phys. stat. sol. (a) 161, 523 (1997)

Subject classification: 77.80.Bh, 77.80.Dj, 77.84.Dy; S11.1

Phase Transition in Pb(Li1=4Nd1=4Mo1=2)O3

Ferroelectric Ceramics

S. Bera and R. N. P. Choudhary

Department of Physics and Meteorology,Indian Institute of Technology, Kharagpur 721302, India

(Received November 18, 1996)

Polycrystalline sample of Pb(Li1=4Nd1=4Mo1=2)O3 has been prepared by the conventional solid statereaction method. The formation of complex oxide was checked by the X-ray diffraction (XRD)technique. The XRD pattern has provided preliminary structural parameters. Dielectric constant�e� and loss (tan d) of the compound have been measured as a function of frequency (400 Hz to10 kHz) at room temperature and temperature (±±100 to 170 �C) at 10 kHz. The spontaneous polar-isation is measured as a function of temperature (27 to 125 �C) at a constant electric field of3.8 kV/cm. The hysteresis loop vanishes at 91.5 �C which confirms the transition temperature ob-tained from our dielectric studies. The dc electrical resistivity �r� is also measured as a function ofboth biasing electric field (30 to 100 V/cm) and temperature (room temperature to 300 �C), whichsuggests that the dc resistivity of the compound decreases with electric field and temperature.

1. Introduction

With the growing interest and suitability for device applications, a large number of fer-roelectric ceramics have been developed in a wide variety of compositions and stablestructures. Oxide ferroelectrics, synthesised by single crystal, thin film and ceramicroutes are now widely used in computer memory and display devices, electro-opticalmodulators, pyroelectric and gas sensors, transducers, hydrophones and other electronicapplications [1 to 8]. Among all the ferroelectric oxides, oxides with perovskite structureof the general formula ABO3 (A: mono- or divalent, B: tri-, tetra-, penta- or hexavalentions) have extensively been studied for the last five decades in the search for new com-pounds for the said devices. It has been found that a wide variety of compositions ofperovskite ferroelectrics can be obtained by making suitable substitutions at A and/orB sites [9, 10] in the above general formula with the following condition [11] on tolerancefactor t,

t � �rA � rO���

2p ��rB � rO�

; 0:8 � t � 1:05 ;

where �rA is the average ionic radius of A site atoms, �rB the average ionic radius ofB site atoms, rO the ionic radius of O2ÿ.

Again among all the perovskite ferroelectric oxides, Pb-based pure and/or complexcompounds (i.e. PbTiO3, PZT, PLZT, PMN, etc.) have very interesting and importantapplications [12, 13]. From a detailed literature survey it has been found that, except afew, not much work has been reported on lead molybdate/tungstate modified with alkaliand/or rare-earth ions because of their high electrical conductivity and dielectric loss at

S. Bera and R. N. P. Choudhary: Phase Transition in Pb(Li1=4Nd1=4Mo1=2)O3 523

low frequency and at high temperature [14, 15]. Therefore, we have carried out system-atic studies of the ferroelectric phase transition in Pb(B0

1=4R1=4B001=2)O3 (B0 alkali ions, R

rare-earth ions, B00 W, Mo) compounds [16 to 20]. In this paper, we report preliminarystructural and detailed dielectric, polarisation reversal and resistive properties ofPb(Li1=4Nd1=4Mo1=2)O3 (hereafter PLNM) for a better understanding of its phase transi-tion.

2. Experimental

Polycrystalline samples of PLNM have been prepared by the high-temperature(�700 �C) solid-state reaction technique using high purity component oxides and carbo-nate: PbO (99.99%, M/s Aldrich chemical company, Inc. (USA)), Nd2O3 (99.99%, M/sIndian Rare Earth Ltd.), MoO3 (99%, BDH, England) and Li2CO3 (99.9%, M/s S.D.fine chemicals Pvt. Ltd.) in the proposed stoichiometry. These oxides and the carbonatewere thoroughly mixed in an agate mortar for 2 h and calcined at 550 �C for 12 h in analumina crucible in air atmosphere. The mixing and calcination were repeated at 680 �Cfor 18 h. Finally, the fine calcined powder of PLNM was used to make cylindrical pellets(10 mm diameter and 1 to 2 mm thickness) in an isostatic hydraulic press at a pressureof 6� 108 Pa. Polyvinyl alcohol (PVA) was used as binder which was burnt during sin-tering. The pellets were sintered in an alumina plate at 700 �C in air atmosphere for 6 h.The quality and formation of a single phase compound were checked by X-ray diffrac-tion (XRD) technique. XRD patterns on calcined powders and sintered pellets were re-corded at room temperature with a diffractometer (Philips PW 1710, Holland) withFeKa radiation (l � 1:9368 �A� in the wide 2q range (20� � 2q � 80�� at scanning rate of2 deg/min. The surface morphology (i.e., grain or particle distribution) of PLNM wasstudied by a computer controlled STEREOSCAN S-180 scanning electron microscope(SEM) at different magnifications. All the elements present in the complex PLNM com-pound were studied by energy dispersive X-ray analysis (EDAX) in SEM.

