06) 1459–1462www.elsevier.com/locate/matlet

Materials Letters 60 (20

Dielectric studies of La and Pb doped SrBi2Nb2O9 ferroelectric ceramic

Vaibhav Shrivastava a, A.K. Jha a,⁎, R.G. Mendiratta b

a Department of Applied physics, Delhi College of Engineering, Delhi-110042, Indiab Department of Physics, ABES Engineering College, Ghaziabad-201001, India

Received 17 June 2005; accepted 14 November 2005Available online 5 December 2005

Abstract

In this paper, structural and dielectric studies of Sr1−xAxBi2Nb2O9 (A=La, Pb and x=0.2) ceramics are reported. Tetragonal straindecreases on introduction of lanthanum while it increases for the lead doped sample. Addition of lead increases Curie temperature and samplewith lanthanum content shows remarkable decrease in Curie temperature with broadened dielectric peak. The sample with x=0.0 shows arapid fall in dielectric constant value above 100 KHz. The lanthanum and lead doped samples show reduced dielectric loss values withfrequency.© 2005 Elsevier B.V. All rights reserved.

Keywords: Tetragonal strain; X-ray diffractograms; Microstructure; Dielectric constant; Dielectric loss

1. Introduction

Aurivillius ceramics are known [1–3] for their excellentfatigue characteristics and high Curie temperature. The unitcell in these ceramics is orthorhombic [4]. The ceramicswith such a unit cell system are potential materials formanufacturing ferroelectric memories with high remnant/spontaneous polarization and piezoelectric coefficients [5,6].These ceramics suffer from high dielectric loss due to theevaporation of bismuth oxide during sample preparation,which limits its use for the above said applications. AmongAurivillius ceramics SrBi2Nb2O9 (SBN) is a material withless distorted octahedron structure. In the present work, thesame structure has been studied for its structural anddielectric properties with doping of lanthanum and lead.The authors felt a need of comparative study to understandthe smart characteristics [7,8] of these ceramics. Thedoping of lanthanum for x=0.1 has been studied by Forbesset al. [9] and insignificant changes are observed in theproperties. In the present work, the studied composition isSr1−xAxBi2Nb2O9; A=La, Pb and x=0.0 and 0.2.

⁎ Corresponding author. Tel.: +91 11 30972376; fax: +91 11 27871023.E-mail address: akj6467@indiatimes.com (A.K. Jha).

0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2005.11.045

2. Experimental

Samples were prepared using solid-state reaction methodtaking SrCO3, La2O3, PbO, Bi2O3 and Nb2O5 (all fromAldrich) in stoichiometric proportions. Mixtures were calcinedat 900 °C in air. Polyvinylalcohol (Aldrich) 2 wt.% solutionwas mixed in all powdered samples in proportions and thenmolded into disc shape pellets by applying a pressure of 270MPa. Pellets were sintered at 1150 °C for 2 h. X-raydiffractograms were taken for all calcined and sintered sampleson Philips X-ray diffractometer PW 1710 using CuKα radiationof wavelength 1.54439 Å in the range 10°≤2θ≤70° at ascanning rate of 0.05°/s. Scanning electron microscopephotographs (SEM) are recorded using Cambridge StereoScan 360 instrument. All pellets were coated using silver pasteand cured at 600 °C for half an hour. Dielectric measurementswere taken on an HP 4192A Impedance Analyzer operating atoscillation level of 1 Vand a frequency of 100 KHz was chosenfor Curie temperature measurements.

3. Results and discussion

Fig. 1 shows the X-ray diffractograms of sintered x=0.0, 0.2lanthanum and lead samples. The formation of desired layeredperovskite phase is confirmed in all the samples. From the observedintensity ratio of different peaks, it is concluded that a slightly different

Fig. 1. X-ray diffractograms of undoped SBN and 0.2 lanthanum and lead doped samples.

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phase is formed in lead doped sample as compared to undoped andlanthanum doped sample. In a report published earlier the set of (hkl)indices in 105 system ((hkl) indices of maximum intensity peak) isreported to be the set of pseudo tetragonal phase and (hkl) indices setwith 115 system is reported to be orthorhombic [10]. Moret et al. [11]have reported about the coexistence of orthorhombic and tetragonalphases in these ceramics. The lattice parameters a and cwere calculatedfrom d-values using standard deviation method. The introduction ofsmaller lanthanum yields a decrease of 0.28% in lattice parameter c andof 0.14% in lattice parameter a. The tetragonal lattice strain, (c /a) ratio[12,13] has been determined for all the three samples (Table 1). Theeffect of c /a ratio on Curie temperature is discussed later.

Fig. 2 shows the variation of dielectric constant (ε) values as afunction of frequency ranging from 50 Hz to 1 MHz at roomtemperature for all samples. The lead doped sample shows almostconstant values of dielectric constant over the frequency range. Theundoped sample shows a rapid decrease in dielectric constant values upto 1 KHz and a slower decrease at higher frequencies. The sample withlanthanum content also shows a small decrease in dielectric constantvalues with increase in frequency. In undoped sample, steep decrease indielectric constant values with increase in frequency up to 100 KHz isattributed to the relaxation and resonance absorption phenomena [15].The range of frequency near 1 KHz is known to show relaxationabsorption in dielectric constant values because of space chargepolarization. The space charge is known to saturate up to 1 KHz;however, a few ceramics show its complete saturation up to 100 KHz.

