47

CHAPTER 3

STRUCTURAL, MAGNETIC, DIELECTRIC AND OPTICAL

STUDIES OF Dy SUBSTITUTED BiFeO3 MULTIFERROIC

CERAMICS

3.1 INTRODUCTION

Amongst all single phase multiferroic materials, BiFeO3 is very interesting and most

studied material due to its ferroelectric and antiferromagnetic transition temperatures above room

temperature [105-107]. BFO can be synthesized in bulk form by using normal synthesis

techniques like solid state reaction method and sol-gel method [108-109]. However, it is still

difficult to prepare phase pure BFO due to the presence of secondary phase. In addition, high

conductivity due to large leakage current, low magnetization, low polarization in BFO is major

obstacles for its potential applications in devices [110-112]. One of the strategies to over these

problems is to substitute some foreign elements (like rare earth ions [113-116] and transition

metal ions [96-97, 1117] etc) at A-site or B-site in BFO lattice. Recently, some researchers

reported Dy-doped BFO ceramics with enhanced multiferroic properties in bulk and thin films

form [118-119]. The functions of partial substitution of Bi3+ions by Dy3+ ions are threefold: (i) to

form phase pure BFO, (ii) to reduce leakage current and (iii) to suppress spiral spin structure for

enhancing magnetization. Due to the size difference between Bi3+ (1.17 Å) and Dy3+ (0.912 Å)

ions, the crystal lattice of BFO is influenced by the substitution in a similar way as by pressure

[120]. Moreover, the introduction of Dy3+ on the perovskite Bi-site is expected to modify the

magnetic properties of BFO because the large magnetic moment of Dy3+ (~10.6 µB) ions which

could result in additional magnetic interactions and ordering and the modification of the structure

[121]. Therefore, we studied the influence of Dy3+ ions substitution on structural, magnetic,

dielectric and optical properties of BFO. This chapter discusses the room temperature and low

temperature magnetic interactions along with detailed structural, vibrational and electrical

properties of Bi1-xDyxFeO3 (x = 0.0, 0.03, 0.05, 0.07, 0.10, and 0.12) ceramics prepared by solid

sate reaction method.

Chapter-3

48

3.2 EXPERIMENTAL DETAILS

Bi1-xDyxFeO3 ceramics with x = 0.0, 0.03, 0.05, 0.07, 0.10 and 0.12 were prepared by

solid state reaction method. High purity powders of Bi2O3, Dy2O3 and Fe2O3 (purity 99.99%,

Sigma Aldrich) were used. The powders were weighed in stoichiometric proportion and then

mixed and grounded in mortar pastel in acetone medium for 4 h to prepare homogenous mixture.

The mixture is then calcined at 7000C for 2 h, after which these were pressed into 1 mm thick

pellets of diameter 10 mm. The pellets were sintered at 800-8200C for 2 h to achieve pure phase

formation. X-ray diffractometer (Brucker D8 Advance) was used to study the crystal structure

and the data was analyzed using FullProf programme for detailed structural parameters. Raman

spectra were recorded by Renishaw Raman spectrophotometer using 514.5 nm Laser beam with

spot size 1 µm. The power of laser was kept below 5 mW in order to avoid any heating of

samples. The surface morphology was studied by using Scanning electron microscope (SEM).

Magnetic properties were measured by superconducting quantum interference device (SQUID).

The room temperature electron spin resonance (ESR) spectra were recorded on Bruker EMX

spectrometer. Ultraviolet-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 and impedance properties

of ceramics were measured on silver coated pellets using Newton’s 4th PSM 1735 impedance

analyzer.

3.3 RESULTS AND DISCUSSION

3.3.1 STRUCTURAL STUDIES

3.3.1.1 X-RAY DIFFRACTION STUDIES

Figure 3.1 (a) shows the XRD patterns of Bi1-xDyxFeO3 (x = 0.0, 0.03, 0.05, 0.07, 0.10

and 0.12) ceramics. The diffraction planes (012), (104), (110), (006), (202), (024), (116), (122),

(018), (300), (208), (220), (131), (036), (128), (134) in the XRD pattern for all samples can be

indexed according to the rhombohedral structure (space group R3c) of BFO (JCPDS card no. 71-

2494). Along with the diffraction peaks belonging to the rhombohedral structure of BFO, few

minor impurity peaks corresponding to Bi2Fe4O9 and Bi24Fe2O39 phases were also detected in

XRD pattern for x = 0.0 and 0.03 samples.

Chapter-3

49

Figure 3.1: XRD patterns of Bi1-xDyxFeO3 ceramics with x = 0.0-0.12.

These impurity phases are accompanied along with rhombohedral phase in the XRD pattern of

pure BFO ceramic due to the kinetics of formation [122]. These impurity peaks suppressed with

increasing Dy concentration in x = 0.0 and 0.03 samples and they almost disappeared for x>0.03

samples. It is evident that the diffraction peaks shifted slightly towards higher 2θ value with

increasing Dy content. This shifting of the XRD peaks is ascribed to the difference in ionic radii

of Bi3+ (1.17 Å) and Dy3+ (0.912 Å) ions. The doublet (104) and (110) around 2θ value of 320 is

gradually suppressed with increasing Dy doping as shown in Figure 3.1 (b). Also the intensity of

(104) peak is decreased compared to that of (110) peak with increasing Dy concentration. In

addition, the splitting behavior of diffraction doublets around 2θ values of 39°, 51°, 56° is

decreased with increasing x, indicating the distortion in the original rhombohedral structure of

BFO.

