Phase transition behavior, dielectric and ferroelectric properties of (1 − x)(Bi0.5Na0.5)TiO3-xBa0.85Ca0.15Ti0.9Zr0.1O3 ceramics

Journal of the European Ceramic Society 36 (2016) 2461–2468

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Phase transition behavior, dielectric and ferroelectric properties of (1 − x)(Bi0.5 Na0.5 )TiO3 -xBa0.85 Ca0.15 Ti0.9 Zr0.1 O3 ceramics Yongping Pu a,b , Mouteng Yao a,∗ , Hairui Liu b , Till Frömling b a b

School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xuefu-Zhonglu 3, Xi’an 710021, PR China Institute of Materials Science, Technische Universität Darmstadt, Alarich-Weiss-Straße 2, Darmstadt 64287, Germany

a r t i c l e

i n f o

Article history: Received 18 December 2015 Received in revised form 7 March 2016 Accepted 8 March 2016 Available online 15 March 2016 Keywords: Solid state synthesis Polar nanoregions Relaxor behavior Energy density High temperature capacitor

a b s t r a c t (1 − x)(Bi0.5 Na0.5 )TiO3 -xBa0.85 Ca0.15 Ti0.9 Zr0.1 O3 ceramics were prepared by solid state route and their phase structure and electric properties were investigated with the focus on optimizing properties for capacitor applications. There are two obvious maxima in the permittivity curves for x = 0.1–0.4 compositions. The first anomaly is suggested to be due to thermal evolution of LT-PNRs and the second one is attributed to the phase transition between the two types PNRs and the thermal evolution of HT-PNRs. The x = 0.3 sample has an operational temperature range from 87 ◦ C to 310 ◦ C. Most of the samples exhibit a dielectric thermal hysteresis. The x = 0.2–0.4 compositions exhibit a significant pinched P-E hysteresis loops. A large d33 * of 427 pm/V was observed in 0.7BNT-0.3BCTZ composition. The x = 0.5–0.6 samples show a slim P-E hysteresis loop, and the sample with x = 0.5 exhibits high energy density of 0.6649 J/cm3 and energy storage efficiency of 83.17%, making it most suitable for application as high energy density capacitors. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Lead-containing ceramics have been used as essential materials for many electronic device. However, the use of lead-containing materials has caused environmental issues due to the high toxicity of lead oxide. Therefore, it is necessary to develop lead-free components for the replacement of these lead-based ceramics [1–3]. (Bi0.5 Na0.5 )TiO3 (BNT) ceramic shows large remanent polarization (Pr = 38 ␮C/cm2 ) and a high Curie temperature (Tc = 320 ◦ C) [1,2]. However, properties like high leakage current and high coercive field (Ec = 7.3 kV/mm) limit possible application in electronic devices. In order to reduce detrimental factors, various NBT-based solid solutions have been developed, such as (1 − x)BNT-xBT [4–6], (1 − x)BNT-xKBT [7,8], and BNT-BT-KNN [9–11]. Additionally, these compositions have attracted considerable attention due to the existence of a morphotropic phase boundary (MPB), accompanied by the enhancement of dielectric permittivity, piezoelectric parameters and E-field induced strain. For example, Zhang et al. [12] found giant strain in BNT-BT-KNN systems and attributed it to the lattice

∗ Corresponding author. E-mail address: [email protected] (M. Yao). http://dx.doi.org/10.1016/j.jeurceramsoc.2016.03.005 0955-2219/© 2016 Elsevier Ltd. All rights reserved.

change by field-induced antiferroelectric-ferroelectric transition. Jo et al. [13], however, showed that the phase transition in 0.92BNT0.06BT-0.02KNN does not involve any notable volume change by monitoring the volume changes from the simultaneously measuring longitudinal and transverse strains on disk-shaped samples. This indicates that there is little contribution from a volume change to the total strain response. Jo et al. proposed that the origin of the large strain observed in the BNT-BT-KNN system can be attributed to a reversible “nonpolar” to ferroelectric phase transformation under electric fields due to their comparable free energies. Recently, Ba0.85 Ca0.15 Ti0.9 Zr0.1 O3 (BCTZ) ceramics with excellent dielectric, piezoelectric and ferroelectric properties were developed by Liu et al. [14] in 2009, which shows better dielectric and piezoelectric properties than many BaTiO3 , (Bi0.5 K0.5 )TiO3 and (Bi0.5 Na0.5 )TiO3 compositions. Gou et al. [15] researched the (1 − x)BNT-xBCTZ (0 ≤ x ≤ 0.1) system, and found that BCTZ can further enhance the piezoelectric properties of BNT. However, the reason is still unclear and information on formation of MPB or the existence of polar, non-polar phases and polar nanoregions is missing in (1 − x)BNT-xBCTZ (0 ≤ x ≤ 0.1) system. (1 − x)NBT-xBCTZ (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) ceramics were prepared to investigate their phase transition behavior, dielectric and ferroelectric properties.

