Materials Research Bulletin 44 (2009) 1405–1410

Investigation of BiFeO3 modified PbTiO3–Bi(MgZr)O3-based complex perovskiteceramics

Seema Sharma a,*, D.A. Hall b, Abhinav Sinha c

a Ferroelectrics Research Laboratory, Department of Physics, A.N. College, Patna 800013, Indiab Materials Science Center, School of Materials, University of Manchester, Manchester, UKc Electronics and Communication Engineering Department, National Institute of Technology, Trichy 620015, India

A R T I C L E I N F O

Article history:

Received 9 May 2008

Received in revised form 13 October 2008

Accepted 25 November 2008

Available online 9 December 2008

Keywords:

A. Ceramics

C. X-ray diffraction

D. Dielectric properties

D. Ferroelectricity

A B S T R A C T

Bismuth containing crystalline solutions of (1 � x)Bi(MgZr)0.5O3–xPbTiO3 (BMZ–PT) and

[(BiFeO3)y � (BiMg0.5Zr0.5)1�y]x � [PbTiO3]1�x (BMZ–BF–PT) have been developed using conventional

ceramic technology. X-ray diffraction analysis reveals that both the systems possess a perovskite

structure, in which tetragonal to rhombohedral phase transformation appears for x = 0.55 in BMZ–PT and

y = 0.20 in BMZ–BF–PT systems. SEM photographs reveal a uniform grain size distribution in the solid

solution matrix with the presence of ferroelectric domains in few of the compositions. Ferroelectric

hysteresis (polarization–electric field, P–E) loops reveal that increase in BiFeO3 in BMZ–PT systems

results in a decrease in residual polarization of the system with change and distortion in the shape of the

(P–E) loops.

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1. Introduction

Bismuth perovskites have been attracting attention as a familyof piezoelectric ceramics in place of the widely used Pb(Zr, Ti)O3

(PZT) system. The advantages of Bismuth perovskites over PZT areenvironmentally more friendly materials, a higher mechanicalstrength and Curie temperature. In this family of materials,bismuth sodium tantanate (BNT) or BNT-based crystalline solu-tions are considered to be one of the best candidates for lead free orlow-lead-content piezoelectric materials due to their strongferroelectricity [1–4]. However, BNT-based ceramics are difficultto pole due to their large coercive field, and they have a limitedrange of working temperatures due to a ferroelectric (FE) toantiferroelectric (AFE) transition in the vicinity of �100 � 22 8C.

Early studies on perovskite-structured Bi(Me)O3–PbTiO3 solidsolutions were mostly concerned with compounds containingtransition metal Me ions such as Fe3+ and Mn3+ [1,2]. Thesematerials show high temperature ferroelectric–paraelectric phasetransitions but the piezoelectric and dielectric properties werelimited or unknown due to their high electrical conductivity, whichprevented poling of the ceramics. Recently, Eitel et al. [3,4]investigated more stable valence B-site cations in Bi-basedperovskites and demonstrated that a material, BiScO3–PbTiO3

(BS–PT), has excellent piezoelectric coefficients (d33–500 pC/N)

* Corresponding author. Tel.: +91 6122261545; fax: +91 6122540482.

E-mail address: 26.seema@gmail.com (S. Sharma).

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doi:10.1016/j.materresbull.2008.11.025

coupled with a high phase transition temperature (Tc = 450 8C) atthe morphotropic phase boundary (MPB) between the rhombohe-dral (R) and tetragonal (T) phases. A recent transmission electronmicroscopy (TEM) investigation revealed that the FE domainstructures and electron diffraction patterns in BS–PT were verysimilar to those in the PbZrO3–PbTiO3 (PZT) solid solution [5]. Theobjective of this article is to interpret the structure–propertyrelations in a ferroelectric system (1 � x)Bi(MgZr)0.5O3–xPbTiO3

(BMZ–PT). In addition to this, the effect of the addition of BiFeO3

(BF) on microstructural and ferroelectric properties of BMZ–PTbinary system has been investigated in this paper.