Chapter-3

50

Figure 3.2: Rietveld refined XRD patterns for Bi1-xDyxFeO3 ceramics with x = 0.0-0.12.

The peak shifting towards higher 2θ values and reduced separation between doublet

indicate the distortion in the rhombohedral structure of BFO without any sign of structure

transformation up to x = 0.12. This behavior is unlike Dy doped BFO nanoparticles in which

structural transformation from rhombohedral to orthorhombic has been reported for x = 0.10

[123]. However, in present Dy doped bulk BFO ceramics, the structural transformation is

expected for x>0.12. The phase coexistence in the broad concentration range has also been

reported for rare earth doped BFO ceramics [114].

Rietveld refinement of XRD patterns for all Dy doped BFO samples have been carried out

for, detailed structural analysis and to extract various structural parameters. The Rietveld analysis

of all samples has been done by considering the rhombohedral structure of R3c space group with

Chapter-3

51

wyckoff positions of Bi/Dy at 6a, Fe at 6a and O at 18b. The final cycle of refinement resulted in

matching between observed and calculated XRD patterns which has been observed with proper

Bragg positions as shown in Figure 3.2. The lattice parameters, lattice volume, bond lengths,

bond angles, refined atomic positions of Bi/Dy, Fe and O and value of R-factors (Rp, Rwp, RBragg

and Rf) obtained from refinement of the XRD pattern were listed in Table 3.1. Refined XRD

patterns indicate that the smaller Dy3+ ions substitution for the bigger Bi3+ ions leads to the

distortion in the BFO lattice which enhances the Fe-O-Fe bond angle together with a contraction

of the unit cell. The magnetization and antiferromagnetic Neel temperature of BFO are closely

related to the Fe-O-Fe bond angle [124-125]. Therefore, the variation in the Fe-O-Fe bond angle

predicts that magnetic properties might improve along with variation in Neel temperature for Bi1-

xDyxFeO3 ceramics.

Table 3.1: Rietveld Refined structural parameters for Bi1-xDyxFeO3 ceramics

Samples Lattice

Parameters Atoms Positions x y z Bond

Length

(Å) Bond

Angle

(°)

R-Factors

(%)

x = 0.0

a = 5.5786 (Å)

c = 13.8667 (Å)

V = 373.73 (Å3)

Bi/Dy

Fe

O

6a

6a

18b

0.0

0.0

0.4230

0.0

0.0

0.0484

0.0

0.2151

0.9598

Bi-O

Fe-O

Fe-O

2.300

1.929

2.211

Fe-O-Fe

O-Fe-O

O-Bi-O

146.27

168.87

142.56

Rp = 6.54

Rwp = 9.16

Rf = 9.86

RBragg = 14.0

x = 0.03

a = 5.5735 (Å)

c = 13.8502 (Å)

V = 372.60 (Å3)

Bi/Dy

Fe

O

6a

6a

18b

0.0

0.0

0.4216

0.0

0.0

-0.0054

0.0

0.2228

0.9545

Bi-O

Fe-O

Fe-O

2.447

1.869

2.199

Fe-O-Fe

O-Fe-O

O-Bi-O

153.47

168.95

147.58

Rp= 5. 07

Rwp = 7.15

Rf = 3.68

RBragg = 6.24

x = 0.05

a = 5.5707 (Å)

c = 13.8337 (Å)

V = 371.78 (Å3)

Bi/Dy

Fe

O

6a

6a

18b

0.0

0.0

0.4205

0.0

0.0

-0.0218

0.0

0.2265

0.9542

Bi-O

Fe-O

Fe-O

2.487

1.766

2.284

Fe-O-Fe

O-Fe-O

O-Bi-O

155.23

168.67

148.47

Rp = 4.09

Rwp = 5.33

Rf = 4.23

RBragg = 5.77

x = 0.07

a = 5.5678 (Å)

c = 13.8277 (Å)

V = 371.23 (Å3)

Bi/Dy

Fe

O

6a

6a

18b

0.0

0.0

0.4392

0.0

0.0

0.01021

0.0

0.2234

0.9581

Bi-O

Fe-O

Fe-O

2.486

1.940

2.103

Fe-O-Fe

O-Fe-O

O-Bi-O

156.07

168.52

143.17

Rp= 3.67

Rwp = 4.87

Rf = 2.32

RBragg = 2.97

x = 0.10

a = 5.5662 (Å)

c = 13.8148 (Å)

V = 370.68 (Å3)

Bi/Dy

Fe

O

6a

6a

18b

0.0

0.0

0.4471

0.0

0.0

-0.0076

0.0

0.2257

0.9512

Bi-O

Fe-O

Fe-O

2.599

1.789

2.235

Fe-O-Fe

O-Fe-O

O-Bi-O

158.33

165.17

146.27

Rp = 3.80

Rwp = 4.93

Rf = 2.16

RBragg = 3.93

x = 0.12

a = 5.5704 (Å)

c = 13.8078 (Å)

V = 371.05 (Å3)

Bi/Dy

Fe

O

6a

6a

18b

0.0

0.0

0.4436

0.0

0.0

-0.0077

0.0

0.2263

0.9561

Bi-O

Fe-O

Fe-O

2.566

1.820

2.199

Fe-O-Fe

O-Fe-O

O-Bi-O

159.33

168.08

144.39

Rp = 4.72

Rwp = 6.56

Rf = 3.55

RBragg = 5.71

Chapter-3

52

The average crystallite size (D) was calculated from XRD peak broadening using Debye

Scherrer’s equation ),cos(θβλKD = where K is a constant, β is the full width at half maxima