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Fig. 1. The density and relative density as a function of BCTZ content for (1 − x)BNTxBCTZ ceramics.

2. Experimental procedure A conventional solid-state reaction method was used to prepare the (1 − x)BNT-xBCTZ ceramics. The oxides or carbonates Bi2 O3 (99.975% purity), BaCO3 (99.8% purity), CaCO3 (99.8% purity), Na2 CO3 (99.5% purity), TiO2 (99.6% purity), and ZrO2 (99.6% purity) (Alfa Aesar GmbH & Co., KG, Karlsruhe, Germany). First, BNT and BCTZ powders were synthesized separately. Prior to weighing, Na2 CO3 powders were dried at 110 ◦ C for 12 h. According to their stoichiometric formula, raw materials for BNT and BCTZ were mixed in planetary ball mill using Y2 O3 -stabilized ZrO2 grinding media for 24 h. After being milled, the mixed powders for BNT and BCZT synthesis were calcined at 850 ◦ C and 1270 ◦ C for 4 h, respectively. After calcinations, the powders were again ball milled separately for 24 h with the above mentioned ball-milling method. According to the chemical formula (1 − x)BNT-xBCTZ (x = 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6), the BNT and BCTZ powders were weighed and mixed in planetary ball mill for 24 h. The (1 − x)BNT-xBCTZ diskshaped samples of 10 mm in diameter and 1 mm in thickness were uni-axially compacted by hand, followed by cold isostatic pressing at 300 MPa (KIP 100 E, Paul-Otto Weber GmbH, Remshalden, Germany). They were then sintered in air at 1150 ◦ C–1180 ◦ C for 2 h in covered alumina crucibles. To minimize the loss of volatile elements, samples were covered with a powder of the same

composition to create an enriched atmosphere of the respective components. Mass densities were measured according to the Archimedes method. Theoretical densities were calculated using the analysis result of X-ray diffraction (XRD), and relative densities were then calculated from mass density and theoretical density. The phase structures of these ceramics were identified by powder Xray diffraction (CD-MAX 2200pc, Rigaku Co., Tokyo, Japan) at a working voltage and current of 40 kV and 30 mA. XRD data was collected in the range of 20–80◦ with a 0.02◦ step and scanning speed of 5◦ /min. Raman spectroscopy (Renishaw-invia, Renishaw, U. K.) was used to further confirm the phase evolution of these ceramics in the region of 100–1000 cm−1 at room temperature. The sintered samples were polished and thermally etched at 1020–1040 ◦ C for 10 min to investigate the microstructure using a scanning electron microscope (SEM, S4800, Rigaku Co., Japan). For the electrical measurements, the sintered pellets were ground, polished, and painted with silver paste. Temperature and frequency-dependent permittivity and dielectric loss were measured with an impedance analyzer (HP 4192A, Hewlett Packard Corporation, Palo Alto, USA). The data were collected at four frequencies (1 kHz,10 kHz, 100 kHz and 1 MHz) for every 2 ◦ C from RT to 400 ◦ C with a heating rate of 2 ◦ C/min. P-E hysteresis loops were obtained using a commercial aix PES setup (Aix ACCT Systems GmbH, Aachen, Germany), which is equipped with a laser interferometer. 3. Results and discussion The densities and relative densities of (1 − x)BNT-xBCTZ ceramics are shown in Fig. 1 as a function of BCTZ content. The densities of these ceramics are in the range of 5.58–5.83 g/cm3 . It can be observed that the densities of these ceramics gradually decrease with increasing BCTZ content. Generally BCTZ ceramics have a lower density than that of NBT ceramics [15,16]. Nevertheless, the relative densities of the synthesized ceramics in this work are still above 97%, as illustrated in Fig. 1. Fig. 2 shows the SEM micrographs of (1 − x)BNT-xBCTZ ceramics after thermal etching. A dense microstructure is developed for all these ceramics, regardless of BCTZ content. Using a linear intercept method, the grain size is about 1–2 ␮m for all samples. No clear relationship between microstructure and BCTZ content was observed.

