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Structural and Ferroelectric Properties of Complex Perovskites Pb(1-

x)Bax(Fe1/2Ta1/2)O3 (x = 0.00, 0.05, 0.1, 0.15)Chandrahas Bharti a; Alo Dutta a; T. P. Sinha a

a Department of Physics, Bose Institute, Kolkata, India

Online publication date: 01 December 2009

To cite this Article Bharti, Chandrahas, Dutta, Alo and Sinha, T. P.(2009) 'Structural and Ferroelectric Properties ofComplex Perovskites Pb(1-x)Bax(Fe1/2Ta1/2)O3 (x = 0.00, 0.05, 0.1, 0.15)', Ferroelectrics, 392: 1, 20 — 32To link to this Article: DOI: 10.1080/00150190903412465URL: http://dx.doi.org/10.1080/00150190903412465

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Ferroelectrics, 392:20–32, 2009Copyright © Taylor & Francis Group, LLCISSN: 0015-0193 print / 1563-5112 onlineDOI: 10.1080/00150190903412465

Structural and Ferroelectric Properties of ComplexPerovskites Pb(1-x)Bax(Fe1/2Ta1/2)O3 (x = 0.00, 0.05,

0.1, 0.15)

CHANDRAHAS BHARTI, ALO DUTTA, AND T. P. SINHA∗

Department of Physics, Bose Institute, 93/1, Acharya Prafulla Chandra Road,Kolkata - 700009, India

We have synthesized the complex perovskite oxides Pb(1-x)Bax(Fe1/2Ta1/2)O3 (where x =0.0, 0.05, 0.10 and 0.15) by two steps Coulombite precursor process in cubic phaseand studied the effect of Ba substitution in Pb(Fe1/2Ta1/2)O3 through the dielectric andferroelectric properties in the frequency range from 100 Hz to 1 MHz and in the temper-ature range from 118 to 363 K. The temperature dependence of the dielectric constantat different frequencies gives diffuse peaks which have been attributed to the occurrenceof relaxor ferroelectric behaviour in Pb(1-x)Bax(Fe1/2Ta1/2)O3. The magnitudes of ε′

m,the maximum value of dielectric constant and Tm, the temperature corresponding toε′

m are decreased with an increase of Ba2+ ion in the materials. There is evidence ofVogel-Fulcher type relaxational freezing in the samples. The analysis of real and imag-inary parts of the dielectric permittivity with frequency has been performed assuming adistribution of relaxation times as confirmed by Cole-Cole plots.

Keywords Perovskite oxide; relaxor; lead barium iron tantalate; Vogel-Fulcher

Introduction

Ferroelectric materials with perovskite like structure attract intense interests due to theirtechnological applications as non-volatile memory devices, multilayer capacitors and opto-electronics devices [1–3]. Most of these materials are lead based and are advantageous to usein technology due to their high dielectric permittivity, relatively low sintering temperaturesand temperature dependent diffuse dielectric maxima. But the presence of lead makes thesematerials toxic and is responsible for ecological hazard. So, there is a need to find theenvironmental friendly materials and considerable studies have been focused on finding oflead free [4–10] and low lead content materials [11, 12].

Many research works have been devoted to lead content perovskite materials. Amongthem, lead iron tantalate, Pb(Fe1/2Ta1/2)O3 (PFT) of general formula A(B′B′′)O3 is a wellknown relaxor ferroelectric having high dielectric constant (∼10000), temperature depen-dent diffuse dielectric maxima and magneto-electric effect [13–15]. When A-site (i.e.,Pb) of PFT is substituted by Ba resulting Ba(Fe1/2Ta1/2)O3 (BFT), it does not show anyferroelectric relaxor properties [16, 17]. Recently, some systematic substitutions of the Aand B sites [18, 19] on PFT have been done to obtain low-lead based relaxors. Thesesubstitutions lead to remarkable changes in structural and dielectric properties. This has

Received May 21, 2009.∗Corresponding author. E-mail: sinha tp@yahoo.com

20

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Complex Perovskites Pb(1-x)Bax(Fe1/2Ta1/2)O3 21

motivated us to study the structural and dielectric properties of lead barium iron tantalate,Pb(1-x)Bax(Fe1/2Ta1/2)O3 [x = 0, 0.05, 0.1, 0.15] (PBFT), where in A-site of PFT, lead ispartially substituted by barium.