Table 1Tetragonal strain (c /a), ionic radius and bond strengths of elements

S. No. Composition c /a Ionic radius withvalence state14

Bond strengths withoxygen (kJ/mole)14

1 SrBi2Nb2O9 6.46 Sr2+ 1.44 Å Sr2+ 4262 Sr0.8La0.2Bi2Nb2O9 6.43 La3+ 1.36 Å La3+ 7993 Sr0.8Pb0.2Bi2Nb2O9 6.47 Pb2+ 1.49 Å Pb2+ 386

The introduction of lanthanum is known to increase dielectric constant[16] compared to that of the undoped sample because of its high ionicpolarazability. The doping of two lanthanum ions onto strontium siteswould introduce two units of free positive charge in the structure, i.e.,one strontium vacancy. Therefore, doping of lanthanum enhances thecreation of strontium vacancies in the structure to maintain chargeneutrality [9]. The absence of pronounced space charge polarization indoped samples is possibly because of charge neutrality in lanthanumdoped sample and similar valence state of lead with that of strontium,respectively [17].

In Fig. 3, dielectric constant versus temperature behavior of studiedcompositions is shown; these observations have been plotted usingMicrocal Origin 5.1. These measurements were taken at frequency 100

Fig. 2. Dielectric constant versus frequency behavior of ceramics.

Fig. 3. Dielectric constant versus temperature plots. Fig. 6. Dielectric loss versus frequency behavior of all samples.

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KHz above which space charge and orientational polarizations ceaseoff and only ionic and electronic contributions persist [15]. Theseceramics are known to have single-phase ferroelectric to paraelectricphase transitions corresponding to Curie temperature where dielectricconstant is maximum (εmax) [18,19]. The Curie temperature decreaseson introduction of lanthanum and increases slightly on doping lead. In

Fig. 4. Dielectric loss versus temperature plots.

Fig. 5. SEM photographs of undoped S

addition, the dielectric peak is broadened on introduction of lanthanum,which is known to be observed due to either as defect induced [20] orrelaxor type phase transition [21] where formation of micro polarregions occurs and each of such regions has its own transitiontemperature. The observed change in Curie temperature is attributed tochange in tetragonal lattice strain [12]. In ferroelectric materials,lanthanum doping is known [22] to reduce lattice distortions byestablishing stable non-ferroelectric phases e.g. tetragonal phase. Thesame is observed in the present work as indicated from tetragonallattice strain (c /a) values in Table 1.

Dielectric loss versus temperature measurements are shown in Fig.4. The dielectric loss values of all the three samples are nearly same upto 320 °C beyond which remarkable changes are observed. Above thistemperature, the lead doped sample shows maximum loss andlanthanum doped sample shows low loss values. The undoped samplebehaves in an intermediate manner. The reason for observed lossbehavior of doped sample could be the high solid solubility oflanthanum [14] and its non-volatile nature, due to which possibly betterconnectivity of grains is obtained and this results in low dielectric lossvalues at high temperatures [23]. This is indeed observed in the SEMphotographs of the undoped SBN sample and lanthanum doped SBNsample (Fig. 5). The lead doped sample shows higher loss valuespossibly because of the defects produced in the system due to theevaporation of volatile lead at high temperatures.

Fig. 6 shows the dielectric loss as a function of frequency at roomtemperature. The undoped SBN sample shows large loss, which

BN and lanthanum doped sample.

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decreases with a dip frequency around 80 KHz and thereafterincreases again; which is known as dielectric dispersion behavior[15]. Among the doped samples, lead doped sample shows low lossvalues throughout the frequency range. The lanthanum doped sampleshows dielectric loss behavior similar to that of undoped samplethough of much lesser magnitude. The increase in loss values forundoped sample beyond 100 KHz is attributed to resonance lossesdue to frequency resonance. The dielectric loss behavior for sampleswith large space charge is described by relaxation loss mechanism,which responds at lower frequencies as ion-jump/electron hopping(b100 KHz) [24]. The resonance losses, if existing in materials,respond at higher frequencies (N100 KHz) as frequency of appliedfield matches with the natural frequency of vibration of an ion/electron [15]. The same is observed in Figs. 2 and 6 where a rapidfall in dielectric constant and dielectric loss values with increase infrequency up to 1 KHz is observed. The lanthanum and lead dopedsamples show reduced loss values beyond dip frequency because ofdepletion of oxygen vacancies by doped element and high frequencyof resonance.

4. Conclusions

The doped samples show better structural and dielectricresponse. The observed changes in tetragonal strain, which isalso known as principal strain, invite impedance and strainversus field measurements with discussion over the piezo-electric response of these ceramics. These measurements arein progress and will be reported soon. The doping oflanthanum results in formation of similar crystal system andlow room temperature dielectric loss of 0.18 compared to thatof 0.6 for undoped sample. The broadened dielectric responsewith temperature offers wide range of temperature for thephase change. The lead doped sample is best for highfrequency applications as it shows low dielectric lossthroughout the frequency range up to 1 MHz. Additionally,this sample offers highest dielectric constant of 1300 at Curietemperature of 445 °C and does not show frequency relaxationup to 1 MHz.

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