Electric field-induced changes of domain structure and properties in La-doped PZT—From ferroelectrics towards relaxors

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Electric field-induced changes of domain structure and properties in La-doped PZT—From ferroelectrics towards relaxors Mojca Otonicar a,b,∗ , Angelika Reichmann a , Klaus Reichmann c a b c

Institute for Electron Microscopy, Graz University of Technology, Steyrergasse 17, 8010 Graz, Austria Jozef Stefan Institute, Jamova 39, Ljubljana, Slovenia Christian Doppler Laboratory for Advanced Ferroic Oxides, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria

a r t i c l e

i n f o

Article history: Received 4 November 2015 Received in revised form 7 March 2016 Accepted 8 March 2016 Available online xxx Keywords: Relaxor ferroelectrics PLZT Domain structure in-situ poling Environmental scanning electron microscopy

a b s t r a c t Electromechanical properties and crystal and domain structures of piezoelectric ceramics from the Pb1−x Lax (Zr0.65 Ti0.35 )1−x/4 O3 (x/65/35 PLZT) solid solution system were systematically analysed by electrical measurements, X-ray diffraction and scanning electron microscopy. The compositionally-driven phase transition from a rhombohedral ferroelectric into a pseudocubic relaxor state is accompanied by a decrease in domain size and changes in domain morphology. SEM analyses of the ex-situ poled samples showed that only in the ferroelectric compositions domain configuration changed, while in the relaxor compositions no domain changes were detected. In order to evaluate the extrinsic contribution to enhanced piezoelectric properties in relaxors, in-situ environmental scanning electron microscopy analyses with applied external electric field were performed. Extrinsic effects as a result of poling were predominant in the ferroelectrics, whereas the intrinsic effect prevailed in the relaxors. Finally, a model of electric field-induced displacements as a result of decreasing domain size for the ferroelectric-to-relaxor PLZT materials was proposed. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Lead-based relaxor ferroelectrics (FEs) were investigated on a large scale during the 1990s and early 2000s for their outstanding electromechanical (EM) performance. Their unique frequencydependent properties were correlated to the temperature-induced structural development, i.e. to the evolution of polar nanoregions (PNRs) in the temperature interval of the broad permittivity maximum. The existence of polar nanoregions and the so-called ‘glassy polarisation behaviour’ was introduced to explain the freezing mechanism of the superparaelectric moments into a nonergodic relaxor state upon cooling [1]. Dispersion of the freezing process was proposed to originate in the distribution of the correlation strengths between PNRs. It was later shown that at freezing temperature an ergodic-to-nonergodic transition is represented by a divergence of the longest relaxation time, thus breaking of ergodicity [2,3]. From experiments of the field-cooled relaxors with a critical electric field applied it was observed that long-range FE ordering is induced. Furthermore, the

∗ Corresponding author at: Jozef Stefan Institute, Jamova 39, Ljubljana, Slovenia. E-mail address: [email protected] (M. Otonicar).

dielectric nonlinearity monotonously decreases when approaching the relaxor-to-ferroelectric transition temperature [4]. The dipole-glass model was upgraded into a spherical randombond–random-field model, where the polarisation dynamics is controlled by random bond characteristics, induced by random fields [5]. In this model various directions of polarisation are allowed and compete with each other. It was further proposed that compositional fluctuations on the B-sites of the perovskite cage form chemical clusters, which induce randomly oriented pinned nanodomains that act as sources of random fields. This model described the relaxor state as reorientable polar clusters embedded in a random array of pinned nanodomains [5]. Moreover, the relaxation dynamics in lead-based components was associated with the existence of two different valence cations on the B-site of the perovskite lattice. For the PMN-PT relaxor system it was shown that two different polar orderings on the A- and B-sites of the perovskite lattice compete with each other (tetragonal and rhombohedral ordering, respectively), which results in a monoclinic structure on a local scale. This short-range monoclinic ordering reconstructs, on average, into a rhombohedral symmetry [6]. In the Lanthanum-doped PZT system a FE-to-relaxor phase transition at RT was observed in a broad compositional range of the Zr/Ti ratio with increasing La content [7]. In the rhombohedral PZT ceramics with Zr/Ti ratio of 65/35 a transition point was determined

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Please cite this article in press as: M. Otonicar, et al., Electric field-induced changes of domain structure and properties in La-doped PZT—From ferroelectrics towards relaxors, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.03.004