Fig. 2. SEM micrographs the polished and thermal-etched surfaces of (1 − x)BNT-xBCTZ ceramics.

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Fig. 3. XRD patterns of (1 − x)BNT-xBCTZ ceramics (a) Full range, the enlarged XRD patterns in the 2 Theta range of (b) 39–40.5◦ and (c) 45–47.5◦ .

Fig. 4. Raman spectroscopy of (1 − x)BNT-xBCTZ ceramics.

Fig. 3(a) provides the XRD patterns of (1 − x)BNT-xBCTZ ceramics with various x values. The data of all the samples show a pure perovskite phase without secondary phases within the resolution of the XRD instrument. This demonstrates that BCTZ has completely diffused into NBT lattices. For better illustration of the (111) and (200) peaks, the enlarged XRD patterns in the 2 Theta range of 39–40.5◦ and 45–47.5◦ are given in Fig. 3 (b) and (c). With increasing BCTZ content, the (111) and (200) reflection peaks shift to a lower 2 Theta angle indicating the expansion of cell volume according to Bragg’s law 2dsin␪ = n␭. The result can be attributed to the substitutions of bigger ions (Ba2+ = 0.161 nm) for smaller ions (Na+ = 0.139 nm, Bi3+ = 0.136 nm) at the A-site and bigger ions (Zr4+ = 0.072 nm) for smaller ions (Ti4+ = 0.605 nm) at B-site [17,18]. BNT-based materials exhibit a complex structure, different models have been reported in the literature, such as single phases (cubic or pseudo-cubic) and two phases (Cc+P4bm and Cc+R3c et al.) [19]. The formation of single (111) and (200) peaks suggests a cubic symmetry as described in the literature [20,21]. In addition, BNT-based relaxor ferroelectrics typically have low symmetry nanoscale domains and show an apparent (average) cubic symmetry [22]. Based on above reports, we can conclude that (1 − x)BNT-xBCTZ having a major pseudo-cubic symmetry and this is typically observed in relaxor materials. Raman spectroscopy was utilized to further confirm the phase transition of (1 − x)BNT-xBCTZ ceramics. The Raman spectra of (1 − x)BNT-xBCTZ ceramics at room temperature is shown in Fig. 4.

Four main active modes were perceived in the (1 − x)BNT-xBCTZ ceramics from 100 cm−1 to 1000 cm−1 , which is consistent with other BNT-based ceramics [23–25]. The modes at wavenumbers ≤150 cm−1 are associated with vibrations of the perovskite Asite, thus involving Na, Bi, Ba, Ca cations. The modes in the wavenumbers range of 200–400 cm−1 relate to B-O vibrations. The modes corresponding to BO6 octahedra vibration are found in the wavenumbers range of 450–700 cm−1 . In the high- frequency range above 700 cm−1 a signal occurs due to overlapping of A1 (longitudinal optical) and E (longitudinal optical) modes [5,8,26]. Compared with the Raman spectrum of pure BNT in literature [23], it seems that the bands corresponding to B-O vibration are broader and the bands corresponding to BO6 octahedra vibration become diffuse. This can be attributed to the B-site disorder in the BNT-BCTZ system, induced by the substituting of Ti4+ by Zr4+ . The mode around 200 cm−1 shows a “valley” for x = 0.1 ceramic, this “valley” gradually transforms into a “slope” when x = 0.2–0.3, and finally turns into a broad peak in the composition range of x = 0.4–0.6. There are two possibilities for a phonon response at around 200 cm−1 : Damping of A1(TO) phonon induced by lattice defects and internal stress in the lattice [27–29]. The modes around 276 cm−1 and 320 cm−1 overlap for x = 0.1 sample, and the mode around 320 cm−1 gradually increase with increasing BCTZ content. A coexistence of two modes (276 cm−1 and 320 cm−1 ) can be observed for x = 0.2. At higher BCTZ content, the mode at 320 cm−1 becomes prominent and the mode at 276 cm−1 becomes fade. We suggest that the weakened intensity of the vibrational modes corresponding to B-O vibration should be ascribed to the lattice distortion induced by the replacement of Ti4+ ions by Zr4+ ion in the B-site, which shorten the Ti O bond dipole moment and thus lead to the decrease in polarizability of the unit cell. The modes corresponding to BO6 octahedra vibration consist of two main broad modes (around 528 cm−1 and 620 cm−1 ) for all samples. In contrast to the mode at 620 cm−1 , the mode around 528 cm−1 becomes more dominant with increasing BCTZ content. Furthermore, the mode at 620 cm−1 for x = 0.2 shifts to slightly lower wavenumbers increasing concentration of BCTZ which may be due to the structural evolution by the Zr4+ substitution into Ti4+ anion [5]. These results are consistent with the XRD results and are also reflected in the electrical properties of (1 − x)BNT-xBCTZ ceramics which will be discussed in the following. The temperature dependence of permittivity and dielectric loss tangent for (1 − x)BNT-xBCTZ ceramics from room temperature to 400 ◦ C are shown in Fig. 5. In recent studies the two dielectric maxima in the frequency dependent permittivity curves, are attributed to the presence of two types of PNR (low-temperature PNRs (LT-