The dielectric constant (e� and loss (tan d) of PLNM were measured using a GR 1620AP capacitance measuring assembly as a function of frequency (400 Hz to 10 kHz) atroom temperature and of temperature (±±100 to 170 �C). A laboratory-made three-ter-minal sample holder, a chromel±alumel thermocouple and a PID temperature controller(Indotherm 401D) were used in the measurements.

Measurement of the spontaneous polarisation (Ps) was carried out as a function oftemperature using a modified ST circuit [21] fabricated in this laboratory and with adual trace oscilloscope (TESTATION ±± 4444 APLAB) at a constant electric field3.8 kV/cm.

The dc electrical resistivity was measured both as function of biasing electric field (30to 100 V/cm) at room temperature (RT) and of temperature (RT to 300 �C) at constantelectric field (80 V/cm) with the help of Keithley-617 programmable electrometer andlaboratory-made sample holder and furnace.

3. Results and Discussion

Fig. 1 shows the comparison of X-ray diffraction patterns of PLNM in form of calcinedpowder and sintered pellet at room temperature. The sharp and single diffraction pat-terns suggest the formation of a single phase PLNM compound. Determination of cellparameters and indexing of all the reflection peaks were carried out in different crystal

524 S. Bera and R. N. P. Choudhary

systems and configurations from the observed d values. Finally, a unit cell in orthorhom-bic crystal system was selected using a standard computer program (PowdMult). Thecell parameters were refined with a least-squares method, and were: a � 7:8562 �A,b � 12:9264 �A and c � 14:8648 �A. It was not possible to determine the space group ofthe compound with the limited powder diffraction data. The linear particle size ofPLNM was determined from a few reflection peaks widely scattered in 2q range usingScherrer's equation [22],

Phkl � kl

b1=2 cos qhkl;

where k is a constant, k � 0:89, and b1=2 the half peak width (in rad), was found to be325 �A which is comparable to that determined from particle size analysis and SEM. Asthere are not much islands/holes in SEM micrographs (Fig. 2), we can conclude that allgrains are uniformly distributed. The elemental analysis in the scanning electron micro-scope (EDAX) has confirmed the presence of all elements in suitable stoichiometry

Phase Transition in Pb(Li1=4Nd1=4Mo1=2)O3 Ferroelectric Ceramics 525

Fig. 1. X-ray diffraction (XRD) patterns of PLNM powder calcined at a) 550 and b) 680 �C andc) pellet sintered at 700 �C

(Fig. 3). The elements with low atomic number like oxygen and lithium could not bedetected as expected.

The room temperature variation of dielectric constant (e) and loss (tan d) as a func-tion of frequency has been shown in Fig. 4 which shows the normal behaviour of a dielec-tric. The dielectric constant e � 58 at 400 Hz decreases to 30 at 10 kHz. In this range offrequency all different types of polarisations (i.e., interfacial, atomic, ionic, dipolar, elec-tronic, etc.) are present [23]. As the frequency is increased, some of the polarisationswere getting ineffective and hence at higher frequency we have a lower value of e. Theloss tan d was found to vary in a similar way as e. Fig. 5 shows the variation of e andtan d as a function of temperature at 10 kHz. A sharp anomaly in e and tan d wasobserved at 91 and 95 �C, respectively. At low temperature (e.g., ±±70 �C) e � 15 whichincreases very sharply with increase in temperature and reaches its maximum value

526 S. Bera and R. N. P. Choudhary

Fig. 2. SEM micrograph of PLNMat 2000-fold magnification

Fig. 3. Energy dispersive X-ray spectrum of PLNM pellet sintered at 700 �C

e � 82 at 91 �C, then decreases to 43at 127 �C. Above 127 �C e increasesagain due to space charge polarisation.The value of dielectric loss has thehighest value of 1.15 at 91 �C. Thishigh value of the loss factor is due totransport of ions at higher thermal en-ergy. The dielectric loss anomaly hasalso been observed in the isomorphouscompound Pb(Li1=4Nd1=4W1=2)O3 withmaximum tan d value of 0.2 atTC � 95 �C [19]. From our observa-

tions we have often found that the dielectric loss of molybdate compounds is higherthan that of tungstate compounds [24 to 26].

Fig. 6a to c show ferroelectric hysteresis loops at 47, 57 and 102 �C, respectively. Asthe ceramic samples have lower density than the single crystal, a higher electric field isrequired to reach saturation polarisation. On the other hand, at higher voltage, the sam-ple breaks into several pieces, therefore, we have optimized the field (i.e., 3.8 kV/cm) atwhich we get the appropriate loops. It has been found that the loop area starts decreas-ing from 57 �C and becomes zero at 91.5 �C. The variation of the calculated spontaneous

Phase Transition in Pb(Li1=4Nd1=4Mo1=2)O3 Ferroelectric Ceramics 527

Fig. 4. Variation of dielectric constant �e�and loss (tan d) of PLNM as a function offrequency at room temperature (20 �C)

Fig. 5. Variation of dielectric con-stant (e) and loss (tan d) ofPLNM as a function of tempera-ture at 10 kHz

