Stabilized antiferroelectric phase in lanthanum-doped (Na1/2Bi1/2)TiO3

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2011 J. Phys. D: Appl. Phys. 44 415302

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IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 44 (2011) 415302 (7pp) doi:10.1088/0022-3727/44/41/415302

Stabilized antiferroelectric phase inlanthanum-doped (Na1/2Bi1/2)TiO3

Jae Yun Yi1 and Jung-Kun Lee2

1 Hynix Semiconductor Inc., Icheon-si, Gyeonggi-do 467-701, Korea2 Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh,PA 15261, USA

E-mail: jul37@pitt.edu (J-K Lee)

Received 31 March 2011, in final form 1 September 2011Published 26 September 2011Online at stacks.iop.org/JPhysD/44/415302

AbstractPhase transition behaviour of La-modified sodium bismuth titanate ceramics[(Na0.5Bi0.5)1−1.5xV0.5xLax]TiO3 (NBLT) was investigated. The two anomalies in εr(T ) andDSC analysis indicated that lower temperature phase transitions below 200 ◦C becamepronounced with La additions. The polarization relaxation of εr(T ) and double hysteresisloops showed that the intermediate region between two dielectric anomalies was theantiferroelectric modulated phase. The origin of the modulated antiferroelectric state wasdiscussed in terms of disordering effects of the La ions and cation vacancies. With increasingLa content, the long-range symmetry of the dipoles in the ferroelectric phase was disturbed inthe intermediate region. The competition between rhombohedral ferroelectric phase andtetragonal paraelectric phase contributed to the formation of a modulated antiferroelectricphase in NBLT ceramics.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Sodium bismuth titanate (Na0.5Bi0.5TiO3) (NBT) is ferroelec-tric at room temperature with a diffuse phase transition (DPT)[1, 2]. However, when compared with prototypical relaxorferroelectrics showing a DPT phenomenon, NBT exhibits afew anomalous behaviours. For example, in the middle of theDPT, a polarization relaxation behaviour is not observed whilea structural change from rhombohedral to tetragonal occurs.Therefore, a number of studies have focused on the peculiar-ities accompanying the phase transition of NBT, such as theaforementioned structural change, dielectric anomalies, opti-cal isotropization and the electrical state of each phase [3–7].Despite the precedent reports, the phase transition behaviour ofNBT is still far from clear. Most of the contradictions concernthe number and the temperature range of different phases aswell as their electrical states. In particular, there are contradic-tory reports on the properties of the intermediate phase between200 and 320 ◦C [2, 5–9]. Experimental results such as a diffusepeak at 200 ◦C, broad maximum of permittivity above 300 ◦Cand double hysteresis in the P(E) curve during DPT suggestthe presence of an antiferroelectric state. However, x-ray and

neutron-scattering studies have not provided a strong proofsupporting antiferroelectricity [8, 10, 11]. Structural studiesindicate the coexistence of tetragonal and rhombohedral struc-tures in this intermediate temperature range due to oxygenoctahedral rotation and cation displacement [8, 11].

There have been extensive studies on the phase transitionand structural modulation in complex perovskite materials.These studies observed a modulated antiferroelectric phasebetween a high-temperature paraelectric phase and a low-temperature ferroelectric phase in highly ordered B-sitecomplex perovskite compounds such as Pb(Co1/2W1/2)O3

(PCW), Pb(Sc1/2Ta1/2)O3 (PST) and Pb(Yb1/2Ta1/2)O3 (PYT)[12–14]. The structural modulation in the ordered B-sitecomplex perovskite compounds motivated the present study.Since NBT has a structure ordering associated with oxygenoctahedral and A-site ions, ambiguous properties observed inthe intermediate phase of the DPT are likely to be associatedwith the formation of an incommensurate phase which werereported in the ordered B-site complex perovskite materials[15, 16]. Transmission electron microscope (TEM) andneutron-scattering studies of pure NBT have shown that aone-dimensional modulated phase nucleates and grows in a