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Fig. 5. Temperature dependence of permittivity and dielectric loss tangent for (1 − x)BNT-xBCTZ ceramics measured at 1 kHz, 10 kHz, 100 kHz and 1000 kHz.

PNRs) and high-temperature PNRs (HT-PNRs)) [9,10,30–32]. The first anomaly, occurring at lower temperatures and showing typical relaxor behavior, is suggested to be due to thermal evolution of LT-PNRs [9]. The second anomaly appears at higher temperatures with a broad peak. It is attributed to the phase transition between the two types PNRs and the thermal evolution of HT-PNRs [9,33,34]. The diffuse behavior caused by the addition of BCTZ in the broad peak can be explained by large differences of ion valences and sizes in both A- and B-site ions, disturbing the long range ferroelectric order of ceramics [35]. The dielectric loss tangent curve also shows two dielectric relaxations responses as shown in Fig. 5 (the inset is the enlarged view near second anomaly). Both relaxations shift to higher temperatures with increasing frequency. The magnitude of first peak increases with increasing frequency, whereas that of second one decreases. Temperature-dependent permittivity measured at 1 kHz for different compositions are summarized in Fig. 6(a). The temperature corresponding to the first dielectric relaxation is defined as T1 and that for the second one is defined as T2 in this work. Fig. 6(b) shows T1 and T2 as a function of BCTZ content. As illustrated in Fig. 6(a), the magnitude of permittivity at T2 decreases remarkably,

whereas that at T1 is almost unchanged with x = 0.1–0.4. The reason is that thermally induced changes of polar structure around T2 are inhibited by defects and compositional disorder [35]. In addition, given that the second anomaly is mainly influenced by the evolvement of HT-PNRs, the depression of the permittivity at T2 indicates that the density of HT-PNRs decreases with the addition of BCTZ [9,33,34]. For x = 0.5–0.6, the magnitude of permittivity at T1 starts to decrease and that at T2 can hardly be observed anymore. The fading out of the second dielectric relaxation for x = 0.5–0.6 suggests that the addition of BCTZ eventually changes the volume ratio between LT-PNRs and HT-PNRs. Fig. 6(b) shows that T1 remains unchanged when x = 0.1–0.4 whereas T2 monotonously shifts to lower temperature for all compositions. T1 begins to shift to lower temperature when x is higher than 0.4 whereas T2 becomes obscure at that point. Note that the magnitude of permittivity for two dielectric relaxation peaks are same at x = 0.3. The variance of permittivity for (1 − x)BNT-xBCTZ ceramics at 1 kHz as a function of temperature are presented in Fig. 6 (c). The permittivity value at 150 ◦ C was taken as the reference point, since it is at the midpoint of the desired operational temperature range and has also been used for other NBT-based high-temperature dielectrics [33,34].

Fig. 6. (a) Temperature dependence of permittivity measured at 1 kHz, (b) T1 and T2 as a function of BCTZ content, (c) The variance of permittivity at 1 kHz for (1 − x)BNT-xBCTZ ceramics.