It must be noted that the introduction of BF to the binaryperovskite BMZ–PT creates a ternary system BMZ–BF–PT which isalso a perovskite (explained later on by the X-ray diffractograms).

Let us introduce a basic formula for the ternary perovskiteaccording to stoichiometry:

½ðBiFeO3Þy�ðBiMg0:5Zr0:5Þ1�y�x� ½PbTiO3�1�x

It is here that we must clearly state that by the notation ‘‘45–20–55’’, we mean x = 0.45 and y = 0.20.

Thus, the concentration of BF is stated as its contribution toBMZ–PT and not to the complete ternary system.

2. Experiment

Polycrystalline samples of BMZ–PT designated as 35–65, 40–60,45–55, 50–50 and BMZ–BF–PT (BMZ = 45%, BF = 0/05/10/15/20/25/50% and PT = 55%) designated as 45–0–55, 45–05–55, 45–10–

S. Sharma et al. / Materials Research Bulletin 44 (2009) 1405–14101406

55, 45–20–55, 45–25–55 and 45–50–55 were prepared by hightemperature solid-state reaction technique. PbO, MgO, TiO2, Bi2O3,Fe2O3 and ZrO2 (all Aldrich AR grade) were used as the startingmaterials. Stoichiometric weights of all starting materials weremixed and ball milled with distilled water for 48 h, using zirconiaballs as the grinding media. After drying in vacuum, calcinationwas carried out at 800 8C for 2 h in air followed by ball milling anddrying. A single-phase formation was confirmed by the X-raydiffraction (XRD) technique. The calcined powder was pressed at100 MPa to form cylindrical pellets using a cold isostatic press(CIP). The pellets were then sintered at 1050 8C for 1.5 h. A PbO-rich atmosphere was maintained by placing PbZrO3 powder in analumina boat near the test samples in closed crucible configura-tions in order to minimize the lead loss during sintering. XRDanalysis was performed on a PW3710 Philips diffractometer usingCu Ka (l = 0.15405 nm) radiation in order to examine the phasespresent in the system. Analysis of the diffraction patterns wasconducted using Celref V3 and XFIT software. Sintered pellets werepolished to a 1 mm finish and thermally etched for 30 min at atemperature 100 8C below the sintering temperature. Polishedsurface microstructure was examined by scanning electronmicroscopy (SEM) (XL30FEG-Philips).

Ferroelectric measurements were performed as a function oftemperature for all BMZ–BF–PT compositions. High field measure-ments were conducted at room temperature (22 � 2 8C) undersilicon oil using a computer-controlled function generator (HP33120A) and a high voltage amplifier (Trek 609D-6). A currentamplifier (Stanford Research Systems Model SR570) was used tomeasure the induced current, which was then integrated numericallyto yield the charge Q and then the polarization P. The appliedfield and induced current waveforms were downloaded to the PCusing a Tektronix TDS 420 DSO. Further details of the measuremethod were given in an earlier publication [6]. The measurementswere carried out using a ‘burst mode’ waveform comprisingtwo complete sinusoidal cycles. This procedure was used inpreference to a continuous waveform in order to avoid field-forcesdeageing effects [7]. The effective high field dielectric coefficientswere determined from the measured P–E loops using the methoddescribed previously [8].

3. Results and discussions

Fig. 1 shows the room temperature XRD (Cu Ka radiation,l = 1.5405 A) patterns of BMZ–PT (35–65, 40–60, 45–55 and 50–50) sintered samples. The peaks in the XRD patterns were found tobe sharp with distinct diffraction peaks. The diffraction lines for thesystem were indexed in different crystal systems and unit cell

Fig. 1. XRD patterns of BMZ–PT (35–65, 40–60, 45–55 and 50–50) samples sintered

at 1050 8C.

configurations using a computer programme package POWD-MULT. Standard deviations, SDd = dobs � dcal where d is the inter-planar spacing, were found to be minimum for tetragonal andrhombohedral structures. All XRDs of BMZ–BF–PT system reveal apure perovskite structure.