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J. Phys. D: Appl. Phys. 44 (2011) 415302 J Y Yi and J-K Lee

three-dimensional host matrix in the intermediate temperaturerange [17, 18]. Moreover, the dependence of dielectricconstant on temperature and time, and thermal hysteresisof the physical properties in the intermediate phase impliesthe presence of the incommensurate phase in NBT [19, 20].Hence, it is worth investigating the intermediate phase of NBTfrom a viewpoint of structural modulation

Doping of aliovalent ions into perovskites would be aneffective way to control the translational symmetry of thestructure. In (Pb1−xLax)(Zr1−xTix)O3 (PLZT) systems, ithas been shown that La ions and accompanying vacanciesoccupy the A-site, relax the oxygen framework, and favourthe anti-phase rotation of the oxygen octahedra, leading tothe suppressed long-range stability of network of BO6 in theABO3 perovskite structure [21]. The effects of aliovalentLa doping on the structural distortion, microstructure andelectromechanical properties are also reported in NBT[22–24]. In a previous study, we have shown that the structuraltransition of La-doped NBT in the intermediate temperaturerange is very sensitive to the location of the vacancy withincation sites.

In this study, we have systematically investigated thedielectric property of the intermediate temperature phase andits structural stability in La-doped NBT where the vacancyis mainly formed in the Na/Bi site. The ratio of La withNa/Bi ranges from 0% to 5%. The effect of La dopingon the phase transition behaviour of NBT is examinedby dielectric spectroscopy, differential scanning calorimetry(DSC), dilatometry, and polarization versus electric field curve(P –E curve).

2. Experimental procedure

The starting materials were Na2CO3, Bi2O3, La2O3 andTiO2 with purities greater than 99.5%. Lanthanum-addedNBT ceramics [(Na0.5Bi0.5)1−1.5xV0.5xLax]TiO3 (NBLT) wereprepared, where x ranged from 0 to 0.05. V indicates avacancy in the A-site of the perovskite structure. Weighedpowders were mixed by ball-milling for 24 h, using stabilizedzirconia media and ethanol. After drying, the mixed powderwas calcined at 800–900 ◦C for 2–5 h and was ball-milled for24 h. The milled powder was uniaxially pressed into pelletsat 1000 kg cm−2 and sintered at 1150 ◦C for 2 h in air. Thecrystal structure of the solid solution samples was investigatedusing x-ray powder diffraction (XRD). Data collection wasperformed in the 2θ range 10◦–60◦ using Cu Kα radiation.The composition of the solid solutions was analysed usingboth physical (electron probe x-ray microanalyzer, EPMA)and chemical (inductively coupled plasma-atomic emissionspectrometry, ICP) methods. The results showed that the realcompositions of the solid solutions were equal to their nominalcompositions.

Fired-on silver paste was used as an electrode materialfor the measurement of dielectric properties. A dielectricconstant versus temperature [ε(T )] curve was measured in thetemperature range from 25 to 300 ◦C with a heating rate of1.5 ◦C min−1 using an impedance analyser. Polarization curves

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Figure 1. XRD results of [(Na0.5Bi0.5)1−1.5xV0.5xLax]TiO3 ceramicsafter 1150 ◦C sintering (x = 0.01, 0.03 and 0.05).

versus applied field (P –E) were investigated using a Sawyer–Tower circuit between 25 and 250 ◦C. The frequency of the acelectric field (E-field) used for P –E measurement was 0.2 Hz.

In order to characterize the thermal properties of thesintered samples, DSC (2920MDSC, TA instrument, USA) inthe temperature range 150–350 ◦C was performed. In addition,a thermal expansion measurement was performed using a high-temperature dilatometer in the temperature range from 50 to450 ◦C at a heating rate of 2 ◦C min−1.