(FWHM), θ is the Bragg and angle λ is wavelength. The average crystallite sizes calculated from

Scherer’s formula are found to be 72, 66, 51, 50, 45 and 36 nm for x = 0.0-0.12 samples,

respectively. The substitution of smaller ionic radii Dy3+ ions at A-site would decrease the

average A-site which can be measured by calculating tolerance factor (t) as

,)(2

)(

OB

OA

RR

RRt

+

+= (3.1)

where RA, RB, and RO are the ionic radii of the A, B, and O sites. The values of tolerance factor

are calculated for Bi1-xDyxFeO3 ceramics by using above formula. The calculated values of

tolerance factor are found to be 0.8909, 0.8883, 0.8865, 0.8847, 0.8820 and 0.8802 Å for x = 0.0-

0.12 samples, respectively, which indicates increased distortion in BFO lattice with increasing

Dy doping. When the value of t is unity, it corresponds to ideal perovskite with undistorted

structure, while the value of t less than unity has a distorted perovskite structure, indicating tilt or

rotation of the FeO6 octahedra. The value of t less than unity indicates compression forces acts on

Fe-O bonds and consequently Bi3+/Dy3+-O bonds are under tension. Further, the oxygen

octahedra tend to rotate cooperatively to alleviate the lattice stress [90]. The relative rotation

angle of the two oxygen octahedra around the polarization [111] axis in the R3c unit cell

increases with increasing Dy substitution for Bi in BFO ceramics. As Dy concentration increases,

the induced lattice distortion suppresses rhombohedral unit cell with reduction in lattice

parameters as well as the overall volume of the unit cell as shown in Table 3.1.

3.3.1.2 RAMAN SPECTROSCOPY STUDIES

Raman spectroscopy was employed to further study the structure modulation in Bi1-x

DyxFeO3 ceramics. As Raman scattering spectra are sensitive to atomic displacement, the

evolution of Raman modes with an increasing Dy concentration in Bi1-xDyxFeO3 ceramics can

provide valuable information about the lattice properties, structural phase transition and spin-

phonon coupling. It has been reported that BFO with distorted rhombohedral structure, R3c space

group and 10 atoms in a unit cell of this structure yields 18 optical phonon modes and can be

summarized using following irreducible representation: EAAopt 954 21 ++=Γ . According to

Chapter-3

53

group theory, 13 observed modes ( EAcRRaman 94 13, +=Γ ) are Raman active, whereas 5A2 are

Raman inactive modes. The A1-modes are polarized along z-axis, and E modes are polarized in x-

y plane. The intensities of A modes are greater than E modes in the present study.

Figure 3.3: Room temperature Raman spectra of Bi1-xDyxFeO3 ceramics with x = 0.0-0.12.

Figure 3.3 shows the room temperature Raman spectra for Bi1-xDyxFeO3 ceramics in the

wavenumber range 50-700 cm-1. Mode assignment in BFO is a complex and often confusing

issue as discussed by Bielecki et. al. [126]. The problem is the strong angular dispersion and the

optical axis in [111] direction. There is no natural surface on a single crystal parallel or

perpendicular to the optical axis due to the general direction of the optical axis. The situation is

even more complex in bulk samples due to the random orientation of grains. As a result, mixed

kinds of modes are observed with shifted frequencies and modified intensities instead of pure

vibrational modes. Raman patterns of Bi1-xDyxFeO3 ceramics correspond to the spectral feature of

typical Raman spectra of rhombohedral BFO as reported by other reports [127-129]. There is no

obvious change in Raman spectra of Bi1-xDyxFeO3 ceramics with increasing Dy indicating the

Chapter-3

54

absence of any structural phase transition unlike Dy doped BFO nanoparticles [123]. To find out

the exact position of Raman modes, the Raman spectra are fitted with the help of peak fit

software in the deconvoluted Raman modes as shown in Figure 3.4. The 12 (4A1 and 8E modes)

Raman active modes for x = 0.0 and 11 (4A1+7E) modes for x = 0.03-0.12 are fitted in the wave

number range 100-700 cm-1 (excluding lower wave number E mode below 100 cm-1). The

deconvoluted modes and their position are shown in Table 3.2. The lower wave number modes

below 170 cm-1 are related to Bi-O bonds and oxygen motion are strongly dominated in the

modes above 262 cm-1. The Fe modes are mainly involved in the modes between 152 and 262

cm-1 and also contribute to some higher wavenumber modes [130].

Figure 3.4: Deconvoluted Raman spectra of Bi1-xDyxFeO3 ceramics with x = 0.0-0.12.

Chapter-3

55

The shifting of lower wave number modes, corresponding to the Bi-O bond, towards higher wave

number in Bi1-xDyxFeO3 ceramics is attributed to the lower atomic weight of Dy (162.50 g/mol)

than that of Bi (208.98 g/mol).

Table 3.2: Observed and reported Raman modes of Bi1-xDyxFeO3 ceramic at room temperature

with wave-number range 100-700 cm-1

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)

A1-1 139 139 138 142 143 144 145

A1-2 171 172 173 174 175 175 176

A1-3 217 220 225 227 232 234 235

E 260 259 257 257 262 267 268

E 274 278 283 285 285 287 296

E 306 304 -- -- -- -- --

E 344 344 333 325 330 320 323

E 368 370 368 375 372 368 370

A1-4 430 434 434 436 435 430 428

E 468 469 473 472 474 476 473

E 520 529 532 533 531 533 532

E 611 616 618 619 620 620 618

3.3.1.3 MORPHOLOGICAL STUDIES

The surface morphology of Bi1-xDyxFeO3 (x = 0.0-0.12) ceramics are shown in Figure

3.5. From the SEM images, it can be observed that all samples are crystallized in micro grains

range with well crystalline nature. The grains are non-uniform and the tendency of decrease in

grain size and increase in grain boundaries are observed, resulting in less porous surface. The

decrease in grain size may be attributed to a reduction in oxygen vacancies with increasing Dy

concentration. Since the strength of Dy-O bond (611 ± 42 kJ/mol) is higher than that of Bi-O

Chapter-3

56

bond (343 ± 6 kJ/mol) [132], the substitution of Dy at Bi-site constrains Bi volatilization and

lower the concentration of oxygen vacancies.