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to be in the region between 6 and 10 at% La added, depending on the synthesis procedure [8–13]. It was suggested that randomly quenched La impurities (random fields) disrupt the long-range FE ordering, resulting in relaxor FE behaviour [14]. Mobile impurities and vacancies, on the other hand, order into chain fragments around domain boundaries, causing pinning of the boundaries and polarisation. Domain structure observation with transmission electron microscopy (TEM) revealed that the x/65/35 PLZT materials with increasing La doping developed from a polydomain rhombohedral FE state into tweed-like domains and further to polar nanodomain state [8,9]. Quenched disorder (i.e. random fields) and competing polar orderings (i.e. frustration) were considered to contribute to the unique structure and behaviour of relaxor PLZT materials. An electric field applied to piezoelectric materials triggers changes in material dimensions—the larger the change, the higher is the piezoelectric coefficient. Volume and shape changes are achieved through intrinsic lattice distortions and/or lattice reorientation and the associated domain wall propagation. The two mechanisms, intrinsic and extrinsic, respectively, can be estimated from high-energy diffraction measurements, where changes in the position of the diffraction peaks represent lattice deformation, whereas relative changes in peak intensities correspond to the texturing of the domains [15]. Furthermore, defects in the lattice cause pinning of domain walls, which results in frequency dependence of piezoelectric coefficients [16]. Moreover, frequency dispersion of electromechanical properties in polycrystalline materials is a result of reversible and irreversible domain wall displacements, i.e. domain wall movement between pinning centres and motion that overcomes pinning centres, respectively. The intrinsic vs. extrinsic contribution to the piezoelectric coefficient is very much dependent on the crystal symmetry. In materials of lower symmetry (or morphotropic phases) possessing more ferroelastic variants of polarisation, domain wall motion with applied fields is promoted, since deformation strain from lattice reorientation is easily accommodated and intergranular constraints are limited [17]. In a tetragonal lattice, on the other hand, which possesses only 3 ferroelastic variants, the crystallographic strain is greatest when the field is applied in parallel to the polarisation direction, causing a mainly intrinsic piezoelectric effect. Analyses of the polarisation switching behaviour in the x/65/35 PLZT relaxor compositions showed that switching occurs through an intermediate nanodomain state even in the ferroelectric regime [18]. It was further concluded that the dielectric nonlinearities, which were observed in the soft ferroelectric state of the low Lacontent compositions and not in the relaxor state, are related to the dynamics of various stable domain-like structures and not to polarisation nonlinearities. For the 8/65/35 PLZT composition it was further proposed that domain wall density is highly increased when the macrodomains decay into nanodomains during reversed poling cycles, resulting in the absolute maximum of permittivity [19]. The ac-field-induced dielectric nonlinearities at lower field amplitudes in the x/65/35 PLZT ceramics were investigated to reveal the characteristics of the dynamic responses of the various domain configurations [18]. It was proposed for the 6/65/35 composition that under moderate fields the tweed-like domains undergo fielddriven fluctuations, contributing to stabilisation of the polar cluster state over the tweed-like state. On the contrary, the 8/65/35 composition showed only weak nonlinearities and the polar cluster state was thus observed to be stable. This implied that the system is beginning to settle into a series of metastable states, while under excitation the system exhibits microhysteresis around these metastable minima. It was postulated that the gradual freezing of the polarisation fluctuations into a metastable minimum is not a local-scale process, but rather a correlated freeze-out of many clusters [18]. Polarisation switching analyses on the 8/65/35 PLZT

ceramics were also performed by Shur et al. [11]. It was predicted that in the poly-domain matrix with isolated inclusions of the nonpolar phase it is the bound charges (induced as a result of polarisation jumps at nanoregion boundaries) that generate depolarisation fields and further stimulate the formation of nanodomain structures. According to PFM results from Pertsev et al., movement of the irregular domain walls in the 9.5/65/35 PLZT relaxor is mainly affected by random-bond disorder caused by cation substitutions and the associated vacancies, which leads to domain wall pinning and weakened dipole-dipole interactions [20]. Since the domain structures of polycrystalline ferroelectric ceramics also depend on mechanical stress fields from threedimensional constraints of neighbouring grains [21], it has to be considered that thin foils used in the in-situ TEM investigations may not represent the actual stress-strain conditions of the bulk material. Furthermore, since relaxors exhibit only relatively small distortions from the cubic equilibrium position of atoms with applied external fields, diffraction techniques cannot always determine the intrinsic vs. extrinsic changes. Many conclusions on the domain switching were also derived from frequency dependent electrical measurements. Visualising the effects of field on domain structures of bulk material would however provide useful information for a proof of concept as well as for materials processing and use. Changes in domain configuration depending on polarisation were studied by XRD and SEM on a tetragonal ferroelectric PZT [22]. Domains were observed before and after ex-situ poled samples. With poling, the lamellar domains oriented perpendicularly to the applied electric field. It was postulated that domain wall motion with applied field occurs in two subsequent modes—domain wall translation and domain reorientation. Furthermore, it was concluded that domain wall translation has zero threshold energy, while domain wall reorientation starts beyond the coercive electric strength, corresponding to a finite threshold energy. In the study of Leach et al. domain switching was directly observed in the SEM on tetragonal PZT material by applying an electric field between two surface-mounted electrodes [23]. With an electric field applied, the domain structure changed from the initial multidomain pattern to a single twinning pattern. When the field direction was reversed the pattern again became multidomain, but different from the initial state. It was again found that twin planes preferentially align perpendicular to the E-direction when the coercive field is exceeded. In this paper, the La-doped PZT x/65/35 system in the compositional range from 5/65/35 rhombohedral ferroelectric towards the 10/65/35 pseudocubic relaxor were systematically analysed by XRD, various electromechanical measurements, SEM and insitu ESEM with applied electric field. The main question addressed in the present study was how the compositionally defined FE-torelaxor phase transition is reflected in the domain structure of bulk samples of PLZT materials. Furthermore, how does an applied external electric field influence the domain structure? Especially in a series of compositions from ferroelectric to relaxor state it is of interest to know in which compositions macroscopic domains appear and how they respond to the electric field. In such cases domain wall movement (and domain wall pinning) would have to be considered for the large signal response of those materials. Another question is, how can extrinsic or intrinsic effects be correlated to the largely enhanced electromechanical response in the relaxor state? In order to answer these questions, an in-situ ESEM method was employed for the first time to directly observe changes in the domain structure with applied external electric field. The main focus of the present study was to determine the effects of the external electric field on the domain structures of the rhombohedral near-relaxor ferroelectric compositions and the changes towards the relaxor compositions. The main drawback of the ESEM is its limited resolution, which is not suitable for the observation of nan-

Please cite this article in press as: M. Otonicar, et al., Electric field-induced changes of domain structure and properties in La-doped PZT—From ferroelectrics towards relaxors, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.03.004

