Dielectric relaxation, lattice dynamics and polarization mechanisms in Bi0.5Na0.5TiO3-based lead-free ceramics Dielectric relaxation, lattice dynamics and polarization mechanisms in Bi 0.5 Na 0.5 TiO 3 -based lead-free ceramics

Dielectric relaxation, lattice dynamics and polarization mechanisms in Bi0.5Na0.5TiO3-based lead-free ceramics Giuseppe Viola, Huanpo Ning, Xiaojong Wei, Marco Deluca, Arturas Adomkevicius et al. Citation: J. Appl. Phys. 114, 014107 (2013); doi: 10.1063/1.4812383 View online: http://dx.doi.org/10.1063/1.4812383 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v114/i1 Published by the AIP Publishing LLC. Additional information on J. Appl. Phys. Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors Downloaded 03 Jul 2013 to 138.37.45.6. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions JOURNAL OF APPLIED PHYSICS 114, 014107 (2013) Dielectric relaxation, lattice dynamics and polarization mechanisms in Bi0.5Na0.5TiO3-based lead-free ceramics Giuseppe Viola,1,2 Huanpo Ning,1 Xiaojong Wei,1 Marco Deluca,3,4 Arturas Adomkevicius,1 Jibran Khaliq,1 Michael John Reece,1,2 and Haixue Yan1,2,a) 1 School of Engineering and Materials Science, Queen Mary University of London, 380 Mile End Road, London E1 4NS, United Kingdom 2 Nanoforce Technology Ltd., 380 Mile End Road, London E1 4NS, United Kingdom 3 Institut f€ ur Struktur- und Funktionskeramik, Montanuniversitaet Leoben, Peter Tunner Str. 5, 8700 Leoben, Austria 4 Materials Center Leoben Forschung GmbH, Roseggerstr. 12, 8700 Leoben, Austria (Received 10 April 2013; accepted 11 June 2013; published online 2 July 2013) In 0.95[0.94Bi0.5Na0.5TiO3-0.06BaTiO3]-0.05CaTiO3 ceramics, the temperature TS (dielectric permittivity shoulder at about 125  C) represents a transition between two different thermally activated dielectric relaxation processes. Below TS, the approximately linear decrease of the permittivity with the logarithm of frequency was attributed to the presence of a dominant ferroelectric phase. Above TS, the permittivity shows a more complicated dependence of the frequency and Raman modes indicate a sudden increase in the spatial disorder of the material, which is ascribed to the presence of a nonpolar phase and to a loss of interaction between polar regions. From 30 to 150  C, an increase in the maximum polarization with increasing temperature was related to three possible mechanisms: polarization extension favoured by the simultaneous presence of polar and non-polar phases; the occurrence of electric field-induced transitions from weakly polar relaxor to ferroelectric polar phase; and the enhanced polarizability of the crystal structure induced by the weakening of the Bi-O bond with increasing temperature. The occurrence of different electric field induced polarization processes with increasing temperature is supported by the presence of additional current peaks in the C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4812383] current-electric field loops. V I. INTRODUCTION Bismuth-based perovskites have attracted large interest in recent years for their intriguing structural, dielectric, and electrical properties. Several solid solutions based on the main constituent bismuth sodium titanate Bi0.5Na0.5TiO3 (BNT) have been developed with many other compounds such as barium titanate BaTiO3 (BT), bismuth potassium titanate Bi0.5K0.5TiO3 (BKT), and potassium sodium niobate K0.5Na0.5NbO3 (KNN), among others. BNT exhibits a rhombohedral R3c polar structure at room temperature,1–3 although reports of monoclinic Cc symmetry recently appeared in the literature.4,5 Temperature variations induce complicated changes in the crystal structure and different phase diagrams have been proposed.2,3 The polycrystalline (1x)BNT-xBT system is one of the most widely studied among lead-free ceramics. Addition of barium titanate BT to bismuth sodium titanate (BNT) induce significant structural modifications. There is a general agreement that in the unpoled state, for x < 0.04, the system has a polar rhombohedral structure with R3c space group up to approximately 150  C.3 In the range 0.05 < x < 0.07, the R3c phase coexists with the weakly polar tetragonal P4bm, forming a morphotropic phase boundary (MPB).3 For x > 0.07, the rhombohedral phase disappears.3 The structural modifications induced with increasing temperature significantly depend on a) Author to whom correspondence should be addressed. Electronic mail: h.x.yan@qmul.ac.uk 0021-8979/2013/114(1)/014107/9/$30.00 the amount of BT additions.3 However, the general effect of increasing temperature consists of the progressive stabilization of