12 and (g) x = 0.15

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From these images it is observed that Eu substitution reduced the grain size and improved

the density of the samples. This further leads to better grain connectivity and reduced porosity.

The decrease in grain size may be attributed to the difference in the bond dissociation energy of

Bi-O (343 ± 6 kJ/mol) and Eu-O (557 ± 13 kJ/mol). In addition, for higher values of x, the

orthorhombic EuFeO3 phase pinning at the grain boundary would hinder the growth of grains,

resulting in high density and smaller grains in all the samples [171].

4.3.2 MAGNETIC STUDIES

4.3.2.1 VSM STUDIES

In order to investigate the magnetic properties of the pure and Eu-substituted BFO

samples, room temperature VSM measurements were carried out as shown in Figure 4.7. The Bi

atoms do not affect the magnetic properties directly but controls the crystalline structure, which

leads to spiral arrangement of spins of Fe atoms. The difference in valence electronic

configuration of Bi (6s26p3) and Eu (4f76s2) gives rise to different magnetic property. Similar to

other rare earth (Sm, La, Gd) [161-163] doped BFO ceramics, remnant magnetization (Mr)

gradually increases with increasing Eu concentration in BFO lattice indicating weak

ferromagnetic (WFM) nature of these samples. The magnetic parameters of all samples are listed

in Table 4.3. The enhancement of magnetization is the collective effect of several factors. The

modification of the spiral spin structure caused by the change in the Fe-O-Fe bond angles

associated with the large distortion in the crystal structure gives rise to increase in remanent

magnetization. In REFeO3 (RE3+, magnetic rare earth ion) orthoferrite, Fe3+-Fe3+, RE3+-Fe3+ and

RE3+-RE3+ exchange interactions coexist due to the presence of two magnetic sublattices of Fe3+

and RE3+. In BFO, Fe3+-O-Fe3+ superexchange interaction dominates, giving rise to AFM

ordering. With increasing Eu3+ concentration in BFO lattice, the interactions between the Eu3+-

Eu3+ and Eu3+-Fe3+ ions decouple the antiferromagnetic interactions between the Fe3+ ions and

contributes to WFM ordering. Moreover, the antisymmetric Dzyaloshinskii-Moriya (DM)

exchange interaction between neighbouring spins induced by the spin-orbit interaction also

contributes to WFM ordering. However, for x = 0.15 sample the appearance of orthoferrite

EuFeO3 phase has also been detected from XRD analysis. The evolution of EuFeO3 phase might

be another reason accounting for the observed higher value of Mr in x = 0.15 sample. In addition,

the coercivity (Hc) also increases with increasing Eu concentration similar to that of Mr. The main

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88

source of large Hc for magnetic materials is shape anisotropy and magnetoelastic anisotropy in

addition to the magnetocrystalline anisotropy [172].

Figure 4.7: Room temperature M-H curves for Bi1-xEuxFeO3 ceramics with x = 0.0-0.15.

Table 4.3: Magnetic parameters for Bi1-xEuxFeO3 ceramics

Compositions Hc (Oe) He (Oe) Mr (emu/g) MH (emu/g) at 15 kOe

x = 0.0 30 -66 0.0003 0.075 x = 0.03 25 -48 0.0005 0.2011 x = 0.05 54 -129 0.0009 0.1574 x = 0.07 229 -288 0.0044 0.2423 x = 0.10 945 -1322 0.0165 0.2263 x = 0.12 1499 -2207 0.0295 0.2454 x = 0.15 5303 -6018 0.08721 0.2153

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4.3.2.2 ESR STUDIES

To further explore the magnetic properties of Eu doped BFO sample, electron spin

resonance (ESR) spectra were measured at room temperature. The line shape of ESR spectra

changes with increasing Eu concentration as shown in Figure 4.8.

Figure 4.8: Room temperature ESR spectra for Bi1-xEuxFeO3 ceramics with x = 0.0-0.15.

Mixture of Gaussian and Lorentzian line shape has been observed in the ESR spectra of x =

0.03-0.07 samples. Lorentzian line shape becomes dominant in ESR spectrum of x = 0.10

sample. For x>0.10 samples, further broadening of the ESR line takes place and the spectra

gradually change from Lorentzian to Dysonian resonances. These changes in line shape of ESR

spectra are attributed to local distortion caused by Eu substitution. The observed ESR signals

were categorized by their g-values, which are defined by equation (3.2). The external magnetic

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90

field H was varied between 0 and 8 kOe. The variation of line width (∆H = peak to peak

distance) with doping is another sensitive indicator of changes in the environment of Fe ions in

FeO6 octahedra. The ESR spectrum of x = 0.0 sample can be described approximately as a sum of

two lines, the broad one with ∆H about 571 Oe and the narrow one with ∆H around 102 Oe as

shown in Figure 4.8. Based on narrow line, the calculated value of g was ~2. Furthermore, the

third broad absorption peak situated around 1706 Oe is assigned to Bi2Fe4O9 impurity phase as

confirmed from the XRD pattern of pure BFO [139]. The simultaneous presence of the broad and

narrow resonances for x = 0.03 sample with g>2 is an indication of its heterogeneous nature,

which reduced for x = 0.05 sample [173]. For x = 0.07 sample, ESR spectrum contain two ESR

signal suggesting the existence of weak inter-cluster interaction of magnetic ions [174]. The ESR

spectrum of x = 0.10 sample with g = 2 and ∆H = 86 Oe is ascribed to a strong exchange

interaction and motional narrowing caused due to the higher flipping rate of Fe3+ ions. The slight

asymmetry in the shape of the spectrum results from the non-equivalence of the positions of the

Fe3+ ions in the BFO lattice. The decrease in exchange and motional narrowing with increasing

Eu concentration may arise due to slight increase in particle size from x = 0.10 to 0.15. The

presence of Dysonian type ESR line shape suggests the formation of ferromagnetically aligned

spin clusters for x = 0.15 sample [175].