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odomains. However, the idea behind these experiments was to directly observe changes in domain structure of the bulk material, where the volume of the grains is constrained. Here it was observed that extrinsic effects were predominant in the ferroelectric PLZT compositions, whereas the intrinsic effect prevailed for the relaxor compositions. Finally, a model of electric field-induced displacements as a result of domain size is proposed for these particular ferroelectric to relaxor compositions. 2. Experimental 2.1. Sample preparation Ceramic samples of B-site deficient La3+ doped PZT with a general formula of Pb1−x Lax (Zr0.65 Ti0.35 )1−x/4 O3 with La3+ content x varying from 0.01 to 0.1 were prepared by conventional solid state reaction route. The following commercially available reagent-grade powders were used as starting materials: Pb3 O4 (99.99%, Penox GmbH), La2 O3 (min. 99%, Treibacher), ZrO2 (99.9%, MEL Chemicals) and TiO2 (99.8%, Tronox Pigments). The raw powders were first dried at 220 ◦ C and then cooled down to RT in a desiccator to avoid water vapour adhesion. In order to obtain the correct stoichiometry, 1 mol% of PbO was added in excess due to the volatilisation of PbO during the heat treatment process. After weighing the powders were ball milled in a planetary mill (Fritsch, Pulverisette 7) at 300 rpm for 30 min in ethanol using stainless steel beakers with a tungsten carbide inlay and 5 mm tungsten carbide balls. The powders were dried at 120 ◦ C after milling and sieved through a 500 ␮m sieve, after which they were put into alumina crucibles, covered with a lid and calcined at 850 ◦ C for 3 h in a box furnace (Nabertherm N7/H with C290 controller) with a heating/cooling rate of 5 ◦ C/min. The calcined powders were sieved and milled again using the same parameters as for the raw powders. After milling the powders were dried and 5 wt% of polyethylene-glycol binder was added, thoroughly mixed with the powder and dried at 80 ◦ C for 50 min. The binder-dried powder was then ground in an agate mortar and sieved through 500, 315 and 180 ␮m sieves. These granulates were then pressed into pellets of 13 mm diameter and about 1 mm height at 150 MPa. To promote decomposition of the binder, the pellets were heat treated at 500 ◦ C for 1 h. For further sintering, the pellets were arranged in a coin roll, separated by ZrO2 powder. Additionally, a 1:1 mixture of Pb3 O4 and ZrO2 powders was spread around the coin roll and covered by an alumina crucible to ensure a PbO rich atmosphere in order to avoid additional lead loss. The samples were then sintered at 1250 ◦ C for 3 h and a heating/cooling rate of 5 ◦ C/min in a box furnace. 2.2. Characterization After calcination and sintering, the calcined powders and the ground sintered pellets were analysed by X-ray diffractometer (XRD; Bruker AXS D5005 y-y, Cu K␣ emitter, graphite secondary monochromator) to ensure a single-phase composition and to compare the changes in crystal symmetry with La addition. All XRD patterns were recorded at room temperature in the 2 range from 10◦ to 100◦ with a step size of 0.02◦ and a time of 2 s/step. Furthermore, XRD patterns of the surface of a sintered pellet with the 8/65/35 PLZT composition were recorded after poling the sample and once again after relaxation with subsequent heat treatment to 900 ◦ C for 30 min. The sintered pellets of all compositions were polished and silver paste was applied to both sides for electrical measurements. Polarisation and strain vs. electric field measurements were performed using an aixACCT aicPES system at a frequency of 0.1 Hz and electric fields up to 2 kV/mm. Changes in polarisation vs. electric field were

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also measured at various temperatures during heating to 200 ◦ C in order to determine the gradual phase transitions from ferroelectric to relaxor state. Impedance spectra were measured with a Novocontrol Concept 80 broadband analyser (Alpha-AN, Novocontrol) using a BDS 1200 cell in combination with an active ZGS cell interface (Novocontrol) allowing temperature-variable two-electrode (dielectric) measurements. Permittivity (␧ ) and dielectric loss factor (tan ␦) vs. temperature (T) were measured on the previously poled samples at frequencies of 1 kHz, 10 kHz, 100 kHz and 1 MHz in a temperature range from 0 to 300 ◦ C. The piezoelectric coefficient d33 was measured by a PiezoMeter System (Piezotest, UK) on the poled samples at 110 Hz mechanical vibration and a dynamic force of 0.25 N. The microstructure of the samples was analysed by a scanning electron microscope (SEM; Zeiss Ultra 55 FEG) in backscattered mode with reduced working distance of 8–3 mm. Reduced working distance was used in order to perform orientation contrast (OC) imaging, which results in various grey-scale levels of differently oriented domains due to orientation anisotropy of backscattered electrons [24]. Samples from PLZT compositions with varying amounts of La added were observed in their virgin state and compared to the samples of the same composition after ex-situ poling. Pellets were covered with epoxy, ground with silica carbide and then polished using 0.25 ␮m diamond paste. Fine mechanical polishing was performed with 0.04 ␮m SiO2 emulsion for 2 h to obtain a smooth surface and to remove any stresses induced during grinding. To prevent charging, samples were sputtered with carbon. In-situ poling experiments were performed in an environmental scanning electron microscope (ESEM; Quanta 600 FEG SEM; FEI, Eindhoven, NL) in a low vacuum at a vapour pressure of 0.3 Torr and an operating voltage of 9 kV. Pellets for the experiment were lapped to 300 ␮m and sputtered on both sides with Cr-Ni-Ag electrodes. Wires were soldered on each side of the pellets. The prepared samples were covered with epoxy resin, ground perpendicular to the pellet surface and finally polished with silica gel. An electric field of up to 3 kV/mm was applied to the samples using an external voltage source (Isolation Tester 1550B, Fluke, Germany) connected to the SEM via cables passed through the chamber housing. OC images were taken before, during and after applying this external electric field.