a weakly polar tetragonal phase and of a cubic symmetry upon further temperature increment.3 BNT-BT solid solutions were previously regarded as antiferroelectric at high temperature,6 but according to more recent studies, direct evidence of antiferroelectric order cannot be found from TEM diffraction patterns.7,8 The microstructural changes induced by temperature variations determine a complicated evolution of the dielectric permittivity with temperature, which has been extensively discussed in the literature but still open for discussion.8–12 It was proposed that the dielectric constant reflects relaxor behaviour and the permittivity anomalies observed in correspondence of particular temperatures can be ascribed to the thermal evolution of coexisting R3c and P4bm polar nanoregions whose microstructural arrangement changes upon temperature variations.8 In particular, based on micro-Raman spectroscopy, it was recently reported that in (Li, Nd)-doped BNT-BKT, the origin of the depolarization temperature (Td), where the ferroelectric long range order disappears, is related to the breaking of Bi-O hybridization, which favours a pseudocubic structure.12 BNT-BT based solid solutions experience changes in the crystal structure under the application of an electric field,13–20 and such structural variations significantly depend on the starting composition.17 In general, the application of an electric field produces the stabilization of a polar order13–20 and increases the unit cell distortion.13,19,20 Reversible formation 114, 014107-1 C 2013 AIP Publishing LLC V Downloaded 03 Jul 2013 to 138.37.45.6. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions 014107-2 Viola et al. of lamellar ferroelectric domains was observed under AC electric field,15 whereas rather permanent modifications of the crystal structure distortion were found in DC poled samples.3,18–20 The response to the application of an alternating electric field depends also on the temperature. Pinched hysteresis loops have been observed in several BNT-based materials under different temperature conditions depending on the composition.9,20–27 This behaviour can be rationalized by the occurrence of a reversible phase transition from a weakly polar (or non-polar) order to a ferroelectric state during the application of an electric field and the re-establishment of the initial weakly polar (or non-polar) state during field removal, which produces large polarization recovery and pinched polarization-electric field loops.8,15,20–27 Recently, it was also proposed that pinched hysteresis is observed under temperature conditions where the application of an electric field produces a transformation from an ergodic relaxor state to a metastable ferroelectric ordered state, and the material returns back to the original ergodic relaxor state during field unloading.28,29 These electric field-induced microstructural modifications are also responsible for the development of large mechanical strains (comparable to lead-based ferroelectrics) during electrical loading, which have indicated BNT-BT based systems as a valid lead-free alternative for actuator applications,9,22–25,27 also based on high fatigue resistance.30 Due to the possibility of a significant polarization recovery during field removal, BNT-BT based materials could also be usefully employed as energy storage capacitors competing with lead-based antiferroelectrics. The energy density release reported for BNT-BT based materials20,31,32 could potentially be further improved by composition optimization, once the mechanisms of the electric field-induced transitions have been further clarified. The main objective of this work is to gain more insights regarding the response of BNT-BT based materials under dynamical electric field conditions, with particular focus on the effect of temperature on the dielectric permittivity and loss and on the current-polarization-electric field loops. The chosen composition is the 0.95[0.94Bi0.5Na0.5TiO3-0.06BaTiO3]-0.05CaTiO3 (BNTBT-5CT) solid solution motivated by the attempt of decreasing the depolarization temperature and the remnant polarization by calcium addition, which may in turn be useful for energy storage applications and can thus support further rationalization for materials selection and development. II. EXPERIMENTAL DETAILS Chemical precursors Bi2O3 (99.9% Sigma-Aldrich), Na2CO3 (99.5% Alfa Aesar), BaCO3 (99.8% Alfa Aesar), CaCO3 (99.5% Alfa Aesar), and TiO2 (99.8% SigmaAldrich) were weighed according to the stoichiometric formula 0.95[0.94Bi0.5Na0.5TiO3-0.06BaTiO3]-0.05CaTiO3. The raw materials were mixed and ball