Figure 3.5: SEM images of Bi1-xDyxFeO3 ceramics with x = 0.0-0.12

3.3.2 MAGNETIC STUDIES

3.3.2.1 SQUID STUDIES

The magnetic properties of Bi1-xDyxFeO3 ceramics have been studied at room and low

temperature by using a SQUID magnetometer. Figure 3.6 shows magnetization-magnetic field

Chapter-3

57

(M-H) loops of B1-xDyxFeO3 ceramics measure at room temperature. Pure BFO is known to be

G-type antiferromagnetic. It can be seen that x = 0.0-0.05 samples show antiferromagnetic nature

similar to pure BFO, while x = 0.07-0.12 samples exhibit weak ferromagnetic nature with higher

values of the magnetization. The M-H loop opening occurs and broadening of MH loops takes

place for x = 0.07-0.12 samples (Figure 3.6). The enhancement in remnant (Mr) and net

magnetization (MH) have been observed with increasing Dy concentration in Bi1-xDyxFeO3

ceramics as shown in Figure 3.7. Magnetic parameters of Bi1-xDyxFeO3 samples are summarized

in Table 3.3. The values of remnant magnetization (Mr) are found to be 0.009, 0.0233, 0.0314,

0.0544, 0.1400 and 0.2103 emu/g for x = 0.0-0.12 samples, respectively.

Figure 3.6: Room temperature M-H curves for Bi1-xDyxFeO3 ceramics with x = 0.0-0.12.

Chapter-3

58

Figure 3.7: Variation of Mr and MH with increasing Dy content.

Table 3.3: Magnetic parameters for Bi1-xDyxFeO3 ceramics

Compositions Hc (Oe) He (Oe) Mr (emu/g) MH (emu/g) at 70 kOe

x = 0.0 1104 -1417 0.009 0.4711 x = 0.03 760 -294 0.0233 0.8465

x = 0.05 837 -1152 0.0314 1.0607

x = 0.07 2163 -1496 0.0544 1.5559

x = 0.10 455 480 0.1400 2.0622

x = 0.12 1156 -1174 0.2103 2.1828

The observed value of remnant and net magnetization are maximum for x = 0.12 sample,

which are much higher than the reported values for A-site doped and codoped BFO bulk ceramics

[114, 116, 133-134]. Several reasons contribute to the improvement of magnetism in

Bi1-xDyxFeO3 ceramics. First, the spiral spin structure of BFO is suppressed by the substitution of

Dy3+ ions for Bi3+ ions which leads to the structural distortion giving rise to an enhanced

magnetization. The Dy substitution only suppressed and could not destroy the spiral spin

Chapter-3

59

structure completely. However, for x = 0.12 sample, larger Fe-O-Fe angle suggests highly

distorted FeO6 octahedra which are responsible for the strong ferromagnetic interaction [124,

135]. The continuous increase in Fe-O-Fe bond angle, which is closely related to magnetic

structure, is in good agreement with observed M-H loops [124, 135]. Second, the enhancement of

magnetization could be attributed to the formation of Bi-O-Dy chains which became magnetic

sub-lattices [134]. The Dy doped BFO system has antiferromagnetic Fe-O-Fe interaction coupled

with a weak ferromagnetic component, which comes from the canted Fe sublattice due to

Dzyaloshinskii-Moriya interaction, which leads to the linear increase with a field in the M-H

curve [136]. Besides this we presume that the ferromagnetic coupling between Dy3+ and Fe3+ ions

also contributes to the enhanced magnetization to some extent.

To further study the enhancement of magnetic properties of Bi1-xDyxFeO3 samples and the

magnetic interaction between Dy3+ and Fe3+ ions, low temperature magnetization measurements

upto 4 K have been carried out. The Field cooled (FC) and zero field cooled (ZFC) magnetization

curves for Bi1-xDyxFeO3 (x = 0.03-0.12) ceramics at applied magnetic field of 500 Oe are shown

in Figure 3.8. Temperature dependent magnetization (M-T) behaviour for x = 0.03 and 0.05

samples exhibited the overlapping of FC and ZFC curves in temperature range (300-4 K)

confirming the AFM nature of these samples. However, M-T plots for x = 0.07, 0.10 and 0.12

samples (Figure 3.8) exhibited an apparent splitting in FC and ZFC curves. This splitting in FC

and ZFC curves increases with increasing Dy concentration, indicating the ferromagnetic

interaction becoming more dominated over AFM interaction at low temperature. The

magnetization for x = 0.07-0.12 samples decreases with decreasing temperature and attains a

minimum. However, on further decreasing the temperature, the magnetization increases sharply,

indicating the magnetic reorientation in these samples. In RFeO3 (R is rare earth ions), the

magnetic interactions are also dependent on R3+-Fe3+ and R3+-R3+ interactions along with Fe3+-

Fe3+ exchange interaction [137]. The spin reorientation transition depends not only on the size of

rare earth ions, but also on magnetic anisotropy. The competing interaction between Dy3+-Dy3+

and Dy3+-Fe3+ ions result in a series of magnetic transition in DyFeO3 [138]. The Fe3+ ions

interact anti-parallel through a super-exchange mechanism and give a G-type AFM ordering, just

below the Neel temperature (~645 K), while the Dy3+-Dy3+ interactions remains dominant in the

lower temperature range (below 60 K). However, at the intermediate temperature ranges the

Dy3+-Fe3+ interaction prevails [138].