3. Results

3.1. Dielectric property versus temperature

Figure 1 shows the XRD results of [(Na0.5Bi0.5)1−1.5xV0.5xLax]TiO3 (x � 0.05) ceramics. All reflections were indexedas the perovskite structure, suggesting that a complete solidsolution was synthesized in the range 0 � x � 0.05. In the(1 1 1) reflection of pure NBT at 2θ = 40◦, a peak split isclearly observed. In addition, the XRD pattern of pure NBThas 1/2(3 1 1) peak at 2θ = 38.4◦, which is attributed to theanti-phase tilting of the oxygen octahedra. They indicate thatpure NBT has a rhombohedral structure with long-range latticedistortion. When La is added to NBT, the split in the (1 1 1)peak and the intensity of the 1/2(3 1 1) peak are decreased[25–27]. Changes in La-doped NBT suggest that the La ionand the accompanying cation vacancy reduce the macroscopiclattice distortion. Figure 2 shows a change in dielectric con-stant and loss as a function of temperature. In NBT, the dielec-tric anomaly was found at 320◦C (denoted as Tmax). Withdecreasing measuring frequency, this anomaly became lessdistinct and the dielectric constant and loss increased rapidlyabove 300 ◦C. The change in the dielectric loss of pure NBTrevealed that the space charge effect became pronounced above200 ◦C and contributed to the frequency dispersion near thedielectric anomaly (320 ◦C). These space charge contributionshave been attributed to the high ionic conductivity of NBT [15].

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J. Phys. D: Appl. Phys. 44 (2011) 415302 J Y Yi and J-K Lee

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In addition to the dielectric anomaly at 320 ◦C, a less pro-nounced peak in εr(T ) was found at about 200 ◦C (denoted asTi) for all measuring frequencies. The phenomenological basisof this less pronounced peak has been in controversy [2–6, 16]and will be discussed later.

With increasing La content, there were noticeable changesin the dielectric behaviour. At Ti, an intermediate peak ofεr(T ) became clear and the temperature for this peak becamemore dependent on the measuring frequency as the La contentincreased. This indicates that the incorporated La changes

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J. Phys. D: Appl. Phys. 44 (2011) 415302 J Y Yi and J-K Lee

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the type of phase transition near Ti and the properties of theintermediate phase between Ti and Tmax. Second, the phasetransition temperatures, Ti and Tmax, responded differently toLa additions. While Ti decreased with increasing La content,Tmax was relatively insensitive to La content and remainedat 320 ◦C. Consequently, the region of the intermediate statebetween two anomalies of εr(T ) broadened with increasingLa content. The third observation is that there is a thermalhysteresis between two peaks of the εr(T ) curves. The degreeof thermal hysteresis was proportional to the La content. Ata composition of x = 0.05, Ti was observed at 130 ◦C onheating and 100 ◦C on cooling. The last observation is thatthe dielectric constant and the dielectric loss significantlydecreased with increasing La additions, suggesting that the Lasuppressed the effects of space charge polarization and ionicconduction.

3.2. Variations in latent heat and thermal expansion

Figure 3 shows the results of DSC analysis of the La-modifiedNBT samples. It is noted that there was a one-to-onecorrespondence between the anomalies of εr(T ) in figure 2and the endotherms in figure 3. In NBT, only one endothermwas found just below 300 ◦C where εr(T ) clearly showeda dielectric anomaly. With increasing La content, anotherendotherm was found near Ti of εr(T ). This correspondencesuggested that the endotherms observed in the DSC curvecan be attributed to the latent heat of phase transitions atTi and Tmax. While the anomaly at 300 ◦C became lessdistinct with increasing La content, another endotherm at180 ◦C became clear for highly La-doped NBT. Figure 4 showsthe thermal expansion behaviour of NBLT as a function oftemperature. The change in the thermal expansion coefficientwas consistent with the DSC results. In NBT, the change inthe thermal expansion coefficient had a singular point near300 ◦C. However, the thermal expansion coefficient changenear 300 ◦C was undetectable at a composition of x = 0.05.These results show that La additions suppressed the structural

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change near Tmax. Given that the macroscopic phase transitionin the middle of DPT is a peculiar property of NBT, thedisappearance of the hump at a composition of x = 0.05indicates that the La addition changed the thermodynamicstability of the intermediate phase. To investigate the electricalcharacteristics of each phase, ferroelectric hysteresis loops(polarization versus field) of NBLT were measured.