milled for 4 h in nylon pots using ethanol and zirconia balls. The slurry was then dried, calcined at 850  C for 4 h and then ball milled again for 24 h to homogenise the particle size. After drying, the powder was sieved through a 250 lm mesh. J. Appl. Phys. 114, 014107 (2013) Ceramics were prepared by Spark Plasma Sintering (SPS) in cylindrical graphite dies of 20 mm diameter at 950  C using a heating rate of 50  C/min, a pressure of 50 MPa, and a dwell time of 3 min. In order to completely remove the effect of any carbon contamination and reduction produced during SPS and to obtain more saturated polarization-electric field loops, the sintered disks were subsequently annealed in air at 1100  C for 24 h in alumina crucibles. The density of the specimens was measured using the Archimedes’ immersion method. Room temperature X-ray diffraction (XRD) was performed on annealed samples using a Siemens D5000 diffractometer (Siemens AG, Karlsruhe, Germany) operating at 40 kV and 30 mA with Cu Ka radiation. After annealing, the disks were cut in several smaller pieces (rectangular shape) for the electrical characterization. Electrodes were fabricated by firing silver paste (Gwent Electronic Materials Ltd., C2011004D5, Pontypool, U.K.) onto the samples cross sections at about 600  C for 20 min. The temperature and frequency dependence of the dielectric constant and loss were measured from room temperature to 600  C, by applying an alternating voltage of 1 V amplitude with frequency in the range 1 MHz-100 Hz, using an LCR meter (Agilent, 4284A, Hyogo, Japan) connected to a tube furnace with controlled temperature. Impedance spectroscopy was performed using an impedance analyser (Agilent 4294A, Hyogo, Japan) in 200 Hz-1 MHz frequency range from room temperature up to 600  C. The heating rate was 5  C/min. Current-polarization-electric field hysteresis loops were measured using a hysteresis tester (NPL, Teddington, U.K.) in a silicone oil bath, at different temperatures in the range 25  C–175  C using triangular voltage waveforms33 with different frequency in the range 0.5–100 Hz. Raman spectroscopy experiments were performed with a LabRAM microprobe system (Jobin-Yvon/Horiba, Villeneuve d’Ascq, France) equipped with a 532.02 nm Nd:YAG laser. The laser beam was focussed on the sample surface with a 100 long working distance objective (NA ¼ 0.8, LMPlan FI, Olympus, Tokyo, Japan) and a maximum effective power density of 3 mW/lm3. Temperaturedependent analyses were performed in a MDS600 heatingcooling stage (Linkam, Tadworth, UK). Spectral parameters (such as peak position and full width at half maximum (FWHM)) were extracted from fits according to multiple Gaussian-Lorentzian peak functions using the commercial softwares LABSPEC 4.02 (Jobin-Yvon/Horiba) and PEAKFIT V4.12 (Systat Software, Inc., San Jose, CA). III. RESULTS AND DISCUSSION The X-ray diffraction pattern of the annealed ceramics is shown in Fig. 1; the diffraction peaks have been indexed based on the pseudocubic structure19 suggesting mainly a single phase with a tiny amount of secondary phase around 30 . The relative density of ceramic was 98%. A. Dielectric studies Figure 2 shows the temperature dependence of the real e0 and imaginary parts e00 of the dielectric permittivity (Figs. 2(a) and 2(b), respectively) together with the loss Downloaded 03 Jul 2013 to 138.37.45.6. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions 014107-3 Viola et al. FIG. 1. XRD of the annealed ceramic. factor tgd (Fig. 2(c)). The real permittivity exhibits the typical features commonly observed in different BNT-BT based materials,8–10,17,22 with a shoulder here indicated as TS (Fig. 2(a)) and a maximum at Tm. The temperature TS increases from approximately 100  C to 130  C when the frequency is increased from 100 Hz to 1 MHz. Based on diffraction and dielectric studies, it has been recently found that the J. Appl. Phys. 114, 014107 (2013) temperature TS in pure BNT may represent the onset of a non-ferroelectric phase with an antiphase tilting which is stable until approximately 40–50  C below Tm.34 The imaginary permittivity e00 and loss factor show visible peaks, approximately 30–40  C below TS, and they also slightly shift towards higher temperature with increasing frequency (Figs. 2(b) and 2(c)). In BNT-based ceramics, the temperature corresponding to the dielectric loss peaks is usually defined as “depolarization temperature” which identifies the point where long range ferroelectric order disappears. However, it should be noted that the dielectric loss curves usually reported in the literature are normally generated at a