Chapter-3

60

Figure 3.8: Field cooled (FC) and zero field cooled (ZFC) curves for Bi1-xDyxFeO3 ceramics with x = 0.03-0.12.

A similar mechanism is adopted to explain the temperature dependent magnetization

behavior in case of Bi1-xDyxFeO3 samples as well. At higher temperature, Fe3+-Fe3+ interactions

are much stronger than Dy3+-Fe3+ and Dy3+-Dy3+ interactions, resulting in AFM ordering. As

temperature decreases, a weak AFM coupling between Fe3+ and Dy3+ moments gives rise to

reorientation transitions around 94, 110 and 113 K for x = 0.07, 0.10 and 0.12 samples,

respectively. On further decreasing the temperature below 40 K, a strong ferromagnetic coupling

between Dy3+-Dy3+ moments results in a sharp increase in magnetization. In Bi1-xDyxFeO3, the

Fe3+ ions at the center of FeO6 octahedra give rise to the splitting of 3d5 orbital into t2g and eg,

which are half-filled orbital. The oxygen 2p orbitals hybridized with half-filled eg orbitals, which

leads to Fe3+-Fe3+ ion interactions via O2- ions in 180° position. However, Dy3+-Fe3+ interaction

via O2- (at 90° position) is weaker and dominates at intermediate temperature range. In Fe3+-O2--

Dy3+ weak AFM interaction, the oxygen p orbital overlaps with half-filled eg orbitals of Fe3+ at

one end and with a more than half-filled f-orbital of Dy3+ at the other end. This interaction results

Chapter-3

61

in the reorientation of G-type anti-parallel aligned Fe3+ component to weak ferromagnetic

interactions around 110 K for x = 0.10 and 0.12 samples. In addition to this weak AFM

superexchange interaction between Dy3+-Fe3+ ions, the DM type anti-symmetric exchange

interaction between Fe3+-Fe3+ ions also plays a crucial role. The Fe3+ spins get canted due to DM

exchange interaction which is also responsible for the splitting of FC and ZFC curves. The Dy3+-

Dy3+ superexchange interaction is ferromagnetic and dominates at the lowest temperatures.

3.3.2.2 ESR STUDIES

To further explore the magnetic properties of Bi1-xDyxFeO3 samples, the measured ESR

spectra of all samples are shown in Figure 3.9.

Figure 3.9: Room temperature ESR spectra for Bi1-xDyxFeO3 ceramics with x = 0.0-0.12.

Chapter-3

62

The double absorption peaks observed in x = 0.03 sample indicates the presence of

Bi2Fe4O9 impurity phase [139]. However, this Bi2Fe4O9 phase disappears with increasing Dy

doping and hence the single absorption peak appears in x = 0.05-0.07 samples. The absorption

peaks around 3 kOe shifts to lower g-value with increasing Dy concentration. Three absorption

peaks have been observed for x>0.07 in which third peak dominates over two other peaks. The g-

value from the ESR signal was calculated from the resonance field by using the relation:

,rH

hg

β

ν= (3.2)

where h is Planck’s constant, ν is the frequency of spectrometer β is the Bohr Magneton and Hr

is the resonance field. The g-values were calculated to be 2.04, 1.99, 2.02, 1.98, and 1.96 for x =

0.03-0.12, respectively. Based on third peak, the g-values were calculated to be 1.03 and 1.04 in x

= 0.10 and 0.12 samples, respectively. The large variation in g-values suggests that the magnetic

environment of Fe3+ ions in BFO gradually changes with increasing Dy concentration. The values

of ∆H (peak to peak distance) were found to be 961, 938 and 993 Oe for x = 0.03, 0.05, and 0.07

respectively. Furthermore, the homogeneous broadening occurs for x = 0.03, 0.05, 0.07 samples,

while inhomogeneous broadening has been observed for x = 0.10 and 0.12 samples. It suggests

that the dipolar interaction between Fe3+ and Fe3+ ions (like spins) dominates for x = 0.03, 0.05,

0.07 samples while the dipolar interaction between Dy3+ and Fe3+ ions (unlike spins) dominates

for x = 0.10 and 0.12 samples [140].

3.3.3 OPTICAL STUDIES

3.3.3.1 UV-VIS DIFFUSE ABSORPTION STUDIES

In recent years, the intense research work has been carried out on electric and magnetic

properties of BFO. However, few works have been done to explore the optical properties of BFO.

UV-visible diffuse reflectance spectra (DRS) were recorded to study the optical properties Bi1-

xDyxFeO3 ceramics. There is a point group symmetry breaking from Oh to C3v as BFO has a

distorted cubic structure (i.e. rhombohedral) [141]. Six possible d-d transitions between 0 to 3 eV

are expected for BFO by considering the C3v local symmetry of Fe3+ ions and using the

correlation group and subgroup analysis for the symmetry breaking from Oh to C3v. Figure 3.10

shows the room temperature UV-Visible absorption for Bi1-xDyxFeO3 ceramics derived from the

diffuse reflectance spectrum using Kubelka-Munk function:

Chapter-3

63

,2/)1()( 2 RRRF −= (3.3)

which is plotted as a function of energy (eV). It is evident that the two transitions around 1.8 and

2.5 eV have been observed. The weak band around 1.8 eV corresponds to on-site d-d crystal field

transition (6A1g→ 4T2g) of Fe3+ ions [142]. The total spin of Fe3+ changes from S = 5/2 to S = 3/2

in this transition due to which it is forbidden. However, the spin selection rule relaxes due to spin

orbit coupling and gives rise to this transition of low intensity.

