Composition-dependent phase evolution and enhanced electrostrain properties of (Bi0.5Na0.5)TiO3–BaTiO3–Bi(Li0.5Ta0.5)O3 lead-free ceramics

Journal of Alloys and Compounds xxx (xxxx) xxx

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Composition-dependent phase evolution and enhanced electrostrain properties of (Bi0.5Na0.5)TiO3eBaTiO3eBi(Li0.5Ta0.5)O3 lead-free ceramics Yunyun Gong a, b, Xiang He a, b, Chen Chen c, **, Zhiguo Yi a, b, c, * a

CAS Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, China c State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 August 2019 Received in revised form 13 October 2019 Accepted 27 October 2019 Available online xxx

Exploring lead-free materials to substitute the commercialized PZT-based compounds is in great demand for the development of miniaturized and nonhazardous actuator devices. Here, a new ternary lead-free (1-x)(Bi0.5Na0.5)1-yBayTiO3-xBi(Li0.5Ta0.5)O3 (BNBTy-xBLT) solid solution system is designed to optimize the electrostrain performance of the well-studied BNT ceramics, and the effects of BLT and Ba substitutions on phase evolution as well as electrical performances of BNT ceramics are systematically investigated. All the compositions prepared by solid-state sintering process exhibit similar pseudocubic phase with rhombohedral/tetragonal distortions in nanoregions. Through optimizing the BLT and Ba contents, the composition with 6.9 mol% Ba and 0.8 mol% BLT exhibits the maximum unipolar strain of ~0.39% (equivalent normalized strain d33*~654 pm/V, 60 kV/cm). Moreover, the origin of the excellent electrostrain response is emphatically analyzed from microscopic (domain structure evolution) and macroscopic (TF-R shifting) perspectives, which results from the electric-field-induced reversible relaxorferroelectric phase transition. These results suggest the composition-optimized BNT-BT-BLT system as a potential alternate to lead-based systems in actuator applications. © 2019 Elsevier B.V. All rights reserved.

Keywords: Bi0.5Na0.5TiO3 Lead-free actuator Composition-dependent phase evolution Electrostrain

1. Introduction Ferroelectric/piezoelectric perovskites have been successfully employed in sensing and actuating field because of their excellent electromechanical coupling ability. As the most commercialized ferroelectric material, lead zirconate titanate (PZT) has been dominating this market for a long period of time, however its limited strain response ~0.2% and environmental harmful issues hinder the sensor and actuator applications towards miniaturization and environmental friendliness, which drives the development

* Corresponding author. CAS Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China. ** Corresponding author. State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China. E-mail addresses: [email protected] (C. Chen), [email protected] (Z. Yi).

of lead-free alternates with high piezoelectricity as well as giant strain response [1,2]. Bi0.5Na0.5TiO3 (BNT)-based ceramics are regarded as a very promising lead-free ferroelectric alternative. Among them, BNT-BT systems with morphotropic phase boundary (MPB) formulated by introducing appropriate amount of BaTiO3 (~6e7 mol% BT) into BNT have been of great interest due to their high electromechanical performances [3e7]. According to recent reports, giant strain response (>0.3%) can be simply achieved by adding trace modifications into BNT-BT systems at MPB regions, such as Nb2O5modified BNBT6 [8], (Zn1/3Nb2/3)4þ-modified BNBT7 [9] or solid solutions formed by adding a third compound AgNbO3 or Sr2MnSbO6 [10,11]. It is suggested that the origin of the excellent electrostrain response in BNT-based ceramics is attributed to composition-dependent phase evolution, which can realize reversible phase transition under external field [12,13]. The unpoled BNT-BT ceramics at MPB regions usually show a macroscopic pseudocubic structure [14,15] with rhombohedral and

https://doi.org/10.1016/j.jallcom.2019.152822 0925-8388/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Y. Gong et al., Composition-dependent phase evolution and enhanced electrostrain properties of (Bi0.5Na0.5) TiO3eBaTiO3eBi(Li0.5Ta0.5)O3 lead-free ceramics, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152822

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tetragonal distortions in nanoregions (polar nanoregions, PNRs), which has been verified by transmission electron microscopy [16]. Once applied external electric field, the PNRs will be irreversibly transformed to long-range correlation, which exhibits typical ferroelectric behavior at room temperature. Through proper doping, its phase transition temperature between ferroelectric and relaxor state (TF-R) is shifted to room temperature or even lower. As a result, long-range correlation is destabilized and a reversible relaxor (zero field)-ferroelectric (under electric field) phase transition is established at room temperature, simultaneously a large field-induced strain can be observed, which is stronger than that associated with piezoelectric effect [8,17,18]. Bi-based compounds (BiMeO3), where Me represents the single or complex B-site cations, have been extensively applied as modifiers to enhance the ferroelectric and piezoelectric performances of lead-free ferroelectrics [19e24]. Recently, it shows that proper BiMeO3 as modifiers into BNT-based ceramics can also disrupt longrange ferroelectric order and result in strong strain response [25]. Nguyen et al. reported that BNT-based ceramics co-doped with Liþ and Ta5þ can obtain a high normalized strain d33*~727 pm/V [26]. Based on the above understanding, Bi(Li0.5Ta0.5)O3 as the third component is introduced into BNBT system to form a novel ternary solid solution. Besides, adjusting Ba content can refine polar domain size [27], resulting in relaxor behavior and large strain. Therefore, in this work, a novel ternary solid solution system (1x)(Bi0.5Na0.5)1-yBayTiO3-xBi(Li0.5Ta0.5)O3 (BNBTy-xBLT) is synthesized through solid-state reaction. The optimum ratio of BLT and BT to obtain high strain response is explored as well. Furthermore, the origin of the giant strain is systematically analyzed from aspects of domain structure evolution and TF-R shifting, respectively.