It is to be mentioned that the main properties of relaxor ferroelectrics can be explainedby the formation of polar nano regions of large distribution of size and shape with shortrange order, which breaks the macroscopic symmetry in cubic phase [20]. Even at lowtemperatures, the interaction strengths of polar-regions are not enough to shift into a longrange order ferroelectric structure as they are inhabited by the quenched compositionalfluctuations and by the induced random fields. At temperatures corresponding to maximumvalue of dielectric constant, the polar nano-regions behave like ideal non-interacting dipoleswhich switch under the fluctuation of random fields and temperatures. At lower tempera-tures, the coupling between the microregions controls the kinetics of the fluctuations andthe relaxor is frozen into polar-glassy state [21, 22].

Experiment

The polycrystalline samples of PBFT were synthesized by two steps Coulombite precursortechnique. In the first step, the powders of Fe2O3 and Ta2O5 were taken in stoichiometricratio and mixed for 8 h in acetone. The mixed powder was calcined at 1200◦C for 7 h. Thecompound thus formed was mixed with PbO and BaCO3 and again the process of mixingwas repeated in acetone medium (here 4 wt% of PbO in excess was added in order tocompensate any weight loss during heating due to evaporation of Pb). For different mol%of BaCO3 such as 0.00, 0.05, 0.10, and 0.15, the mixed powders were calcined at differenttemperatures as 1000◦C, 1050◦C, 1075◦C and 1100◦C respectively for 7 h. The calcinedpowders were grinded and used to make pellets adding poly vinyl alcohol (PVA) as binder.The pellets were sintered for 5 h at the temperatures approximately 50◦C higher than thecalcination temperatures of the corresponding compositions.

The x-ray diffraction (XRD) patterns of the samples were taken at room temperatureusing a Rigaku Miniflex-II automatic X-ray powder diffractometer. Scanning electronmicrographs (SEMs) of the samples were taken at room temperature using FEI Quanta 200scanning electron microscope. The sintered pellets of the samples were polished and silverelectroded and connected to an LCR meter (Hioki-3532) for dielectric measurement. Thefrequency dependence of capacitance and loss tangent was measured in the frequency rangefrom 100 Hz to 1 MHz and in the temperature range from 118 to 363 K. All the dielectricdata were collected while heating at a rate of 0.5◦C min−1. The temperature was controlledby Eurotherm 2216e programmable temperature controller connected with the oven.

Results and Discussion

The XRD patterns of PBFT are shown in Fig. 1 where all the reflection peaks of the x-rayprofiles are indexed and lattice parameters are determined by using a least-squares methodwith the help of a standard computer program (Crysfire). A good agreement between the ob-served and calculated inter-planer spacings (d-values) suggests that the materials have cubicstructure with lattice parameters a = 4.077, 4.012, 4.015 and 4.015 for x = 0.00, 0.05, 0.10and 0.15 respectively. XRD analysis illustrates that all the compositions have single phase.

The SEMs of the sintered samples are shown in Fig. 2 which indicates that the distri-bution of elements in the materials is uniform and no Pb-rich phase is occurred at the grainboundaries. The average grain size of the samples is found to be ∼10 µm.

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22 C. Bharti et al.

Figure 1. X-ray diffraction pattern of Pb(1-x)Bax(Fe1/2Ta1/2)O3 taken at room temperature for x =0.00 (a), 0.05 (b), 0.10 (c), 0.15 (d).