3. Results and discussion 3.1. XRD The XRD patterns of ground sintered pellets from 5/65/35 to 10/65/35 PLZT compositions reveal a rhombohedral to cubic phase transition (Fig. 1). In the 7/65/35 PLZT spectra the (111)C and (211)C peaks are not split, but exhibit shoulders at positions where characteristic splitting of a rhombohedral distortion appears. A gradual reduction of the distances between the split peaks can be observed, indicating a decreasing rhombohedral distortion towards the pseudocubic phase. In the 8/65/35 PLZT composition no characteristic splitting of the peaks is observed, however, most of the peaks in its diffraction pattern are broadened. Furthermore, from the broadening of the (200)C peak additional lattice distortions away from the rhombohedral symmetry are proposed, possibly into an orthorhombic symmetry [12]. In the 9/65/35 PLZT roots of the peaks are broader than in the 10/65/35 PLZT composition, so the two materials were defined as pseudocubic and cubic, respectively. Since the XRD patterns were measured on the samples that were ground after sintering, it is most likely that the 8/65/35 PLZT composition was distorted due to the mechanical stress applied during grinding. The applied pressure might have induced various distortions away from the essentially cubic symmetry, not limited to a

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Fig. 1. XRD patterns of ground pellets from the Pb1−x Lax (Zr0.65 Ti0.35 )1−x/4 O3 (x/65/35 PLZT) system with x = 5–10 at% La. On the right hand-side enlarged reflections from compositions with x = 6–9 at% La show a gradual phase transition from a rhombohedral to a pseudocubic perovskite structure. Arrows point to broadening of the peaks in the 8/65/35 composition, implying distortions away from the cubic symmetry.

defined lower symmetry. Such an observation corresponds to characteristics of the relaxor-type materials. These results further imply that the 8/65/35 PLZT composition is most influenced by external fields, which would correspond well to the idea of a nonergodic relaxor state of the material. Fig. 2 shows XRD patterns of the 8/65/35 PLZT composition taken from the surface of a poled pellet and of the same pellet after heat treatment to 900 ◦ C. Shifts in diffraction peaks towards lower angles were observed for the poled sample (view inset) and the magnitude of shifts was larger for specific peaks. This implies changes in the dimensions of the crystal lattice with increased distances between specific crystallographic planes in the direction of the applied electric field [15]. Furthermore, the relative intensities of the peaks in the poled sample changed in comparison to the unpoled sample, indicating some degree of crystallographic texturing. No apparent peak splitting was detected that would correspond to a lower crystal symmetry of the lattice, which could again be interpreted as texturing, with highly aligned lattice distortions with the applied electric field. It could further be argued in a similar way to Tan et al., who proposed the establishment of metastable states for the 8/65/35 PLZT relaxor composition, where the system, when excited, exhibits correlated microhysteresis around these metastable minima and no large-scale distortions [18]. Thus, the 8/65/35 composition is defined as a distorted pseudocubic phase.

Fig. 2. XRD patterns from poled and heat-treated (unpoled) pellets of the 8/65/35 PLZT composition, representing increased distances between atomic planes with poling in the direction of the applied electric field.

3.2. Electromechanical properties The main feature of relaxor materials is the frequency dependence of the temperature of maximum permittivity (Tm ). Measurements of permittivity vs. temperature (␧ /T) at various frequencies from unpoled PLZT samples changing from ferroelectric to relaxor state are presented in Fig. 3. With increasing La concentration from 5 to 10 at% La in the x/65/35 PLZT the values of permittivity maximum decrease towards lower temperatures. Furthermore, permittivity maximum becomes frequency dependent at the 7/65/35 composition and the frequency dependence increases towards the 10/65/35 composition (represented by arrows). The results show the emerging relaxor character at 7/65/35 PLZT, becoming more pronounced towards the 10/65/35 composition. Room temperature hysteresis loops of polarisation vs. electric field (P/E) show a gradual transition from ferroelectric to relaxor state at compositions ranging from 8/65/35 (nonergodic relaxor—induced FE) to 10/65/35 PLZT (ergodic relaxor). When heated, all compositions gradually turn into an ergodic relaxor state (Fig. 4). Transition temperatures correlate well with the depolarisation peak from the permittivity vs. temperature of the poled samples at 100 kHz (Fig. 5); 110 ◦ C for 7/65/35, 80 ◦ C for 8/65/35 and 40 ◦ C for 9/65/35. For the 10/65/35 composition the transition from an ergodic to a nonergodic relaxor state is predicted to be below room temperature [10]. The temperature-dependant permittivity measurements of the poled samples enable the prediction of phase transitions from the induced ferroelectric state to the high-temperature ergodic relaxor state. Peaks that correspond to the depolarization temperature (also denoted as the freezing temperature) in 7/65/35, 8/65/35 and 9/65/35 PLZT compositions are pointed out in Fig. 5. The highest depolarization peak is observed for the 8/65/35 composition. Additionally, the highest peak for the 8/65/35 composition is seen from the graph of temperature vs. dielectric losses (tan␦/T), implying increased disturbance of the atoms, associated with rearrangement of their position. It is believed that the poling field induces the largest change in ferroelectric ordering in the nonergodic relaxor phase, in which random bonds/fields caused by La impurities weaken the dipole interactions [14,20]. Furthermore, easy polarisation paths that are mainly unrestricted by symmetry promote highly aligned dipoles with applied external fields in relaxors, leading to higher ferroelectric ordering. No depolarisation peak was observed for the 10/65/35 composition, again implying that the ergodic-to-nonergodic relaxor transition takes place below RT. For the 9/65/35 PLZT composition, the inflection point in the ␧ /T curve is relatively close to RT, which results in the highest RT permittivity of the studied system.

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Fig. 3. ␧ /T curves at 1 kHz, 10 kHz, 100 kHz and 1 MHz of FE-to-relaxor compositions with 5–10 at% La in the x/65/35 PLZT. Maximum permittivity shifts to lower temperatures with increasing La and becomes frequency dependent at 7/65/35; frequency dependence increases towards ergodic relaxor at 10/65/35.