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Table 1 ␧150 ◦ C and temperature range of (1 − x)BNT-xBCTZ ceramics. x ε150 ◦ C temperature range (◦ C)

±10% ±15%

0.1 3700 101–191 92–204

0.2 3851 98–194 90–215

The permittivity value at 150 ◦ C (ε150 ◦ C ) and the temperature range for ±10% and ±15% tolerance of (1−x)BNT-xBCTZ ceramics are summarized in Table 1. With increasing BCTZ content, for ±10% tolerance, the temperature range increases first and then decreases, showing the widest temperature range of 89–310 ◦ C for the sample with x = 0.3. The variation tendency of ±15% tolerance is similar with that of ±10% tolerance, exhibiting the widest temperature range of 81–330 ◦ C for the sample with x = 0.3, making it most suitable for application as high-temperature capacitor. The permittivity curves of (1 − x)BNT-xBCTZ ceramics upon heating and cooling show a notable difference. Most of the samples exhibit this dielectric thermal hysteresis in this work as shown in Fig. 7. According to previous studies by Chen et al. this indicates a gradual first-order diffuse phase transition [4,36]. They found dielectric thermal hysteresis in BNT7BT single crystal and Zr-doping BNT7.5BT ceramics. It was proposed that the thermal hysteresis is due to the diffuse phase transition from mixed phase of rhombohedral (R) and tetragonal (T) to single tetragonal (T) phases. Zang et al. [9], however, suggest that the phase transition from LT-PNRs to HT-PNRs is the reason for the hysteresis which should be the most likely explanation for the results in this work. The transition between LT-PNRs and HT-PNRs upon heating or cooling might be different. The temperature region of dielectric thermal hysteresis becomes narrower with increasing BCTZ content and the dielectric thermal hysteresis phenomenon gradually disappears for x = 0.5–0.6 ceramics. This implies that the added BCTZ favors a predominant existence of HT-PNRs at lower temperatures, which weakens the contribution of the phase transition from LT-PNRs to HT-PNRs and the subsequent relaxation of transformed HT-PNRs at high temperatures [9]. The P-E hysteresis loops of (1 − x)BNT-xBCTZ ceramics with the maximum applied electric field of 5.5 kV/mm are shown in Fig. 8(a),

0.3 3698 89–310 81–330

0.4 3465 74–246 67–272

0.5 2980 52–211 46–235

0.6 2424 40–193 32–214

(b) and (c). For x = 0.1, the P-E loop illustrates a large coercive field and high remnant polarization Pr for the sample, which is similar to BNT-based ceramics as described in the previous literature [8,25,37–40]. With increasing BCTZ content, the coercive field and Pr decrease and samples feature pinched polarization loops for x = 0.2–0.4. Zhang et al. considered that this pinched polarization loop is due to the formation of some antiferroelectric phase in BNT-based materials [12]. However, recent studies indicate that the existence of “non-polar” phase contributes to this pinched polarization loop rather than antiferroelectric phase [10,41,42]. This implies that the ferroelectric order is disturbed with the increase of BCTZ content, leaving a “non-polar” phase at zero electric field for samples with x = 0.2–0.4. Moreover, increase of x reduces Pr , but has little influence on the maximum polarization value Pmax , as shown in Fig. 8(d). This indicates that the “non-polar” phase at zero electric field can easily transform into a ferroelectric phase by an external electric field of 5.5 kV/mm [43]. For x = 0.5–0.6, Pmax starts to decrease, which is due to the increasing amount of “non-polar” phase. This decrease in the absolute polarization value inducible at given electric field can also be explained by the fact that the addition of BCTZ increases the threshold field required to induce a long-range ferroelectric order [9,30]. From the P-E hysteresis loop, the area between the polarization axis and the discharge curve can be calculated, which is related to the stored energy. Typically, the area of this slim P-E hysteresis loop is much higher than that of normal ferroelectrics [44]. To evaluate the energy density of sample with x = 0.5, its P-E loop at the field up to its breakdown voltage is shown in Fig. 8 (e). Generally, the energy storage density J could be calculated according to [44]:



J=

EdP

Fig. 7. Thermal hysteresis at f = 1 kHz of (1 − x)BNT-xBCTZ ceramics.