The temperature dependence of dielectric constant (ε′) at various frequencies forpure and Ba-doped PFT is shown in Fig. 3. The diffusiveness of the dielectric peaks forall compositions suggests a microscopic heterogeneity in the compounds with differentlocal Curie points. It is observed that with increasing frequency, the peak value of dielectric

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Complex Perovskites Pb(1-x)Bax(Fe1/2Ta1/2)O3 23

Figure 2. Scanning electron micrographs of Pb(1-x)Bax(Fe1/2Ta1/2)O3 with x = 0.00 (a), 0.05 (b),0.10 (c), 0.15 (d) taken at room temperature.

constant, ε′m decreases, which is a typical nature of relaxor behaviour [23]. The temperature

(Tm) corresponding to ε′m at 100 kHz for all the compositions is listed in Table 2, which

indicates that both Tm and ε′m are dependent on percentage of doping. The magnitudes of

ε′m and Tm decrease with the increase of Ba2+ ion substitution at Pb-site which prevents its

neighbouring TaO6 octahedra from being coupled to the ferroelectric network. Since theBa2+ ions will affect the coupling of TaO6 octahedra to the network uniformly, irrespectiveof their B ion environment, the decrease of ε′

m is indicative of a general decrease in thenumber of TaO6 octahedra coupled together in the ferroelectric network. The decreasein the value of Tm due to the substitution of Ba2+ for Pb2+ ions can be interpreted asthe progressive breakdown of relatively large lead rich micro-regions into smaller leadrich regions. The strength of the ferroelectric coupling within a lead rich micro-region isdiminished as its size decreases [24, 25]. At temperatures far above Tm, a monotonousincrease in the value of ε′ caused by electrical conduction is observed.

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24 C. Bharti et al.

Figure 3. Temperature variation of dielectric constant of Pb(1-x)Bax(Fe1/2Ta1/2)O3 with x = 0.00 (a),0.05 (b), 0.10 (c), 0.15 (d) at various frequencies.

For PBFT to be relaxors, the degree of diffuseness can be estimated from ε′(T) datafor T > Tm using a modified Curie Weiss law proposed by Uchino and Nomura describedas [26]

1

ε′ − 1

ε′m

= (T − Tm)γ

c, (1)

where c is the Curie constant. The value of γ lies in between 1 and 2, where γ = 1 corre-sponds to the normal ferroelectric phase and γ = 2 indicates an ideal relaxor nature. Thehigher the γ values, the more relaxor behaviour the materials display [27]. The logarithmicplots related to the Eq. (1) at two different frequencies for all the compositions are shownin Fig. 4, where the slope of the curves gives the value of γ . The values of γ are foundto be 1.82, 1.853, 1.904, and 1.79 at 175 kHz for x = 0, 0.05, 0.1 0.15 respectively. Theobserved values of γ indicate the relaxor behaviour of Pb(1-x)Bax(Fe1/2Ta1/2)O3.

Figure 5 shows a plot of the inverse of ε′ as a function of temperature at 635 kHz. Adeviation from Curie-Weiss law starting at the temperature Tdev can be clearly seen. Theextrapolation of the un-deviated part of the curve meeting at temperature axis gives theCurie-Weiss temperature, Tcw. The values of Tdev are listed in Table 2 along with Tcw. A

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Complex Perovskites Pb(1-x)Bax(Fe1/2Ta1/2)O3 25

Figure 4. log (1/ε′–1/εm′) vs. log (T-Tm) plot for Pb(1-x)Bax(Fe1/2Ta1/2)O3 with x = 0.00 (a), 0.05

(b), 0.10 (c), 0.15 (d). The symbols represent the experimental data points and the solid lines show alinear fit to the data points.

large deviation from Curie-Weiss behaviour and the observed values of γ suggest that thesystem Pb(1-x)Bax(Fe1/2Ta1/2) is relaxor.

In relaxor materials, an empirical Vogel-Fulcher (VF) relationship can be used toaccount for the dielectric relaxation which appears as a result from thermally activatedpolarization reversals between two equivalent variants. Based on this model, the polarizationflipping frequency υm is related to the activation energy Ea as [28, 29]

υm = υ0 exp

[− Ea

kB (Tm − Tf )

](2)

where, υ0 is the attempt frequency, kB is the Boltzmann constant, Tf is freezing tem-perature and Ea is the activation energy (the barrier energy between two equivalent po-larization states). Figure 6 shows the temperature dependence of relaxation frequency,plotting Tm vs. lnυm, and the best fitting to the experimental data obtained using Eq. (2)

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26 C. Bharti et al.