Fig. 4. P-E hysteresis loops of x/65/35 PLZT samples with x = 7–10 at% La measured at various temperatures during heating to 150 ◦ C. The 10/65/35 PLZT sample shows a relaxor P-E loop already at RT, while the rest of the samples undergo a FE-to-relaxor phase transition at 40 ◦ C (9/65/35), 80 ◦ C (8/65/35) and 110 ◦ C (7/65/35).

At approximately 180 ◦ C there are local peaks in the dielectric loss curves (tan␦/T), which correlate with humps in the permittivity curves (␧ /T) for the relaxor compositions and as a peak in the permittivity curve of the 7/65/35 (ferroelectric) composition. This anomaly is frequency independent and is observed at the same temperature for all relaxor and ‘near-relaxor’ compositions. Since only in the ferroelectric 7/65/35 composition permittivity peaks at this anomaly, while in the relaxor compositions the highest peak in the ␧ /T curve is from increased interatomic hopping within the dynamic polar nanoregions of the ergodic relaxor state, it seems that this difference is distinguishable between ferroelectric and relaxor states in the x/65/35 PLZT ceramics. Furthermore, this frequency and La-concentration independent anomaly above the permittivity maximum in relaxors can be explained by unit-cell groupings of different Pb/La concentrations [13]. While the temperature of spontaneously polarised nanoregions depends on the amount of La within the locally correlated unit cells (increased amount of La decreases the temperature of spontaneously induced dipoles), the highest temperature of polar ordering corresponds to the Pb-only-containing unit-cell groupings, which

is most likely represented by the anomaly at 180 ◦ C. Hence, this anomaly represents an ordering temperature. At the same temperature Keve et al. observed an anomaly in the DTA curves for the 6/65/35 PLZT composition, interpreted by these authors as transition from a nonpolar to a polar phase [12]. It is not reasonable to argue that no distortions are present above this temperature and that the material transforms to cubic (Burns temperature for this type of relaxor is much higher) [13], nevertheless, it may indicate that those distortions are not polar or are AFE in nature, or that the local dipoles are no longer correlated. The strain vs. electric field loops (Fig. 6) confirm what has been deduced from polarization curves at room temperature. For the samples with La concentration below 9 at% pronounced butterfly curves are measured, which are typical for ferroelectric materials (also for the 8/65/35 nonergodic relaxor composition). The coercive field (indicated by the minimum in strain) decreases with increasing La concentration, which might be due to the initially higher degree of long range ordering in ferroelectric compositions (with lower La concentration). Regarding strain, the difference between the maximum strain and the remanent strain (which is set as zero

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Fig. 7. Piezoelectric coefficients d33 of the x/65/35 PLZT compositions measured by piezometer; d33 is highly increased in the nonergodic relaxor state.

Fig. 5. Permittivity and dielectric losses vs. temperature curves measured at 100 kHz during heating of the previously poled x/65/35 PLZT samples with x = 7 to 10 at% La. The arrows point out the depolarisation peaks where the induced long-range ordering is broken; the dashed line marks the frequency- and temperature-independent peaks/humps, which possibly originate from the loss of correlated dipoles.

Fig. 6. S-E bipolar loops of x/65/35 PLZT samples with x = 1, 6, 8 and 9 at% La. The negative strain increases towards the nonergodic relaxor and decreases in the ergodic state. The positive strain is the highest in the ergodic state, while the overall displacement is the largest in the 8/65/35 nonergodic relaxor.

in Fig. 6) increases with La concentration. The maximum strain in the 8/65/35 nonergodic relaxor composition, however, is not higher than in the 6/65/35 ‘near-relaxor’ ferroelectric composition. Considering the ‘negative strain’ (the difference between remanent strain and the minimum strain at coercive field), this feature increases up to the 8 at% La concentration. For the 9/65/35 sample the ‘negative strain’ decreases abruptly and the strain vs. electric field loop rather gets sprout shaped, exhibiting the highest ‘positive strain’. The described ‘negative strain’, which represents shrinkage in the subcoercive field range, can be explained by the disturbance and contraction of the negatively aligned dipoles from the previous poling cycle [25]. After contraction, the dipoles flip into an

opposite direction and start to couple and grow as the applied external field increases, promoting a higher degree of ordering. It can be argued that in ferroelectrics it is the polarisation reorientation mechanism (domain switching) that mainly contributes to the extent of distortion. In other words, the pre-established crystal and domain structures contribute to strains that prevent larger displacements of the bulk ferroelectric ceramics. In the relaxor materials, on the other hand, inducing the interatomic excitations requires lower fields and the final displacements in the ergodic state are higher due to the reduced restrictions from crystal symmetry or domain structure (which are pseudocubic and nanosized, respectively). Furthermore, the ‘negative strain’ that corresponds to reorientation of the previously induced polar directions is the smallest in the ergodic composition, which suggests that structural changes induced by poling are to a great extent reversible and that correlation between induced polar regions is not significant. Measurements of the piezoelectric coefficient d33 show increasing values from ferroelectric towards relaxor materials (Fig. 7). A pronounced increase in the d33 value is observed for the 8/65/35 PLZT composition (560 pC/N), which is in accordance with the previously proposed idea of external field induced nonergodic relaxor-to-ferroelectric phase transition. Thus, the highest electromechanical coupling is reached in the nonergodic relaxor due to reduced intragranular strains (no long-range ordering), increased domain wall density (nanosized domains) and a reduced potential barrier, promoting various easy paths of polarisation. It is believed that in the pseudocubic relaxor state with localized random orderings of atoms the poling field triggers irreversible correlated shifts in the direction of the applied field, thus largely increasing the overall net polarisation. Furthermore, reversing the poling direction in relaxor materials leads to easier reorientation of polarisation through initial contraction and subsequent extension of the volume ceramics. Additional to lattice strain, intergranular elastic effects that accommodate for the unfavourably oriented lattices with respect to the field direction were determined to influence the strain characteristics of a material [26]. For the relaxor materials it is proposed that the field-induced lattice distortions are accompanied by distortions of grains throughout the volume of ceramics, leading to minimized strains at grain boundaries. In the 7/65/35 and 9/65/35 compositions the d33 values are similar, but in the 7/65/35 ferroelectric composition it is the mechanism of reorientation (domain texturing) that is believed to contribute most to the effect. As was previously defined for the rhombohedral PZT compositions, the remanent strain and the degree of texturing are high, with subsequent cycling, however, domain switching is relatively low [27]. Thus, with decreasing amounts of La added and an increasing rhombohedral distortion the piezoelectric coefficient decreases on account of reduced domain switching after initial poling. In the 9/65/35 relaxor composition, on the other hand, it is the triggering of highly aligned small magnitude dipoles within