35 physica (a) 161/2

polarisation as a function of temperature hasbeen shown in Fig. 7. The spontaneous polar-isation decreases with increasing temperatureuntil it becomes zero or very small. The tem-perature at which the spontaneous polarisa-tion becomes almost zero or almost constantin a small temperature interval, is calledCurie temperature (TC). The phase transi-tion of PLNM is confirmed by both experi-mental evidences ±± dielectric anomaly andpolarisation measurement [27]. In ceramicsamples it has been found that there remainsa constant spontaneous polarisation aboveTC instead of zero. This can be explained bythe nature of domains present in the ceram-ics. The ceramic sample has a large numberof domains having different directions ofpolarisation in spite of a single polarisingaxis as in the single crystal. Due to applica-tion of an external ac field the dipoles inevery domain experience a force to oscillateabout their mean rest position with the fre-quency of the applied ac field. Due to thethermal energy they may be agitated and re-

gain their randomness. But some of the domains still have a definite value of dipole mo-ment which results in a small value of the spontaneous polarisation above TC (Fig. 7).

528 S. Bera and R. N. P. Choudhary

Fig. 6. Trace of hysteresis loops of PLNM at a)47, b) 57 and c) 102 �C at a constant electric fieldof 3.8 kV/cm

Fig. 7. Variation of spontaneous polarisation(Ps) of PLNM as a function of temperature

The ac dielectric conductivity �s�and activation energy �Ea� ofPLNM have been calculated fromthe measured dielectric data andusing the formulae [28]

s � ee0w tan d �1�and

s � s0 exp �ÿEa=kT � ; �2�where e0 is the vacuum dielectricconstant, w the angular frequency,k the Boltzmann constant.

The plot of equation (2) as ln sversus 103=T (Fig. 8) shows ananomaly exactly at the transitiontemperature TC � 91 �C of the ma-

terial. This type of anomaly has been observed in many other ferroelectric ceramics [29].The activation energy has been obtained to be 0.10 eV. This low activation energy canbe explained as follows: (i) Typical ionic solids possess a limited number of mobile ionswhich are hindered in their motion being trapped in relatively stable potential wells.Due to a rise in temperature the donor cations take a major part in conduction. The

donors create a level (usually called donorlevel) in the vicinity to the conductionband. Therefore, to activate donors a smallamount of energy is required. (ii) On theother hand, a slight change in stoichiome-try (i.e. the metal to oxygen ratio) in mul-ti-metal complex oxides gives rise to a largenumber of donors or acceptors which maycreate donor or acceptor like states in the

Phase Transition in Pb(Li1=4Nd1=4Mo1=2)O3 Ferroelectric Ceramics 529

Fig. 8. Variation of ac dielectric conduc-tivity (ln s) of PLNM as a function ofinverse absolute temperature �1=T � at10 kHz

Fig. 9. Variation of dc resistivity (ln r) of PLNMas a function of the biasing electric field at roomtemperature (32 �C)

35*

vicinity of conduction or valence band. These donors or acceptors may also be activatedby a lower energy [30].

At room temperature, the variation of dc resistivity as a function of biasing fieldis shown in Fig. 9. It has been found that the resistivity of the compound decreaseswith increasing biasing field. This may be due to the following reasons: a) Ionisationoccurs in inhomogeneous dielectric solids mainly through the mechanism of partial dis-charge of gases or moisture from the pores or islands present in the ceramics. Theelectric field results in the generation of local heat which in turn results in the genera-tion of thermal stresses and increase of local conduction. The stresses can generatemore pores/cracks leading to further ionisation up to a certain field. b) Electrons maybe ejected from the electrode material. These electrons are then accelerated throughthe sample and collide with ions or atoms in the solid, knocking out other electronsand thus ionisation takes place. So, resistivity comes down with increasing biasingfield [30].

The temperature dependence of dc resistivity of PLNM at a constant biasing field of80 V/cm is shown in Fig. 10. It has been observed that the dc resistivity decreases withincreasing temperature. Due to addition of thermal energy the electrons could be set freefrom O2ÿ ions. When an electron is introduced in the sample it might be associated withcations and hence results in an unstable valence state [30]. This type of resistivity beha-viour has been found in semiconductors [31].

4. Conclusion

Finally it can be concluded that PLNM is a ferroelectric material (TC � 91 �C) havingan orthorhombic structure at RT and low activation energy in the paraelectric state. Athigher temperature �>100 �C) it shows semiconducting behaviour (NTC).

530 S. Bera and R. N. P. Choudhary

Fig. 10. Variation of dc resistivity (ln r) of PLNM asa function of inverse absolute temperature �1=T � atconstant biasing field (80 V/cm)

Acknowledgements The authors are thankful to Prof. C. L. Roy for his kind encour-agement, to Mr. B. P. Mirdhya and Mr. B. Das of CRF for their kind help in X-rayanalysis, to Mr. S. Mitra and Mr. D. Dutta of CRF for their help in SEM and EDAXstudies.

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Phase Transition in Pb(Li1=4Nd1=4Mo1=2)O3 Ferroelectric Ceramics 531