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Fig. 8. (a), (b) and (c) P-E hysteresis loops of (1−x)BNT-xBCTZ ceramics, (d) Ec , Pr and Pm as a function of BCTZ content, (e) P-E hysteresis loops of x = 0.5 ceramic.

where E is the applied electric field and P is polarization. The 0.5BNT-0.5BCTZ sample exhibits high energy density of 0.665 J/cm3 and energy storage efficiency of 83.17%, making it most suitable for application as high energy density storage applications. The energy density of 0.5BNT-0.5BCTZ sample is higher than that of widely studied Ba0.4 Sr0.6 TiO3 ceramics (∼0.37 J/cm3 ) although the breakdown strength of 0.5BNT-0.5BCTZ is lower [45,46]. To further improve the energy density of 0.5BNT-0.5BCTZ, one way could be the enhancement of its breakdown strength by the glass additives [47] or spark plasma sintering [48]. Therefore, future work will mostly focus on the enhancement of breakdown strength for 0.5BNT-0.5BCTZ ceramic. Fig. 9 shows the temperature dependent P-E hysteresis loops of (a) x = 0.2 and (b) x = 0.4 in the range of room temperature to 100 ◦ C. The maximum and remanent polarization progressively decrease with increasing temperature, suggesting decreasing stability of the field-induced long-range ferroelectric order and the increasing amount of “non-polar” phase [13,30]. A slimmer P-E loop can be observed at the temperature of 100 ◦ C, at which the first dielectric anomaly appears with typical relaxor behavior, which is consistent the reported literature [30].

The compositionally induced ferroelectric to “non-polar” phase transformation can be well verified by the bipolar strain measurements, as provided in Fig. 10(a) (b) and (c). It is evident that the samples with x = 0.1 and 0.2 display a typical ferroelectric feature with “W-shaped” butterfly curve [4]. In contrast, x = 0.3–0.6 compositions show a relaxor ferroelectric feature with “V-shaped” curve [4], which is due to the reduction in the negative strain that is closely related to the domain back switching during bipolar cycles with the appearance of “non-polar” phase [5,26,43]. The large signal d33 * (Smax /Emax ) is considered as an idea reference value when compared to different actuator materials [4]. The d33 * as a function of BCTZ content is shown in Fig. 10 (d). The d33 * significantly increases with increasing BCTZ content up to x = 0.3 and then it decreases. The presence of “non-polar” phase which can be converted to ferroelectric phase under the electric field contributes to the large field-induced strain [43]. Moreover, for x ≥ 0.4, the amount of “non-polar” phase further increases which will causes a decrease in d33 *. A large electric field-induced strain of 0.23% was perceived at a low applied field 5.4 kV/mm which corresponds to d33 * of 427 pm/V in 0.7BNT-0.3BCTZ composition.

Fig. 9. Temperature dependent P-E hysteresis loops of (a) x = 0.2 and (b) x = 0.4.

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Fig. 10. The bipolar strain hysteresis loops of (1 − x)BNT-xBCTZ ceramics (a), (b) and (c), d33 * as a function of BCTZ content.

4. Conclusions Solid state route was used to prepare (1 − x)BNT-xBCTZ ceramics. The two dielectric maxima in the frequency dependent permittivity curves, are attributed to the presence of LT-PNRs and HT-PNRs. The magnitude of permittivity at T2 decreases remarkably, whereas that at T1 is almost unchanged with x = 0.1–0.4. For x = 0.5–0.6, the magnitude of permittivity at T1 starts to decrease and that at T2 can hardly be observed anymore. The 0.7BNT0.3BCTZ sample can be used as temperature stability capacitors in the temperature range of 81–330 ◦ C, in which the maximum permittivity variation is within ±15%. The dielectric thermal hysteresis, observed in most of the samples, is due to the phase transition between LT-PNRs and HT-PNRs. The significant pinched P-E hysteresis loops for x = 0.2–0.4 is attributed to the existence of “non-polar” phase at zero field. A large d33 * of 427 pm/V in 0.7BNT-0.3BCTZ composition is attributed to the “non-polar” to ferroelectric phase transition. The 0.5BNT-0.5BCTZ sample exhibits higher energy density of 0.665 J/cm3 and energy storage efficiency of 83.17%.

Acknowledgements This research was supported by the National Natural Science Foundation of China (51372144) and the Key Program of Innovative Research Team of Shaanxi Province (2014KCT-06).

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