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polar nanodomains that contribute to a relatively large d33 . In the 10/65/35 composition d33 is nearly zero, implying a nearly cubic structure and some reversible reorientation changes within isolated polar nanoregions of the ergodic relaxor state. The described mechanisms that explain increased values of piezoelectric properties lead to the proposition that in relaxor materials the intrinsic structural contribution to the enhanced EM response is predominant, whereas in the ferroelectric compositions the extrinsic effect prevails. The intrinsic structural instability induced by PNRs, which is manifested as an asymmetry in the lattice dynamics and further contributes to ultrahigh piezoelectric properties, was also proposed by Xu et al. [28]. 3.3. Scanning electron microscopy Fig. 8 shows a series of backscattered electron images of unpoled (a) and ex-situ poled samples (b) from compositions with 1 and 5 to 10 at% La added in the x/65/35 PLZT solid solutions. Based on the XRD analyses, samples with 1–7 at% La added exhibit a rhombohedral symmetry. The domain pattern of the unpoled samples changes from lamellar domains with various orientations within a single grain (1/65/35) towards a ‘square-patched’ domain pattern, where domains decrease in size with increasing La content. In the 8/65/35 composition the size of the ‘square-patched’ domains is reduced down to about 100–200 nm (similar domains were also termed as ‘nanoscale quasi-regular maze or finger-print domains’ [29], or ‘labyrinth-type nanoscale domains’[20]). In the 9/65/35 composition the domains seem to have the same texture as in the 8/65/35 ceramics, however, due to their small size (below 100 nm) they become hard to distinguish. Observation at this scale is limited by the resolution limit of the SEM. In the 10/65/35 composition no domains are visible. In the ex-situ poled samples (Fig. 8b) the domain patterns of ferroelectric compositions change significantly compared to the unpoled samples. In compositions from 1 to 7 at% La added to the x/65/35 PLZT system the domain structure changes from a ‘square-patched’ pattern to a lamellar domain structure, where the lamellas are mainly oriented perpendicular to the applied electric field (as was previously observed by other authors [12,22,23]. A high degree of texturing after initial poling in the rhombohedral PZT compositions was already proposed by Kungl et al. [27]. The relative orientation of lamellas with respect to the direction of the applied field (perpendicular) can be explained as the most effective arrangement of polar directions within a defined symmetry. In this arrangement the majority of polar axes are oriented at about 45◦ to the field direction [26], resulting in an optimum net polarisation alongside minimized strain from the neighbouring grains. In the 8/65/35 PLZT composition the ‘square-patched’ domains become more distinct after poling, although the pattern type remains similar to the domain pattern observed prior to poling. The absence of induced lamellar domains indicates a small degree or even no domain texturing with poling, which implies that a highly strained crystal lattice contributes to the enhancement of the electromechanical properties. However, it could also be a result of a reversible texturing process (as defined for an ergodic relaxor state). In the 9/65/35 PLZT sample some domain features are observed, partly similar to domains of the 8/65/35 composition, only smaller in size. Furthermore, some parts of the grains possess irregular stripe-like domains similar to those observed by Pertsev et al. in the 9.5/65/35 PLZT sample, indicating some irreversible domain texturing [20]. No distinct changes are observed in the 10/65/35 composition. Results from SEM imaging apparently show irreversible changes in the domain texture of the poled (as compared to the unpoled) rhombohedral ceramics, which were designated as ferroelectric based on the electrical measurements. In the 8/65/35 nonergodic relaxor composition, which shows a ferroelectric P-E hysteresis

Fig. 8. Domain structures of unpoled (a) and poled (b) samples from the x/65/35 PLZT FE to relaxor compositions with x = 1–10 at% La. In the rhombohedral ferroelectric samples, the domain structure changes from a ‘square-patched’ pattern into lamellar domains (electric field is applied from the left to the right-hand side). In the 8/65/35 nonergodic relaxor nanosized ‘square-patched’ domains become more pronounced with poling, while in the ergodic samples domain size is decreased and the SEM resolution limit prevents domain structure observation.

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loop, the size of the domains is decreased and its ‘perturbed’ domain pattern remains similar, yet more pronounced, after poling. This implies that polarisation may be induced within the nanoscale domains with pinned domain walls, most probably as rotation and extension of the locally defined dipoles along the applied field direction, leading to a highly strained crystal lattice. Since there are no obvious changes in domain pattern before and after poling, however, this raises the question of whether the changes are reversible and should be observed in-situ, during poling, or whether most changes occur within the nanosized domains through an intrinsic deformation of the crystal lattice. Thus, in-situ ESEM was employed

to directly observe changes in domain structures during a poling process.