few frequencies within a range of several order of magnitudes. In addition, in unpoled ceramics, dielectric loss peaks are usually broad and their corresponding temperature often varies with frequency, suggesting that the identification of the depolarization temperature from the dielectric loss peaks could not be accurate and that the presence of loss peaks may reflect different underlying mechanisms. For unpoled BNT-based ceramics, impedance spectroscopy with a broader and more continuous frequency spectrum turns useful to obtain additional details on the frequency dependence of the dielectric behaviour, overcoming the limitations related to a discrete frequency spectrum. Figure 3 shows the impedance spectroscopy analyses at T < TS (Figs. 3(a)–3(d)), TS < T < Tm (Figs. 3(e)–3(h)), and T > Tm (Figs. 3(i)–3(l)), respectively. Real and imaginary permittivities are plotted against the logarithm of the frequency (log f). The Cole-Cole plot relative to permittivity and plot of real part electric modulus (M0 ) against the imaginary electric modulus (M00 ) are also presented for a better interpretation of the spectroscopic data. At T < TS, the real part of the dielectric permittivity exhibits an approximately linear decrease with the logarithm of frequency (Fig. 3(a)). The reduction of the permittivity with increasing frequency is in line with previous reports on BNT35 and 0.94BNT-0.06BT36 bulk ceramics and 0.88BNT0.08BKT-0.04BT thin films.37 Logarithmic dispersion can be quantified by estimating the parameter a from the following relationship:38 e0 ¼ e00  alogf ; FIG. 2. (a) Real part of permittivity, (b) imaginary part of permittivity, (c) loss factor as a function of temperature at different frequencies in the range 100 Hz-1 MHz. (1) where e00 is the intercept and a is the slope of the e0 -logf plot. Table I shows the variation of a with the temperature. It can be noticed that in the range 25  C-94  C, a monotonically increases denoting a higher frequency dispersion of e0 with increasing temperature. At room temperature, the logarithmic dispersion of our BNT-BT-5CT ceramics (a ¼ 78.87) is greater than the soft Pb(Zr0.52Ti0.48)0.99Nb0.01O3 (a ¼ 35.7 in the range 102 < f < 106 Hz).38 This is in agreement with what observed in Ref. 36, where the permittivity of BNT-BT based materials was found to have higher frequency dispersion than soft lead zirconate titanates (PZT). It was proposed that a higher a indicates a higher domain wall mobility and a more disordered spatial configuration of pinning points.38,39 Logarithmic dispersion in the permittivity has been previously observed also in other relaxor ceramics, for instance lanthanum modified lead zirconate titanate in the temperature range 150 K < T < 250 K (Ref. 40) and in fine grains Downloaded 03 Jul 2013 to 138.37.45.6. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions 014107-4 Viola et al. J. Appl. Phys. 114, 014107 (2013) FIG. 3. (a) e0 -logf, (b) e00 -logf, (c) permittivity Cole-Cole plot, (d) electric modulus Cole-Cole plot, in the range T < TS; (e) e0 -logf, (f) e00 -logf, (g) permittivity Cole-Cole plot, (h) electric modulus Cole-Cole plot, in the range TS < T < Tm; (i) e0 -logf, (j) e00 -logf, (k) permittivity Cole-Cole plot, (l) electric modulus M0 -M00 plot, in the range T > Tm. Pb(Mg1/3Nb2/3O3)-35%PbTiO3 (PMN-35%PT) at T < 200 K.41 Furthermore, logarithmic-type frequency dispersion was also observed in other properties of PZTs such as the piezoelectric constant42 and the mechanical compliance,43 both having a significant extrinsic domain wall contribution. It was suggested that such dependence originates from domain wall motion in the presence of randomly distributed pinning points.36,40,44 Pinning points perturb the potential energy of the medium giving rise to a multi-well energy profile TABLE I. Temperature dependence of the parameter a. Temperature (  C) 25 29 50 75 94 a 78.87 81.20 101.51 125.14 140.91 reflected in a broad relaxation times distribution,36 which determines a logarithmic-type frequency dependence. At T < TS, the imaginary part of the permittivity shows visible peaks at a particular frequency fR which increases with increasing temperature (see peaks signed by arrows in Fig. 3(b)). In unpoled soft and hard PZTs at room temperature, loss peaks were observed at higher frequencies (in the order of GHz) and attributed to domain wall relaxation.38 However, dielectric loss peaks can be also found in the kHz range as for instance in the case of barium titanate.45 It was reported that at frequencies in the range 102–107 Hz, dielectric loss originates from the energy dissipation due to the interaction of domain walls with defects, while at higher frequencies (>108), a significant contribution to dielectric loss is given by a Doppler effect of phonons interacting with oscillating domain walls.46 The interpretation of the origin of loss peaks in the kHz region in our BNT-BT-5CT ceramics is not straightforward and it can be ascribed to different relaxation mechanisms which may be active in the Downloaded 03 Jul 2013 to 138.37.45.6. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions 014107-5 Viola et al. ceramics. It is possible that these peaks are associated to the presence of defect dipoles whose flipping may have a contribution to the permittivity: approaching fR, it becomes more difficult for the defect dipoles to follow the driving field, contributing to the reduction of the permittivity with increasing frequency (Fig. 3(a)). In addition, there is the possibility that domain wall movement is influenced by the presence of octahedral tilting (in rhombohedral PZTs47,48) and defects complexes.49,50 In this case, fR could represent the relaxation frequency of domain wall movement. The increment of the relaxation frequency fR with increasing temperature supports both the relaxation scenarios since thermal energy would provide additional driving force to defects dipoles flipping and domain wall mobility. However, the presence of interfacial Maxwell-Wagner (MW) polarization effects, which could also be the mechanism responsible for the presence of loss peaks in the kHz range,51,52 cannot be excluded. Figures 3(c) and 3(d) show the permittivity Cole-Cole plot and M0 vs M00 plot, respectively, where relaxation regime corresponds to the arch-like shaped segments, indicated with letter R in the curve relative to 94  C as an example. In the intermediate temperature range TS < T < Tm, the dielectric behaviour changes as evidenced in Figs. 3(e)–3(h). It can be noticed that the real permittivity is no longer a quasi-linear function of logf, but it shows a more complicated trend (Fig. 3(e)). The imaginary permittivity still shows visible peaks whose corresponding frequency again increases with increasing temperature (Fig. 3(f)). The permittivity Cole-Cole plot in Fig. 3(g) shows a gradual change with increasing temperature. The arch-like region shifts towards the high frequency regime and it shrinks with increasing temperature (see letter R in the curve relative, for instance, to T ¼ 255  C), leaving room to a creep-like behaviour where real and imaginary permittivities are proportional (linear portion of the plots in Fig. 3(g)). Above 255  C, relaxation peaks are no longer clearly visible in the imaginary permittivity in the range of frequency studied (Fig. 3(f)), although relaxation processes are probably still active as suggested by the persistence of arch-like region in the ColeCole plot above 255  C (see segment indicated with letter R in Fig. 3(g)). The gradual change in the dielectric behaviour with increasing temperature is also apparent in the M0 vs M00 plot as shown in Fig. 3(h). In the temperature range T > Tm, the real permittivity decreases with increasing frequency (Fig. 3(i)) and the loss peaks are no longer neatly visible in the studied frequency range (Fig. 3(j)). The arch-like region in the Cole-Cole plot progressively shrinks with increasing temperature (see segment indicated with R in Fig. 3(k)) and the creep-like region in the permittivity Cole-Cole plot (Fig. 3(k)) becomes progressively predominant with increasing temperature and it can be associated with an increasing contribution of electrical conductivity. This is also evidenced by the approaching of the electric modulus plot to a perfectly semi-circle shape with increasing temperature (Fig. 3(l)). Figure 4 shows the temperature dependence of the relaxation frequency in an Arrhenius-like plot. It can be easily noticed the visible change in slope across TS which suggests different energy barrier for the relaxation process below and J. Appl. Phys. 114, 014107 (2013) FIG. 4. Arrhenius plot of the relaxation frequency. above TS. The visible change across TS of the temperature dependence of fR and the progressive suppression of the loss peak with increasing temperature may suggest that the relaxation peaks is associated with domain wall movement (domain/ polar cluster size shrinking with increasing temperature). B. Raman Spectroscopy In order to more directly correlate the observed temperature dependent dielectric behaviour to underlying structural modifications, the temperature evolution of the lattice vibrational modes was studied by Raman spectroscopy. Figure 5 shows the