Figure 3.10: UV-Visible diffuse absorption spectra for Bi1-xDyxFeO3 ceramics with x = 0.0-0.12.

The increase in intensity of this band with increasing Dy concentration indicates the

enhancement of the spin-orbit coupling in Bi1-xDyxFeO3 samples. Above 1.8 eV, the absorption

gradually increases and shows a broad charge transfer (CT) transition band around 2.5 eV. This

Chapter-3

64

CT band around 2.5 eV is ascribed to Fe1 3d-Fe2 3d intersite electron transfer [143]. This charge

transfer band shifts towards lower energy with increasing Dy concentration in BFO is attributed

to the changes in local environment of FeO6 octahedron. As a consequence of contraction in unit

cell volume with Dy substitution in BFO, the chemical pressure is induced in BFO unit cell

which in turn distorted FeO6 octahedron.

Figure 3.11: Tauc’s plots for Bi1-xDyxFeO3 ceramics with x = 0.0-0.12.

To extract the band gap information from UV absorption spectra, we use Tauc’s relation

n

gEhAh )( −= ννα (3.4)

with n = 1/2 since BFO is a direct band gap material. Figure 3.11 displays (αhν)2 versus hν

curves to calculate band gap of Bi1-xDyxFeO3 samples. The band gap is calculated by

extrapolating the linear portion of the curve to the x-axis. The calculated optical band gap values

Chapter-3

65

are 2.25, 2.21, 2.20, 2.19, 2.12 and 2.09 eV for x = 0.0-0.12 samples, respectively. The obtained

values of the band gap for Bi1-xDyxFeO3 samples are consistent with the reported values for pure

and doped BFO [144-145] and smaller than BFO thin film [146] and BFO nanoparticles [147].

The decrease in the band gap of Bi1-xDyxFeO3 ceramics may be attributed to the rearrangement of

molecular orbital and distortion induced in the FeO6 octahedra [148]. The energy band gap in the

visible region makes Bi1-xDyxFeO3 ceramics suitable for photocatalytic and solar cell

applications.

3.3.3.2 FTIR STUDIES

Infrared spectroscopy has been a powerful technique for materials characterization for

over many decades. An Infrared (IR) spectrum represents a fingerprint of a sample with an

absorption peak corresponding to the frequencies of bonds vibration of the atoms making up the

material. No two materials have an exact IR spectrum because each different material is a unique

combination of atoms. IR spectroscopy can identify bonding between various atoms present in

the material. Therefore, Fourier transform infrared (FTIR) spectra of all samples were recorded

using KBr pellet methods and are shown in Figure 3.12.

Figure 3.12: FTIR spectra for Bi1-xDyxFeO3 ceramics within a range of 400-1000 cm-1.

Chapter-3

66

Two broad absorption bands around 450 and 550 cm-1 has been observed. The broad

nature of the observed vibration bands is due to the occurrence of absorption peaks of both Fe-O

and Bi/Dy-O bonds at nearly the same wavenumber. The bending vibration of O-Fe-O bond and

stretching vibration of Fe-O bond in the FeO6 octahedron unit falls at 439 and 548 cm-1

respectively [149]. Moreover, the absorption peaks due to vibrations of Bi/Dy-O bond in the

BiO6 octahedral unit appears at 450 and 550 cm-1. In addition, the peak at 638 cm-1 corresponds

to the bending mode of vibration of O-Bi-O bond. The IR active modes shift towards higher

wavenumber with increasing Dy concentration in BFO samples. This shifting is attributed to the

distortion in the crystal lattice with increasing Dy substitution in BFO samples. In order to

determine the change in the position of Bi-O and Fe-O bands induced by Dy substitution,

Gaussian peak fitting was carried out in the spectral region 500-630 cm-1, for all samples (Figure

3.13). The fitted peaks around 525, 550 and 590 cm-1 are associated to Bi/Dy-O bond, Fe-O bond

and phase vibrations of basis atoms of oxygen [149] respectively for x = 0.0-0.12 samples.

Figure 3.13: Gaussian peak fitting of the FTIR spectra for Bi1-xDyxFeO3 ceramics with x = 0.0-0.12

The vibrational frequency of Fe-O bond can be determined using the relation

Chapter-3

67

,2

1 21

=

µπν

k

c (3.5)

where ν is the wave number, c is the velocity of light, k is the average force constant of the Fe-O

bond and µ is the effective mass of the Fe-O bond given by

,)(

)(

FeO

FeO

MM

MM

+

×=µ (3.6)

where MFe and MO are atomic weight of Fe and O, respectively. The force constant can be

correlated to the average Fe-O bond length (r) via relation

3

17

rk = . (3.7)

With the help of above relations, force constant (k) and bond length (r) of Fe-O bond was

calculated for all samples as listed in Table 3.4. The variation of Fe-O bond length calculated

from fitting of FTIR spectra is consistent with average Fe-O bond length obtained from Rietveld

refinement of XRD pattern.

Table 3.4: IR band, force constant, and Fe-O bond length calculations for Bi1-xDyxFeO3 ceramics.