Figure 5. The inverse dielectric constant (1/ε′) as a function of temperature at 635 kHz forPb(1-x)Bax(Fe1/2Ta1/2)O3 with x = 0.00 (a), 0.05 (b), 0.10 (c), 0.15 (d). The symbol represents theexperimental data points and the solid line shows fitting of the Curie-Weiss law.

is shown by solid line. The values of υ0 and Tf obtained from the fitting are given inTable 1.

One of the characteristics of the relaxor ferroelectrics is the appearance of a lowfrequency dielectric loss peak below Tm in the ε′′ versus logω plots [30–35]. Figure7 shows the ε′′ versus logω plots at various temperatures for PBFT. The loss peaksshift to lower frequencies on decreasing the temperature, indicating the thermally acti-vated nature of the dielectric relaxation. An increase in the value of ε′′ in the lower-frequency region may be due to the increase in ionic conductivity resulting from thedisordering of mobile cations in the oxygen-octahedral skeleton. It seems clear that thewidth of the loss peaks in Fig. 7 can not be accounted for in terms of a monodesper-sive relaxation process, and points towards the possibility of a distribution of relaxationtimes.

One of the most convenient ways for checking the polydispersive nature of dielectricrelaxation is through complex Argand plane plots of ε′′ against ε′, usually called Cole-Coleplots [36]. For a pure monodispersive Debye process, one expects semicircular plots withthe centre located on the ε′-axis, whereas for poly-dispersive relaxation, these complexArgand plane plots are close to circular arcs with end-points on the axis of reals and the

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Tabl

e1

Var

ious

para

met

ers

obta

ined

from

the

anal

ysis

ofre

laxo

rbe

havi

our

ofPB

FT.

Sam

ples

Pb1-

xB

a x(F

e 1/2

Ta1/

2)O

3

Tm

(K)

at10

0kH

zε

′ mat

100

kHz

Tf

(K)

Tcw

(K)

100

kHz

υ0

(Hz)

Tde

v(K

)E

a(e

V)

x=

0.00

265.

377

1074

5.9

256

282

8.5

×10

1231

20.

46x

=0.

0518

7.93

2123

.87

173.

512

0.43

2×

1011

214

0.18

x=

0.10

173.

512

75.2

715

4.7

65.4

5×

1010

209

0.21

x=

0.15

142

775.

613

215

.34

8×

1012

182

0.2

27

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28 C. Bharti et al.

Figure 6. Frequency dependence of Tm for Pb(1-x)Bax(Fe1/2Ta1/2)O3 with x = 0.00 (a), 0.05 (b),0.10 (c), 0.15 (d). The symbols indicate the experimental data points and the solid line is the fit toVogel-Fulcher relationship.

centre below this axis. The complex dielectric constant in such situations is known to bedescribed by the empirical relation [36]

ε∗ = ε′ − iε′′ = ε∞ + εs − ε∞1 + (iωτ )1−α

(3)

where εs and ε∞ are the low-and high–frequency values of ε′, α is a measure of thedistribution of relaxation times, τ ( = ω−1) is the most probable relaxation time and ω =2πυ. The parameter α is determined from the location of the centre of the Cole-Cole circlesof which only an arc lies above the ε′-axis. The Cole-Cole plots as shown in Fig. 8 confirmthe polydispersive nature in the dielectric relaxation of PBFT.