temperature dependence of the Raman spectrum of BNT-BT-5CT in the range 25–350  C. In BNT-based materials, the Raman spectrum is characterised by a broad appearance, which might underline the presence of secondorder effects or non-k ¼ 0 modes activated by local disorder, so that a traditional mode assignment based on group theory is challenging.35 The spectrum has been decomposed into its main features by a collection of Gaussian-Lorentzian curves, and their peak position, intensity, and FWHM are represented in Figs. 6(a)–6(c), respectively. Note that each point is the average of three different measurements made at the same temperature, and the error bars represent the standard deviation. The spectrum was decomposed into 8 main modes, two of which (at 200 cm1 and 480 cm1) however were used to represent the background and are thus not displayed. This spectral signature is consistent with FIG. 5. Temperature-dependent Raman spectra of BNT-BT-5CT. Downloaded 03 Jul 2013 to 138.37.45.6. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions 014107-6 Viola et al. J. Appl. Phys. 114, 014107 (2013) FIG. 7. Temperature dependence of the two modes at 270 cm1 and 380 cm1. FIG. 6. Spectral analysis of BNT-BT-5CT in dependence of temperature. (a) Peak position; (b) intensity; and (c) FWHM. Each point is the average of three experimentally collected data points. The standard deviation is represented on the graphs as error bars. rhombohedral BNT,53 although due to the intrinsic transitional disorder of this material, monoclinic structure cannot be excluded.54 Modes in the frequency range up to 400 cm1 are associated with Ti-O vibrations.54,55 The main peak at 270 cm1 is a polar vibration of A1 character55 and it has already been demonstrated that such mode is highly sensitive to the local polar order in BNT-based materials.54 The mode at 380 cm1 probably originates from the zone-centre A1(TO2) mode of Ti-based perovskites, which is due to the motion of the titanium ion against the BO6 octahedra.56 However, its broad appearance might underline also second order effects and thus makes it sensitive to spatial disorder on the nanometer scale. The modes above 500 cm1 involve mainly polyhedral oxygen vibrations.57 Figure 7 shows the shifts of the peaks at 270 cm1 and 380 cm1 with increasing temperature, choosing a scale that improves the visibility of their behaviour. Between room temperature and 125  C, the peak at 270 cm1 undergoes softening, which is compatible with anharmonic effects of the crystal lattice due to enhanced thermal vibration. The softening, however, ceases at 125  C (here around TS). The intensity of this mode (Fig. 6(b)) gradually decreases with increasing temperature, but it still shows significant intensity above Tm, suggesting the presence of local polar clusters beyond the dielectric maximum.58 The mode at 380 cm1 displays hardening with increasing temperature, probably due to an increased interaction between oxygen atoms within the octahedral framework. This could be caused by changes in the octahedral environment surrounding the B-site upon shrinking of the unit cell or changes in the polar axis. This latter mode has an anomaly at 125  C, and at 245  C its hardening ceases. In summary, two characteristic temperatures could be identified from the Raman data: 125  C which in this case seems related with the permittivity shoulder around TS and 245  C which may correspond to the temperature above which dielectric loss peaks at fR started to be significantly dampened (Fig. 3(f)). The former is distinguished by the ceasing of the 270 cm1 mode softening along with a sudden jump in the wavenumber corresponding to the mode at 380 cm1, a sudden drop in the intensity of the modes at 270, 380, 520, 590 cm1 (Fig. 6(b)), and a clear jump in the FWHM of the same modes (Fig. 6(c)). At 245  C, a visible drop in the intensity of the modes at 270, 520, 590 cm1 (Fig. 6(b)) and anomalies in the FWHM of most of the modes indicated in Fig. 6(c) can be observed. These phenomena can be interpreted in the following way. Below 125  C, the ferroelectric phase is dominant. In the absence of any changes in the microstructure, a monotonous decreasing of the wavenumber correspondent to the 270 cm1 is expected with increasing temperature. Above 125  C, the softening of the 270 cm1 mode ceases, indicating a sudden increase in the spatial disorder of the material, which can be ascribed either to the presence of a nonpolar phase or to a loss in the interaction between polar nanoregions (shrinkage of polar clusters). This would give rise to a weakly polar state that persists up to 245  C, where a stationary state in the disorder of the material is reached. Downloaded 03 Jul 2013 to 138.37.45.6. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions 014107-7 Viola et al. J. Appl. Phys. 114, 014107 (2013) C. Current-polarization-electric field hysteresis loops The temperature and frequency dependent response of BNT-BT-5CT was also studied under high electric field (60 kV/cm), by generating current-polarization-electric field (I-P-E) loops in the range of temperature 25  C–175  C at different frequencies in the interval 0.5–10 Hz (Figs. 8 and 9). Frequency dependent P-E loops were recently reported for another ternary BNT-based system.59 For simplicity, only the hysteresis loops relative to 0.5 Hz and 10 Hz are shown here. It can be seen that from 25  C up to 100  C, in agreement with Raman data, BNT-BT-5CT behaves as ferroelectric with typical domain switching current peaks and large remnant polarization. Although the P-E loops may not be completely saturated, the electric field Ec corresponding to the current peak, the maximum polarization Pmax, and the remnant polarization Pr can still be considered to study rate effects under the same electric field amplitude. The characteristic field Ec increases, while the maximum polarization Pmax and the remnant polarization Pr decrease with increasing frequency (see Fig. 10). This is due to a smaller contribution of domain wall movement with increasing frequency similarly to what observed in PZT 5H.60 The increasing difficulties in obtaining saturated P-E loops with increasing frequency (Fig. 8) and the pronounced frequency dependence of the Pmax in the low temperature range (Fig. 10(a)) support the idea that the relaxation at fR is linked to domain wall movement. The higher saturation of the P-E loops and the lower frequency dispersion with increasing temperature confirm that in our BNT-BT-5CT, thermal energy may help in overcoming domain wall hindrance by pinning sites as suggested above. At 125  C, the I-E loop shows four peaks, two for each electrical loading quadrant (Fig. 9). Similar current peaks have already been observed in other BNT-BT based FIG. 8. Current-Polarization-Electric field loop at different frequencies in the range 25  C–75  C. FIG. 9. Current-polarization-electric field loop at different frequencies in the range 100  C–175  C. materials,27,61 although not discussed in details. In order to distinguish the I-E loop at 125  C from the ones at lower temperatures, the current peaks have been named 6E1 and 6E2, which indicate the corresponding electric fields. It can be seen that the magnitude of both E1 and E2 slightly increases with increasing frequency (Fig. 9). Interestingly, at 125  C, the frequency dependence of Pmax and Pr is at the minimum (Fig. 10). These observations may be interpreted as follows. At 125  C, there is the coexistence of a polar and a weakly polar phase. The origin of the current doublets can be better FIG. 10. Frequency dependence of Pmax and Pr in the range 25  C–175  C. Downloaded 03 Jul 2013 to 138.37.45.6. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions 014107-8 Viola et al. understood by monitoring the I-E signals during the entire electrical history. During experiments at increasing electric field amplitude (not shown here), it was noticed that the peaks at 6E1 are not present at small electric field amplitudes until the peaks at 6E2 appear at higher electric field amplitudes. During the application of a positive electric field, the peak at þE2 may indicate the critical field that produces a polar phase. Upon electric field unloading, there is a little polarization recovery leading to a pronounced remnant polarization. During field reversal (application of a negative electric field), the remnant polarization is completely recovered at the field correspondent to the current valley between E1 and E2, where the polarization becomes zero. Therefore, at electrical cycling regime conditions, the field þE1 (or E1) can be regarded as the field that needs to be applied to recover the polarization effects produced in the previous electrical cycle in correspondence of E2 (or þE2). Sequential electric-field induced polarization events in BNT-based materials associated with coexistence of weakly polar and polar phases are supported by recent in-situ diffraction analyses.62 The I-E loops in the range 150  C175  C also show four peaks corresponding to 6EF and 6ER (Fig. 10). The difference compared to the current doublets at 125  C is that two current peaks indicated with 6ER appear during electric field unloading. In this case, at electrical cycling regime, the