Compositions Wave number

(cm-1)

Force Constant

(N/cm)

Bond Length (Å)

(Fe-O)

(From FTIR)

Bond Length (Å)

(Fe-O)

(From Rietveld)

x = 0.0 544.18 2.011 2.037 2.070

x = 0.03 547.90 2.081 1.972 2.034

x = 0.05 548.89 2.225 1.970 2.025

x = 0.07 549.83 2.233 1.967 2.022

x = 0.10 550.24 2.236 1.966 2.012

x = 0.12 551.25 2.244 1.964 2.010

3.3.4 ELECTRICAL STUDIES

3.3.4.1 DIELECTRIC STUDIES

The room temperature frequency dependent dielectric constant and dielectric loss for Bi1-

xDyxFeO3 ceramics are shown in Figure 3.14 and Figure 3.15, respectively. The value of

dielectric constant decreases with increasing Dy substitution in BFO samples. The low frequency

Chapter-3

68

dielectric relaxation behaviour has been observed for x = 0.03 and 0.05 samples. However,

dielectric constant for x = 0.07-0.12 samples remain stable in the whole frequency range.

Figure 3.14: Room temperature frequency dependent dielectric constant of Bi1-xDyxFeO3 ceramics with x = 0.0-0.12.

Figure 3.15: Room temperature frequency dependent dielectric loss of Bi1-xDyxFeO3 ceramics with x = 0.0-0.12.

Chapter-3

69

This indicates that dipoles with small effective mass mainly contribute to dielectric

constant, instead of charge defects with large effective masses [124]. The dielectric loss also

decreases with increasing Dy concentration in BFO samples. This indicates that Dy substitution

at A-site effectively suppressed the formation of oxygen vacancies which results in decreased

conductivity.

The temperature dependent dielectric constant measurements at selected frequencies (25,

50, 75, 100 and 1,000 kHz) are shown in Figure 3.16. Two broad diffused peaks have been

observed for x = 0.03, 0.05 and 0.07 samples, while only one peak has been observed for x = 0.10

and 0.12 samples. The low temperature (100-230°C) broad peak observed for x = 0.03, 0.05 and

0.07 samples gradually suppress with increasing frequency. Similar to other perovskite ferrites,

this low temperature peak is not intrinsic and could be related to the defects such as ⋅⋅

OV and

'23

++

FeFe which are usually created during the sintering process [150]. The complete suppression of

low temperature dielectric peak for x = 0.10 and 0.12 samples has been attributed to the

suppression of oxygen vacancies and defects. This type of reduction in oxygen vacancies and

defects could be associated with the decrease in grain size and the interaction between grain and

grain boundaries in x = 0.10 and 0.12 samples. The high temperature characteristic peak

associated with antiferromagnetic transition temperature (TN) has been observed around 285, 280,

300, 335, and 350 0C for x = 0.03-0.12 samples, respectively. This anomaly in the dielectric

constant indicates the coupling between the ferroelectric and magnetic orders which is essential

for the multiferroic materials. Further, the high temperature peak corresponding to TN shifts

towards higher temperature in x = 0.05-0.12 samples. This shift in antiferromagnetic transition

temperature could be correlated to change in the Fe-O-Fe bond angle due to structural

modification. The variation of antiferromagnetic transition temperature (TN) with the Fe-O-Fe

bond angle is given by the following relation [151]:

),cos()1( θ+= SJZSTN (3.8)

where J is the exchange constant, S is the spin of Fe3+; Z is the average number of linkages per

Fe3+ ions and θ is the Fe-O-Fe bond angle. Rietveld refinement of XRD patterns has shown an

enhancement in Fe-O-Fe bond angle with increasing Dy concentration. Therefore, the

enhancement of TN (except x = 0.03 sample) may be attributed to the increase in the Fe-O-Fe

bond angle [125].

Chapter-3

70

Figure 3.16: Variation of dielectric constant with temperature for Bi1-xDyxFeO3 ceramics with x = 0.03-0.12 at

different frequencies.

3.3.4.2 IMPEDANCE STUDIES

Figure 3.17 shows modulus spectra (imaginary part M" versus real part M') for Bi1-

xDyxFeO3 ceramic with x = 0.05, 0.07, 0.10 and 0.12 in the temperature range 120-280°C. The

direction of increasing frequency is represented by the arrow in Figure 3.17. There is only one

arc in modulus spectra for x = 0.05 and 0.07 samples at all the temperatures, suggesting the

dominance of one type of relaxation process (equivalent to a parallel RC circuit). However, two

arcs for x = 0.10 and 0.12 samples, one at low frequency and another at high frequency indicate

the presence of two relaxation processes, which is equivalent to an electrical circuit comprising

two parallel RC elements in series. The low-frequency arc represents the grain boundary response

Chapter-3

71

(Rgb and Cgb) and the high frequency arc represents the bulk response (Rg and Cg) corresponding

to the grain boundary response.

Figure 3.17: Complex modulus spectra (M' vs. M") for Bi1- xDyxFeO3 ceramics as a function of frequency at

different temperatures.

The peak in the modulus spectra is described by [152]

( ),

1 20"

+=

RC

RC

CM

ω

ωε (3.9)

where ε0 is the free space permittivity and ω = 2πf is the angular frequency. The frequency at the

maxima of semicircular arc for each RC element is given by

.

21

maxRC

fπ

= (3.10)

From equations (3.10) and (3.11), the magnitude of M"max (maximum value of M") at the peak

maxima is given by

Chapter-3

72

.