3.3. Electric field induced phase transition

The temperature dependence of the hysteresis loops (P –E

curve) is shown in figure 5. The P –E curve of NBTindicates that NBT behaves as a normal ferroelectric atroom temperature. With increasing temperature, the coercivefield (Ec) and the remanent polarization (Pr) of NBTdecreased and the P –E curve above 200 ◦C deviated fromthe P –E curve of typical ferroelectrics. At a compositionof x = 0.01, spontaneous polarization (Ps) and remanentpolarization (Pr) at room temperature increased, whichwas attributed to the relief of the internal stress by Laadditions. However, as La content increased further, Ps and Pr

decreased and the P –E hysteresis became small. Moreover,double loop hysteresis, characteristics of electrically inducedantiferroelectric–ferroelectric (AFE-FE) transformations, wasobserved in the intermediate temperature range between twoanomalies of εr(T ). The origin of the double hysteresis loop inthe intermediate temperature range may be ambiguous wherethe dielectric loss of the sample is large like pure NBT [28].However, the low dielectric loss at compositions of x = 0.01shown in figure 2 strongly supports that the double hysteresisof La-added NBT is due to the antiferroelectricity of theintermediate phase rather than the artificial effect of the spacecharge. Hence, it can be concluded that the intermediatephase of La-doped NBT, particularly at x = 0.03 and 0.05,exhibits antiferroelectricity. At temperatures between 150 and200 ◦C, the P –E curves of the samples with x = 0.03 and 0.05become slightly different from that of typical antiferroelectricmaterials. As the E-field becomes larger than 40 kV cm−1,the slope of the P –E curve increases rather than beingsaturated. This suggests that La addition may not reduce

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Figure 5. Hysteresis loops for NBLT samples at various temperatures; (a) x = 0 at 25 ◦C, (b) x = 0 at 100 ◦C, (c) x = 0 at 210 ◦C,(d) x = 0.01 at 25 ◦C,(e) x = 0.01 at 100 ◦C, (f ) x = 0.01 at 160 ◦C, (g) x = 0.03 at 25 ◦C, (h) x = 0.03 at 100 ◦C, (i) x = 0.03 at 190 ◦C,(j ) x = 0.05 at 25 ◦C, (k) x = 0.05 at 100 ◦C and (l) x = 0.05 at 160 ◦C.

the dielectric loss at a high E-field as effectively as at alow E-field. A major source of dielectric loss of NBT athigh temperatures is an increase in space charge polarization,which comes from the increased mobility of charged ionicspecies such as oxygen vacancy at high temperatures [15]. Atlow E-fields, doped La can effectively decrease the mobilityof the ionic species at high temperatures and suppress thedielectric loss. However, as the magnitude of the E-fieldincreases at high temperatures, the ionized defect can beelectrically activated, leading to the release of free carrierseither in the conduction band or in the valence band. Newlyproduced carriers of La-doped NBT under high ac E-fields

increase space charge polarization which is overlaid with theantiferroelectric property of La-doped NBT.

4. Discussion

The crystal structure and electrical properties of theintermediate phase present in NBT ranging from 200 to320 ◦C have not been well understood. One of the difficultiesin identifying the electrical properties of the intermediatephase is the high ionic conductivity above 200 ◦C. Hence,substituting A-site cations for Sr2+ was attempted to decreasethe phase transition temperature and rule out the effect of

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J. Phys. D: Appl. Phys. 44 (2011) 415302 J Y Yi and J-K Lee

high conductivity [4, 16]. Using double hysteresis loopof p = f (E), it has been suggested that there wasan intermediate antiferroelectric phase between the low-temperature ferroelectric phase and the high-temperaturetetragonal phase. Despite this experimental suggestion, theantiferroelectric intermediate phase is still in dispute forthe lack of elementary cell doubling, the dependence ofoxygen octahedral tilting on temperature [29] and the lack ofmacroscopic structural change near 200 ◦C.