3.4. In-situ environmental scanning electron microscopy To observe the response of the domain structure to the application of an external field in the 8/65/35 nonergodic relaxor material, in-situ measurements were performed in an environmental scanning electron microscope (ESEM). In order to test the experimental setup, a classical tetragonal ferroelectric composition (45/55 PZT with 1 at% La added) with a well-defined lamellar domain struc-

Fig. 9. Sketch shows arrangement of the pellet with Ag side electrodes, soldered wires, all covered with epoxy resin and polished perpendicular to the pellet surface. Tetragonal ferroelectric material 45/55 PZT before, during and after poling: circles show changes in domain configuration. Obvious is the reorientation of domains during poling and only small changes after poling.

Fig. 10. Poling of the 8/65/35 PLZT nonergodic relaxor shows enhanced ‘perturbed’ domain pattern during poling and no obvious changes in domain configuration after poling.

Fig. 11. Schematic representation of domain reorientation/reconfiguration with an external electric field applied to (a) ferroelectrics with lamellar domains, (b) ferroelectrics with a ‘square-patched’ domain pattern and (c) relaxors with polar nanodomains. The largest displacement/distortion was observed for the relaxor materials due to the easy reorientation of polarisation vectors throughout the volume of the ceramics.

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ture (Fig. 9) was analysed first. Application of an external electric field (3 kV/mm) led to a reorientation of the lamellar domain pattern, which is comparable to a poling process. Furthermore, prior to that poling process, grains consisted of multi-domain patterns of variously oriented lamellas, while during and after poling these lamellas were observed to extend across the length of the grains. After removal of the field most domains retained the same pattern as during poling, so the changes were observed to be mainly irreversible. This experiment showed that in tetragonal ferroelectric ceramics the electromechanical properties are mainly influenced by extrinsic effects (domain wall movement). Fig. 10 represents results from in-situ poling of the 8/65/35 nonergodic relaxor material in the ESEM. In the unpoled state the domain structure shows ‘square-patched’ or ‘perturbed’ domain patterns with domains up to 100–200 nm in size. During poling at 1.5 kV/mm the domain structure remained very similar to the structure observed prior to poling, but the domains became more pronounced, as if the field caused the lattice to strain and distort coherently within the already existing domains. After removing the electric field, the domains were less pronounced than during poling, but more enhanced than prior to poling, and mainly of the same configuration as prior to and during poling. It can be argued that the domain walls, which were observed to be irregular and rough, are pinned by the defects resulting from the intrinsic random-bond disorder, associated with the cation substitution and the compensating vacancies [20]. Furthermore, it can be argued that in the ground state the coupling of the oxygen octahedra is disturbed due to the random local strain fields caused by La impurities [14]. Since the oxygen octahedra interact only on a short range and the Pb/La atoms are randomly displaced, this creates nanosized domains of various distortions [30]. It was previously suggested that under excitation the polarisation process in relaxors is driven by correlated microhystereses around the metastable minima [18], which would designate the polarisation process as an intrinsic effect. Thus, the applied poling field forces the atoms throughout the volume of the bulk to mainly arrange along with the applied field regardless of the local lattice orientation, causing the crystal lattice to become highly strained. From the in-situ ESEM experiments and the XRD data of the 8/65/35 PLZT composition it can be concluded that the enhanced electromechanical properties in the nonergodic relaxor material are mainly caused by intrinsic distortions of the crystal lattice and not by domain wall movement, as may be the case in ferroelectric materials [17]. A simple model is introduced, explaining the changing poling mechanisms from ferroelectric towards relaxor compositions in the PLZT system. Fig. 11a represents characteristic changes in the domain structure with applied electric field in a normal displacive ferroelectric, where the width of domains is altered, depending on the relative orientation of polar vectors towards the direction of the applied electric field. When domain size is reduced and domains do not extend through the whole volume of the grains, the poling field triggers growth of domains with favourable orientations with respect to the applied electric field (Fig. 11b). In this way, more defined lamellar domains are induced, increasing the volume distortion in the direction of the field. The preferred orientation of the grown lamellas is perpendicular to the applied electric field, as this arrangement is optimal for reaching the largest volume extension, additionally compensating grain constraints. In a nonergodic relaxor, on the other hand, where domains are as small as below 100 nm and there is only short-range ordering within clusters of differently oriented small-scale dipoles, the applied electric field triggers reorientation and growth of the locally correlated dipoles in the direction of the field (Fig. 11c). Since the crystal structure of a relaxor material in its ground state is pseudocubic and strains arising from lattice distortions between neighbouring clusters or polar nanoregions are minimized, the

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atoms can easily shift in any direction, according to the direction of the applied field. In this way, most of the dipoles within the volume of the polycrystalline material orient themselves and grow with the field, while these distortions do not require a phase transition from pseudocubic to a defined lower symmetry state. Additionally, since the polar distortions are not limited by well-defined domain boundaries and there are no symmetry restrictions between the polar clusters, changing of the poling direction results in easy reorientation of polarisation vectors. The mechanisms described are assumed to contribute to the largest electromechanical response in the nonergodic relaxor material. The reversible vs. irreversible distortions after poling of the ergodic and nonergodic relaxors, respectively, are believed to be connected to a critical amount of randomly quenched and mobile impurities and/or vacancies, which are the cause of random fields and competing polar orderings [9,20,30,31]. In the nonergodic relaxors domain wall pinning prevents long-range ordering, but the field-induced lattice strain is retained, resulting in the high remanent polarisation. Above the critical concentration of La added (9 at%) bound charges at the nanoregion boundaries generate depolarisation fields [11], which further prevent any correlation between the induced dipoles. Thus, in ergodic relaxor materials almost no remanent polarisation is induced by external fields, which makes the poling process reversible. A similar amount of displacement (observed from the d33 measurements) in the ‘near-relaxor’ ferroelectric (7/65/35) and in the ‘near-ergodic’ relaxor (9/65/35), as well as the larger electromechanical strain in the nonergodic relaxor (8/65/35) can be explained in terms of a decreasing domain size from ferroelectric towards relaxor compositions (displacements are similar but the contribution changes from extrinsic to intrinsic). It is argued that in a pseudocubic nanodomain state of a nonergodic relaxor applied stress produces more charge by deforming the crystal lattice (intrinsic) than by domain reorientation and growth and the associated domain wall movement (extrinsic), which prevails in ferroelectric materials. Even though the intrinsic effect becomes more pronounced towards the relaxor state, the amount of positive displacements (extension) triggered by the electric field becomes limited due to stronger pinning of domain walls with increased La-impurities and vacancies in the system. In a ‘near-ergodic’ relaxor state (9/65/35) a material is free from initial constraints from lattice distortions and domains so the intrinsic effect prevails, giving the largest positive displacement. Moreover, the piezoelectric coefficient in an ergodic state is lower than in a nonergodic state due to the (mainly) reversible structural distortions.