20"

maxC

Mε

= (3.11)

The values of R and C were calculated using equations (3.11) and (3.12), and are listed in Table

3.5. It is observed that the grain resistance (Rg) is much lower than the grain boundary resistance

(Rgb). The heterogeneous conduction in the grains and grain boundaries of the compounds is

closely associated with Maxwell-Wagner polarization effect [153]. Both Rg and Rgb decrease with

increasing temperature indicating a negative temperature coefficient of resistance (NTCR)

behavior, which implies a thermal activated conductivity of the material.

Table 3.5: Comparison of grain resistance (Rg) and capacitance (Cg) and grain boundary resistance (Rgb) and

capacitance (Cgb) for Bi1-xDyxFeO3 ceramics

Compositions Temperature

(°C) Rg (k Ω ) Cg (pF) Rgb (M Ω ) Cgb (pF)

120 2044 6.8 -- --

160 776 6.9 -- --

x = 0.05 200 336 7.1 -- --

240 146 7.2 -- --

280 65 7.1 -- --

120 1442 4.8 -- --

160 692 5.1 -- --

x = 0.07 200 390 5.3 -- --

240 303 5.2 -- --

280 120 5.1 -- --

120 117 17.8 476 22.2

160 41 17.2 79 20.0

x = 0.10 200 18 17.0 57 18.4

240 8 15.9 18 16.6

280 5 15.1 2 16.6

120 218 11.0 7913 20.1

160 77 10.4 418 16.9

x = 0.12 200 31 9.9 119 15.3

240 16 9.6 16 14.6

280 9 8.9 1 15.8

Chapter-3

73

The frequency dependence of Z" and M" at 200°C are shown in Figure 3.18. The

magnitude of the mismatch between the peaks of Z" and M" indicates the space charge

polarization in the samples.

Figure 3.18: Variation of Z" and M" with frequency at 200°C for Bi1- xDyxFeO3 ceramics with x = 0.05-0.12.

A significant mismatch was observed in Z" and M" peaks for x = 0.10 and 0.12 samples,

indicating the presence Maxwell-Wagner type space charge polarization arising at grain boundary

and sample-electrode interfaces [154]. The two peaks observed for M" for x = 0.10 and 0.12

samples indicate the presence of grain boundaries. The frequency dependant Z" and M" curves

indicate ideal Debye behavior for x = 0.05 and 0.07 samples and non-Debye type behavior for x

= 0.10 and 0.12 samples.

3.3.4.3 CONDUCTIVITY STUDIES

Figure 3.19 shows the ac conductivity of Bi1-xDyxFeO3 ceramics as a function of

frequency at varying temperatures from 120° to 280°C. The frequency dependence of the

conductivity can be described by the power-law relation proposed by Jonscher [155]:

Chapter-3

74

,)0()( sAωσωσ += (3.12)

where )(ωσ is the total conductivity, )0(σ is the frequency-independent dc conductivity, ω is

the angular frequency, and coefficient A and exponents are temperature and material dependent

parameters. It is observed that x = 0.05 and 0.07 samples possess a broad dc conduction region.

The conductivity of these samples increases with rising temperature due to the increased mobility

of oxygen vacancies or other structural defects [156]. The flattened region moves upward and

dominates over a larger range of frequencies. However, the step-like curves were observed with

increasing frequency for x = 0.10 and 0.12 samples. The low-frequency region corresponds to

grain boundaries and the high-frequency region is associated to grains.

Figure 3.19: Variation of ac conductivity with frequency for Bi1-xDyxFeO3 ceramics at different temperatures.

3.4 CONCLUSIONS

In this chapter, we have synthesized Dy doped BFO ceramics (i.e. Bi1-xDyxFeO3; x = 0.0,

0.03, 0.05, 0.07, 0.10 and 0.12) by the solid state reaction method. Effects of Dy substitution on

Chapter-3

75

structural, magnetic, optical, vibrational and dielectric properties of BiFeO3 were examined.

Rietveld refinement of the XRD patterns demonstrated the pure phase formation of Bi1-xDyxFeO3

ceramics for x≥ 0.05 without any structural transition up to x = 0.12. The variation in Fe-O-Fe

bond angle and intensity of Raman modes related to Fe-O bonding predicted the improvement of

magnetic properties of Dy doped samples. Dy substitution resulted in weak ferromagnetism at

room temperature and exhibited enhanced in magnetization with decreasing temperature due to

Dy3_Fe3+ and Dy3+-Dy3+ interactions for x = 0.07-0.12 samples. The remnant magnetization

varied from 0.009, 0.0233, 0.0314, 0.0544, 0.1400 and 0.2103 in x = 0.0-0.12 samples with

maximum remnant magnetization of 0.2103 emu/g for x = 0.12 sample. Significant change in

ESR spectra of x = 0.10 and 0.12 samples also suggested the existence of strong ferromagnetic

coupling between Dy3+ and Fe3+ ions. UV-visible absorption spectra showed one d-d and one C-T

bands for all samples. The intensity of 1.8 eV band increase, indicating the enhanced spin-orbital

coupling with increasing Dy concentration in BFO samples. The band gap values varied from

2.25 eV to 2.09 eV for x = 0.0-0.12 samples, in order. The decrease in the band gap is ascribed to

the breaking symmetry due to distortion in the FeO6 octahedra with increasing Dy content. FTIR

results showed the reduction in Fe-O bond length as also confirmed from Rietveld analysis. The

complex impedance study enabled us to separate the grain and grain boundary contributions in

the materials. Both the grain and grain boundary resistances decreased with increasing

temperature for all samples. Maxwell-Wagner polarization effect is responsible for the

heterogeneous conduction in the grains and grain boundaries of the compounds.