The experimental results of this study enable us toelucidate this controversial intermediate phase. As La isincorporated into the A-sites of NBT, (1) the temperature forthe local maximum of εr(T ) (Ti) increases as the measuringfrequency increases, (2) an endothermic peak appears aroundTi, (3) the intermediate temperature region between Ti and Tmax

increases from 120 to 170 ◦C and (4) the dramatic change inthe thermal expansion coefficient near Tmax disappears. Theseresults suggest that there is a structural transition around Ti

in NBT and that La additions into NBT manifest the phasetransition of Ti and stabilize the intermediate phase. In NBT,the presence of the antiferroelectric phase is also explained bythe occurrence of 1D modulated phase which is observed inTEM and neutron scattering studies [17, 18]. This indicatesthat the structural modulation in the intermediate temperaturephase is strengthened by substituting Na/Bi with La. Themodulation in NBT is due to a reordering of the cationdisplacements and a subsequent oxygen octahedra tilting [30].Since the polar vectors of the tetragonal phase are reoriented inan antiparallel way, the antiferroelectricity becomes dominantabove Ti where the rhombohedral symmetry (R3c) is notmicroscopically observed [17].

The substitution of aliovalent La ions for Na/Bi of NBTproduces cation vacancies, weakens the anti-phase tilting of theoxygen octahedra and suppresses the ordered distribution ofA-site ions [23, 24, 30]. As a consequence of weakened long-range translational symmetry, the stability of the ferroelectricphase and the energy barrier for the modulation are decreasedbetween Tmax and Ti. If structural modulation occurs, theantiferroelectricity would be preferred to the ferroelectricitysince the symmetry length of the antiferroelectric phase isshorter than that of the ferroelectric phase. The previousstudies on PLZT have shown that the antiferroelectric phasewith relatively short-range order replaces the ferroelectricphase when the long-range translational symmetry becomesunstable [22, 31].

The other role of La addition is the production of a cationvacancy that compensates for the positive charge of La inthe Na/Bi site. This cation vacancy is known to pin thedomain walls of the modulated antiferroelectric phase. Oncethe domain wall is decorated with the vacancy, the stabilityof the intermediate antiferroelectric phase is increased. Asshown in the insets of figure 2, La addition causes the thermalhysteresis in the transition temperature of ε′(T ) and ε′′(T )

near Ti. This suggests that the cation vacancy is adsorbed onthe domain walls of the modulated phase. When the mobiledefects pin the domain walls, the wavelength of structuralmodulation decreases and the stability of the modulated phaseincreases [32, 33]. Moreover, the effect of cation vacancies in

the NBT explains the inconsistency in the literature regardingthe structure of the intermediate phase. The volatilizationof Bi which significantly affects the vacancy concentrationand the non-stoichiometry is very sensitive to the processingparameters. Differences in the stoichiometry in previousstudies may produce the different driving force for the creationof the modulated phase in NBT, which varied the phasetransition behaviour between Ti and Tmax.

5. Conclusion

The transition of the crystal structure and the dielectricproperty of La-added NBT [(Na0.5Bi0.5)1−1.5xLax]TiO3 (0 �x � 0.05) was explored. The dielectric anomalies ofε′(T ) and ε′′(T ) and the endotherms of the DSC indicatedthat there was an intermediate phase between Ti and Tmax.La addition manifested the occurrence of the intermediatetemperature phase between the ferroelectric rhombohedralphase and paraelectric tetragonal phase. The hysteresis loopand the polarization relaxation showed that the intermediatephase is antiferroelectric. The effect of the defects suchas substitutional La and cation vacancy was to facilitate thefrustration of the translational symmetry. When La was doped,the oxygen octahedral tilting and the alignment of dipoles weresuppressed in the rhombohedral phase. This may favour theappearance of the modulated phase and increase their stabletemperature range.

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