A change of structure from rhombohedral to tetragonalbetween 45–55 and 40–60 compositions is evident from theXRD patterns. Hence, it is inferred from this that morphotropicphase boundary (MPB) lies between these two compositions. Thecrystal structure as determined from the XRD technique showsthat there was a change in structure from rhombohedral totetragonal structure for BiFeO3 from 15% to 20%. Thus it may beinferred that MPB lies within this region. Tetragonality increases asthe BiFeO3 content increases in the respective compositions (asshown in Fig. 2(a) and (b)). Increase in tetragonality with BFcontent has also been reported earlier [9–11]. The relative densityof all the samples were found to be�95% of the theoretical density.

Fig. 3 shows SEM photographs of BMZ–BF–PT samples. Thesefigures show the uniform grains in the solid solution matrix. Here,black areas are pores and the thin white filaments are a liquidphase of PbO–Bi2O3 that appeared during sintering situated at thegrain boundary region. The liquid phase is in fact an amorphousphase resulting from sintering. The average grain size of all thecompositions was found to be in the range of 1.25–1.45 mm.Ferroelectric domains are visible in some of the compositions andthey diminish with increase in BF content.

Ferroelectric hysteresis curves for different compositions ofBMZ–BF–PT at different temperatures are shown in Fig. 4(a)–(d).P–E loops are more predominant at low temperatures. Theremanent polarization Pr (=2.75 cm�2 for BF = 0.05, at 50 8C)was found to increase and coercive field was found to decreasewith temperature till 100 8C with constriction in the loops. Afterthat at 150 8C the spontaneous and remanent polarizationdecreases and the loops become more and more conductive asthe temperature increases. The decrease in coercive field is due tothe defects created near the domain walls. Similar effects wereobserved by Hagemann [12] and Godefroy et al. [13] for Fe dopedBaTiO3. They showed that Fe addition acts as an obstacle to domainwall motion. At 150 8C the loops have a rounded appearance; this isdue to the increase in conductivity at higher temperatures. In thiscase, the ferroelectric behaviour starts to become less clearbecause the electrical response is dominated by the conductioneffects.

Effect of BF on BMZ–PT is to decrease the spontaneous andresidual polarization of the system. This is expected as BF is ahardener and hence the loops get constricted. For the compositionswith increasing Fe content, we never really approach saturation sothat the polarization values generally remain low and the loopsthat we observe are obtained in the sub-coercive field region.Presence of the hardener impedes the domain wall mobility ordomain switching leading to which is evident from the curves forthe various compositions. As the BF is introduced in the crystallattice of BMZ–PT the cut-off low field value increases significantly.

Fig. 5 shows the dielectric-temperature curves of[(1 � x)Bi(MgZr)0.5O3 � xPbTiO3, x = 0.55–0.80], BMZ–PT samplesat different frequencies. Samples with compositions nearest to theMPB did not display the largest dielectric constant, but highestdielectric constant was measured in samples with a compositioncloser to high value of PT. This phenomenon has previously beenobserved for high Curie temperature BS–PT system, where it wasexplained in terms of the connectivity between the TiO6

octahedron and ScO6 octahedron in the perovskite structure [7].Fig. 6(a)–(c) corresponds to the typical compositions withBF = 20%, 25% and 50%, respectively for BMZ–BF–PT system. Curietemperature increased with increasing BiFeO3 content in BMZ–BF–PT compositions. Dielectric permittivity decreases with increase in

Fig. 2. (a) XRD patterns of BMZ–BF–PT (BF = 0, 0.05, 0.10 and 0.15) samples sintered at 1050 8C. (b) XRD patterns of BMZ–BF–PT (BF = 0, 0.20, 0.25 and 0.50) samples sintered

at 1050 8C.

S. Sharma et al. / Materials Research Bulletin 44 (2009) 1405–1410 1407

frequency and the same type of frequency dependence is found inmany ferroelectric ceramics [14,15]. Maximum dielectric permit-tivity increases with increase in BF content. Dielectric permittivityincreases at higher temperatures for different compositions due to

Fig. 3. SEM photographs of

the presence of space charge polarization present at lowfrequencies and high temperatures.