4.3.3 OPTICAL STUDIES

4.3.3.1 UV-VIS DIFFUSE ABSORPTION STUDIES

The effect of Eu substitution on optical properties of BFO ceramics is studied by UV-

visible diffuse reflectance spectroscopy. Figure 4.9 shows the room temperature UV-Visible

absorption spectra for Bi1-xEuxFeO3 ceramics in the spectral range 1.12-3.5 eV. Six possible

transitions between 0 and 3 eV have been reported for BFO by taking into account the R3c or C3ʋ

local symmetry of Fe3+ ions and using the correlation group or subgroup analysis of the

symmetry breaking [141]. The two broad bands around 1.3 and 1.8 eV correspond to 6A1g→4T1g

and 6A1g→ 4T2g transitions, respectively. These bands are assigned to on site d-d transitions of

Fe3+ ions. Formally, these excitations are forbidden; however, they have small strengths due to

the relaxation of the selection rule caused by spin-orbit coupling [142]. Above 1.8 eV, the

absorption gradually increases up to 2.5 eV and shows two broad charge transfer (CT) transition

bands, one around 2.5 eV and another broad band around 3.2 eV.

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Figure 4.9: UV-Visible diffuse absorption spectra for Bi1-xEuxFeO3 ceramics with x = 0.0-0.15.

Figure 4.10: (αhν)2 versus energy (hν) curves for Bi1-xEuxFeO3 ceramics with x = 0.0-0.15.

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92

The CT band around 2.5 eV is associated with Fe13d-Fe23d inter site electron transfer,

while band at 3.2 eV is ascribed to interatomic O 2p-Fe 3d transitions [143]. Obviously,

increasing Eu concentration changes the FeO6 local environment which results in the small shift

in d-d and Charge transfer (CT) transition bands. This change in octahedron arises due to

increased chemical pressure in BFO with Eu substitution as a consequence of the contraction in

unit cell volume. Figure 4.10 indicates the (αhν)2-(hν) curves for all samples in which energy

band gap was determined by extrapolating the linear portion of this curve to zero. The optical

band gap was calculated to be 2.25, 2.24, 2.22, 2.20, 2.18, 2.17 and 2.16 eV for x = 0.0-0.15

samples, respectively.

4.3.4 ELECTRICAL STUDIES

4.3.4.1 DIELECTRIC STUDIES

Figure 4.11 and Figure 4.12 illustrate the frequency dependence of dielectric constant (ε)

and dielectric loss (tanδ ) for Eu doped BFO ceramics.

Figure 4.11: Room temperature frequency dependent dielectric constant for Bi1-xEuxFeO3 ceramics with

x = 0.0-0.15.

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The inhomogeneous behavior of grain and grain boundary regions leads to high dielectric

constant in doped samples [176]. The dielectric constant decreases with the increase of frequency

and is found to be nearly constant at higher frequencies (>100 kHz) for all samples. It is so

because at low frequencies the space charges follow the frequency of the applied field, whereas,

at higher frequencies, they may not have time to undergo relaxation process.

Figure 4.12: Room temperature frequency dependent dielectric loss for Bi1-xEuxFeO3 ceramics with x = 0.0-0.15.

The appearance of the relaxation peak in dielectric loss (Figure 4.12) at higher

frequencies for x = 0.03 and 0.05 samples can be associated to the strong correlation existing

between the conduction mechanism and dielectric polarization of the samples [164]. This

relaxation peak is suppressed for x>0.05 samples. The enhancement in frequency independent

region of dielectric constant and dielectric loss has been observed with Eu substitution and it is

maximum for x = 0.15 sample. This behavior can be associated with the reduction of oxygen

vacancies and hence associated leakage current with increasing doping [177]. Therefore,

improved dielectric properties are observed with increasing Eu substitution.

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4.4 CONCLUSIONS

In this chapter, Eu substituted BiFeO3 (Bi1-xEuxFeO3; x = 0-0.15) polycrystalline ceramics

were synthesized by the solid state reaction method. Rietveld refinement of X-ray diffraction

patterns reveals that samples crystallize in R3c structure for x ≤ 0.10 and (R3c + Pn21a) phases

coexist for x≥0.12-0.15. The magnetic measurements show weak ferromagnetic nature of Eu

substituted BiFeO3 samples due to ferromagnetic coupling between Eu3+ and Fe3+ ions. The

remnant magnetization is found to increase from 0.0003 emu/g for x = 0.00 to 0.087 emu/g for x

= 0.15. The gradual change in line shape of electron spin resonance spectra has been attributed to

local distortion induced by Eu substitution. UV-Visible absorption spectra in the spectral range

1.12 to 3.5 eV were dominated by two Charge transfer transitions and two doubly degenerate d-d

transitions. The optical band gap is found to decrease from 2.25 to 2.16 eV with increasing Eu

concentration. Improved dielectric properties with enhancement in frequency independent region

in dielectric constant and loss have been observed.

CHAPTER 4 INFLUENCE OF Eu SUBSTITUTION ON STRUCTURE ...

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