We can look at the distribution of relaxation times from yet another angle. If g (τ ,T) isthe temperature-dependent distribution function for relaxation times, the complex dielectric

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Complex Perovskites Pb(1-x)Bax(Fe1/2Ta1/2)O3 29

Figure 7. Frequency dependence of ε′′ of Pb(1-x)Bax(Fe1/2Ta1/2)O3 with x = 0.00 (a), 0.05 (b), 0.10(c), 0.15 (d) at various temperatures.

constant can be expressed as [37]

ε∗ − ε∞ = ε (0, T) ∫ g (τ, T )d(ln τ )

1 − iωτ(4)

where ε(0,T) is the low frequency dielectric constant. For a broad distribution of relaxationtime, ε′′(ω,T) can be approximated as [38, 39]:

ε′′ (ω, T) ∼= π

2ε(0, T )g(τ, T ). (5)

Thus the spectrum of dielectric loss gives direct information about g(τ, T ). In the limitof τmin ≤ 1/ω ≤ τmax, one can also obtain an important simple relation between real andimaginary parts of the dielectric permittivity [40, 41]:

ε′′ (ω, T) ∼= π

2

∂ε′(ω, T)

∂(lnω). (6)

We have used our experimental data to verify the validity of the assumptions madeto obtain Eq. (5). The results obtained are shown in Fig. 9. A good agreement betweendirectly measured values of ε′′ and those calculated from the dispersion of ε′ using Eq. (6)suggests that the spectrum g(τ, T ) is broad in nature.

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30 C. Bharti et al.

Figure 8. ε′ vs. ε′′ plots (Cole-Cole plots) of Pb(1-x)Bax(Fe1/2Ta1/2)O3 with x = 0.00 (a), 0.05 (b),0.10 (c), 0.15 (d).

In perovskite type compounds the relaxor behaviour appears when at least two cationsoccupy the same crystallographic site A or B. In PBFT, the ionic radius of Fe (3+)is larger than that of Ta (5+). Therefore, an inhomogeneous distribution results at theB-site of the structure. A cationic disorder induced by B-site substitution is always re-garded as the main derivation of relaxor behaviour. Also, in the studied composition ofPb(1-x)Bax(Fe1/2Ta1/2)O3, both Pb2+ and Ba2+ ions occupy the A-sites of the perovskitestructure and are ferroelectrically active. These cations are off-centered in the octahedralsite giving rise to a local dipolar moment. The replacement of Pb2+ by Ba2+ ions leadsto the formation of dipolar impurities and defects which have a profound influence on thestatic and dynamic properties of this material.

The relaxor behaviour as observed in PBFT can be induced by many reasonssuch as microscopic compositions fluctuation, the merging of micro-polar regions,or a coupling of order parameter and local disorder mode through the local strain.Vugmeister and Glinichuk [42] have reported that the randomly distributed electri-cal field of strain in a mixed oxide system is the main reason leading to the relaxorbehaviour.

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Complex Perovskites Pb(1-x)Bax(Fe1/2Ta1/2)O3 31

Figure 9. Comparison of the measured frequency dependent imaginary part of the dielectric constantof Pb(1-x)Bax(Fe1/2Ta1/2)O3 having x = 0.00 (a), 0.05 (b), 0.10 (c), 0.15 (d) with the calculated onesfrom Eq. (6) at different temperatures, where the symbols represent the experimental data and thesolid lines represent the calculated data points.

Conclusions

The relaxor properties of complex perovskite oxide Pb(1-x)Bax(Fe1/2Ta1/2)O3 prepared bytwo steps Coulombite precursor process have been studied in the frequency range from100 Hz to 1 MHz and in the temperature range from 118 to 363 K. The temperature depen-dence of the dielectric constant at different frequencies gives diffuse peaks which have beenattributed to the occurrence of relaxor ferroelectric behaviour in Pb(1-x)Bax(Fe1/2Ta1/2)O3.The magnitudes of ε′

m, the maximum value of dielectric constant and Tm, the temperaturecorresponding to ε′

m are decreased with an increase of Ba2+ ion in the materials. Thedecrease in the values of ε′

m and Tm with an increase of Ba2+ ion has been attributed toprevent the TaO6 octahedra from being coupled to the ferroelectric network. There is anevidence for Vogel-Fulcher type relaxational freezing for all the compositions. The analysisof real and imaginary parts of the dielectric permittivity with frequency has been performedassuming a distribution of relaxation times as confirmed by Cole-Cole plots.

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