The temperature variation of loss tangent of the BF modifiedBMZ–PT compounds is shown in Fig. 7. The sharp rise in dielectric

BMZ–BF–PT samples.

Fig. 4. P–E hysteresis loops of BMZ–BF–PT ceramics. (a) Room temperature P–E curves for BMZ–BF–PT samples. (b) P–E curves for BMZ–BF–PT samples at 50 8C. (c) P–E curves

for BMZ–BF–PT samples at 100 8C. (d) P–E curves for BMZ–BF–PT samples at 150 8C.

Fig. 5. Dielectric curves of BMZ–PT compounds.

S. Sharma et al. / Materials Research Bulletin 44 (2009) 1405–14101408

loss may be due to the scattering of thermally activated chargecarriers and some defects in the samples. At higher temperaturethe conductivity begins to dominate, which in turn is responsiblefor the rise in loss tangent (i.e. conductivity a loss tangent). At hightemperature (paraelectric phase) the contribution of ferroelectricdomain walls to loss tangent decreases, which is also a responsiblefactor for the rise in loss tangent at high temperature [16,17].

The temperature variation of AC conductivity of BMZ–BF–PTcompounds is shown in Fig. 8. The AC conductivity of thecompound, a thermally activated process is expressed assAC = so exp[�Ea/kT] where so = pre-exponential factor, Ea = acti-vation energy, k = Boltzmann constant and T = absolute tempera-ture. The AC electrical conductivity has been calculated bys = veoetan d, where eo is the vacuum dielectric constant and vis the angular frequency.

AC conductivity was found to increase with increase intemperature showing a NTCR effect. A negative temperaturecoefficient (NTC) occurs when the thermal conductivity of amaterial rises with increasing temperature, typically in a definedtemperature range. This behaviour has been found in many

Fig. 6. Dielectric curves of (a) 45 BMZ–20 BF–55 PT compounds, (b) 45 BMZ–25 BF–

55 PT compounds and (c) 45 BMZ–50 BF–55 PT compounds.

Fig. 7. Dielectric loss tangent curves of BMZ–BF–PT compounds.

Fig. 8. Temperature–AC conductivity curves of BMZ–BF–PT compounds.

S. Sharma et al. / Materials Research Bulletin 44 (2009) 1405–1410 1409

ferroelectric materials. With respect to the increase in BF contentin the BMZ–PT compound, AC conductivity was found to increase.This is due to the fact that bismuth ferrite has a large leakagecurrent and its inclusion effects the conductivity of the parentcompound [18,19]. At high temperature, the donor cations have amajor part to play in the conduction process. The donors havecreated a level (i.e. the band donor level), which is much nearer tothe conduction band. Therefore, only a small amount of energy isrequired to activate the donors. In addition to this, a slight changein stoichiometry in the multi-metal complex oxides causes thecreation of a large number of donors or acceptors, which createsdonor- or acceptor-like states in the vicinity of conduction orvalence bands. These donors or acceptors can also be activatedwith a little energy [20].

4. Conclusions

A new solid solution ferroelectric system BMZ–BF–PT wasinvestigated and was found to possess pure perovskite structure.Tetragonality increases with increase in BF content. Morphotropicphase boundary was found to lie in the region for x = 0.40–0.45 forBMZ–PT system and for y = 0.15–0.20 for BMZ–BF–PT system.Effect of BF on BMZ–PT is to decrease the spontaneous and residualpolarization and increase the Curie temperature of the system.Ferroelectric behaviour starts to become less clear at hightemperatures as electrical response is dominated by the conduc-tion effects. Temperature dependence of AC conductivity showsthe NTCR effect in the compounds. This system may be a potentialcandidate for its application in electronic industries.

Acknowledgements

The authors wish to thank Royal Society, London, UK andDepartment of Science and Technology, Govt. of India for providingthe financial support under INDO-UK networking scheme.

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