4. Conclusions Ferroelectric relaxor materials from the x/65/35 PLZT system with x = 5–10 at% lanthanum were systematically studied by various complementing methods to reveal the effects contributing to enhanced electromechanical (EM) properties at the composition-dependent transition from ferroelectric to relaxor state. In the rhombohedral PLZT compositions with increasing Ladoping, domain patterns change from lamellar to ‘square-patched’ domains and further towards nanosized domains. It was shown that the 8/65/35 composition with pseudocubic symmetry and nanosized ‘perturbed’ domains exhibits highly enhanced electromechanical strain, and was defined as a nonergodic relaxor phase with field-induced transition into a ferroelectric phase. In order to identify the contribution of domain wall motion to EM properties, a unique in-situ method was employed to observe changes in the domain patterns during electrical poling in the environmental scanning electron microscope (ESEM). These in-situ poling experiments and XRD comparison of the poled and unpoled

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sample led to the conclusion that effects from intrinsic distortions of the highly strained crystal lattice mainly contribute to the enhanced piezoelectric response in the nonergodic relaxor phase. The electric field-induced lattice distortions are proposed to be a collective process of correlated dipole alignments throughout the volume of the polycrystalline ceramics, giving the largest volume displacements along the field direction. In the ‘near-relaxor’ ferroelectric compositions, on the other hand, the applied electric field causes domain wall movement and rearrangement from ‘squarepatched’ domains into well-defined lamellar domains. The main contribution to the EM response is thus considered to be extrinsic. Further high-energy diffraction studies would be necessary to confirm the change from extrinsic to intrinsic contributions to EM response with increasing La-impurities and decreasing lattice volume. Furthermore, studies of coupling between oxygen octahedral tilting and polarisation [32] may be of great relevance to understanding the mechanisms of field-induced ordering and lattice distortions. Acknowledgements This research was performed as part of a postdoctoral project and was supported by the JECS Trust Fund, under the contract number 201473. M. Otonicar wishes to thank Theresa Kainz for her introduction to the solid state synthesis of PLZT and Florian Preishuber-Pflügl for temperature-dependent permittivity measurements. K. Reichmann acknowledges the support of EPCOS OHG, a group company of the TDK-EPC Corporation and funding by the Christian Doppler Research Association, Austria, and the Ministry of Science, Research and Economy, Austria. We all thank Manuel Paller and Daniel Schreiner for the skilful preparation of SEM samples and Margit Wallner and Selina Haingartner for graphics design. References [1] D. Viehland, J.F. Li, S.J. Jang, L.E. Cross, M. Wuttig, Glassy polarization behaviour of relaxor ferroelectrics, Phys. Rev. B 46 (13) (1992) 8013–8017. [2] Z. Kutnjak, C. Filipiˇc, R. Pirc, A. Levstik, R. Farhi, M.E. Marssi, Slow dynamics and ergodicity breaking in a lanthanum-modified lead zirconate titanate relaxor system, Phys. Rev. B 59 (1) (1999) 294–301. [3] V. Bobnar, Z. Kutnjak, R. Pirc, A. Levstik, Electric-field-temperature phase diagram of the relaxor ferroelectric lanthanum-modified lead zirconate titanate, Phys. Rev. B 60 (9) (1999) 6420–6427. [4] V. Bobnar, Z. Kutnjak, A. Levstik, Nonlinear dielectric response of relaxor PLZT ceramics in a dc bias electric field, J. Eur. Ceram. Soc. 21 (2001) 1319–1322. [5] R. Blinc, J. Dolinˇsek, B. Zalar, C. Filipiˇc, Z. Kutnjak, A. Levstik, R. Pirc, Local polarization distribution and Edwards-Anderson order parameter of relaxor ferroelectrics, Phys. Rev. Lett. 83 (2) (1999) 424–427. [6] B. Dkhil, J.M. Kiat, G. Calvarin, G. Baldinozzi, S.B. Vakhrushev, E. Suard, Local and long range order in the relaxor-ferroelectric compounds PbMg1/3 Nb2/3 O3 and PbMg0.3 Nb0.6 Ti0.1 O3 , Phys. Rev. B 65 (2001) 024104. [7] G.H. Haertling, C.E. Land, Hot-pressed (Pb, La)(Zr, Ti)O3 ferroelectric ceramics for electrooptic applications, J. Am. Ceram. Soc. 54 (1) (1971) 1–11. [8] X. Dai, Z. Xu, J.-F. Li, D. Viehland, Effects of lanthanum modification on rhombohedral Pb(Zr1−x Tix )O3 ceramics: part I. Transformation from normal to relaxor ferroelectric behaviors, J. Mater. Res. 11 (3) (1995) 618–625. [9] D. Viehland, X.H. Dai, J.F. Li, Z. Xu, Effects of quenched disorder on La-modified lead zirconate titanate: long- and short-range ordered structurally incommensurate phases, and glassy polar clusters, J. Appl. Phys. 84 (1998) 458–471.

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