polarization effects produced at þEF (or EF) can be significantly recovered during unloading at þER (or ER). In the range 150  C-175  C, the remnant polarization significantly decreased compared to 125  C (see Fig. 10(b)), suggesting higher stability of a weakly polar state in the absence of an applied electric field with increasing temperature. This is further confirmed by the increasing of EF and ER from 150  C to 175  C (see Fig. 9). To note, also the weaker frequency dispersion of EF and ER compared to Ec (Figs. 8 and 9). A final comment is deserved by the temperature dependence of Pmax. From Fig. 10(a), it is apparent that Pmax initially decreases in the interval 25  C–30  C, as typical ferroelectrics would normally experience with increasing temperature. At T > 30  C, instead, BNT-BT-5CT shows a remarkable increment of Pmax with increasing temperature, with a maximum at 150  C (Fig. 10(a)). In addition to the aid of thermal energy to overcome eventual domain wall pinning, this phenomenon can be attributed to the existence or co-existence of the following polarization mechanisms favoured with increasing temperature: (a) an increasing contribution of polarization extension mechanism; (b) an electric field-induced transition from weakly polar ergodic relaxor to non-ergodic state, and (c) an enhanced polarizability of the unit cell under electric field due to the weakening of the Bi-O hybrid orbitals with increasing temperature as previously proposed.12 The presence of additional current peaks found in the I-E loops with increasing temperature can be attributed to a single or to an overlapping of the above polarization mechanisms. Polarization extension mechanisms have already been invoked to justify the enhancement of piezoelectric properties in different systems near a critical triple point where MPBs coexist with a third non-polar phase.63 It was suggested that the coexistence of rhombohedral (R)-tetragonal (T) MPB with a non-polar cubic phase (C) produces the flattening of the energy profiles between the coexisting phases J. Appl. Phys. 114, 014107 (2013) which allows easier polarization extension mechanisms from cubic-to-rhombohedral and from cubic-to-tetragonal, together with R-T polarization rotation under the application of the field.63 In BNT-BT system with the morphotropic phase boundary composition, polarization extension mechanisms were not found significant at the depolarization temperature Td, because the Raman studies did not support the local coexistence of MPB with a third non-polar cubic phase.64 However, in our BNT-BT-5CT, the presence of calcium on the A-site may induce the coexistence of polar and non-polar phases of different symmetries with increasing temperature as suggested by the Raman measurements. This would favour polarization extension mechanisms under electric field with increasing temperature. The other possible polarization mechanism is related to the existence of a reversible phase transition induced by the electric field as observed in other BNT-based systems.8,15,28,65 It is likely that with increasing temperature, the application of an electric field produces a metastable polar order during the application of the field. The latter disappears during field unloading, where the material recovers a significant amount of polarization. In order to gain insight into the weakening of the Bi-O bond (scenario c), a detailed analysis of the modes below 100 cm1 would be necessary. However, it was demonstrated12 that such changes related to the A-site are then reflected into anomalies of the octahedral vibrations. This could be at the basis of the hardening observed for the 380 cm1 mode and the other anomalies observed for the high-frequency (>500 cm1) modes. IV. CONCLUSIONS The temperature TS corresponding to a visible shoulder in the dielectric permittivity of 0.95[0.94Bi0.5Na0.5TiO3– 0.06BaTiO3]0.05CaTiO3 represents a characteristic threshold between two dielectric relaxation processes. Below TS, the permittivity decreases in a quasi-liner fashion with the logarithm of the frequency, while above TS shows a more complicated dependence. With increasing temperature from TS onwards, the softening of the Raman mode at 270 cm1 and the significant reduction of the remnant polarization suggest the significant presence of a weakly polar order. Three possible polarization mechanisms during the application of an electric field were identified in the weakly polar phase based on the frequency and temperature dependence of the P-E response and the temperature dependence of the Raman modes. 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