2. Experimental procedures High purity carbonate powders Li2CO3 (98%), BaCO3 (99.99%), Na2CO3 (99.8%) and metal oxide powders TiO2 (98%), Bi2O3 (99.86%), Ta2O5 (99.99%) as starting materials were used to prepare

lead-free BNBTy-xBLT ceramics by solid-state method. All starting powders were stoichiometrically weighed followed by ball-milling in ethanol for 10 h at 300 rpm. Then, the well-mixed powders were calcined for 2 h at 850  C in air. The product was re-milled in ethanol for 10 h at 300 rpm. After drying and then granulating using a binder of 5 wt% polyvinyl alcohol (PVA), the powders were pressed into pellets of 410 mm  H1 mm under 4 MPa. After PVA is burned for 1 h at 650  C, the pellets were embedded in corresponding calcined powders in a sealed alumina crucible and then sintered for 2e4 h at 1120e1160  C. X-ray diffractometer (XRD, Rigaku Co., MinFlex 600, Tokyo, Japan) was used to identify the phase structure of BNBTy-xBLT ceramics. Surface microstructural analysis of sintered ceramics was conducted using a scanning electron microscope (SEM, SU-8010, Tokyo, Japan). Scanning probe microscope (SPM, Dimension Icon, Bruker) was used to observe domain morphology of polished ceramic samples. Prior to electrical characterization, both surfaces of ground ceramics were covered by silver paste and then heated for 20 min at 550  C. The temperature-dependent permittivity and loss curves were collected by using an LCR meter (TH2816A, Changzhou, China). The frequency-dependent impedance and phase angle at room temperature were tested by the same equipment, where resonance frequency fr and antiresonance frequency fa were read. Then the planar electromechanical coupling factor kp could be calculated according to the following equation:

1 kp ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi fr 0:395 f f þ 0:574 a

(1)

r

A ferroelectric testing equipment (TF Analyzer 2000 FE-module, Germany) was employed to measure the ferroelectricity of the ceramics, including ferroelectric hysteresis loops, current curves and bipolar/unipolar strain curves. To observe the phase evolution under electric field, field-dependent dielectric permittivity εr was measured at room temperature under 400 Hz using the same ferroelectric testing equipment. A quasi-static piezo-d33 apparatus

Fig. 1. (a) Room temperature XRD patterns of unpoled BNBT0.065-xBLT ceramics; (b) The variation of lattice parameter a as changing x; (ced) Surface morphology of selected BNBT0.065-xBLT ceramics: (c) 0.005, (d) 0.011.

Please cite this article as: Y. Gong et al., Composition-dependent phase evolution and enhanced electrostrain properties of (Bi0.5Na0.5) TiO3eBaTiO3eBi(Li0.5Ta0.5)O3 lead-free ceramics, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152822

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Table 1 Measured density, theoretical density and relative density of BNBT0.065-xBLT ceramics. Composition

Measured density (g/cm3)

Theoretical density (g/cm3)

Relative density (%)

x¼0 x ¼ 0.005 x ¼ 0.008 x ¼ 0.011 x ¼ 0.013 x ¼ 0.015

5.645 5.874 5.747 5.805 5.791 5.816

5.941 5.954 5.960 5.978 5.973 5.936

95.02 98.66 96.43 97.11 96.95 97.98

(ZJ-3AN, Institute of Acoustics, Beijing, China) was used to read piezoelectric constant d33 value of poled ceramics. 3. Results and discussion

(x ¼ 0e0.015) ceramics. The εr curve is strongly affected by BLT contents. A significant hysteretic behavior can be observed when BLT content locates at 0  x  0.011, which indicates the existence of field-induced ferroelectric. The local maximum εr near Ec is attributed to the ferroelectric domains switching. When x is 0.013,

3.1. Structure and electrical performance of BNBT0.065-xBLT The room temperature XRD patterns of unpoled BNBT0.065xBLT ceramics (Fig. 1a) reveal all the compositions as solid solutions with a pure pseudocubic phase, evidenced by no splitting of (2 0 0)pc and (2 1 1)pc peaks due to the resolution limit of the XRD technique. To reveal the structure changes with increasing BLT contents, the lattice parameters were calculated based on XRD peak refinement fitted using a pseudocubic phase. It is noticed that the lattice parameter a gradually decreases when x is below 0.011, and then increases when x is above 0.011 (Fig. 1b). The variations of lattice distortions can be explained by the difference among ionic radii. Compared with Ba2þ (1.61 Å), Naþ (1.39 Å) and Bi3þ (1.36 Å), Liþ (1.24 Å, CN ¼ 12) has smaller radius [28]. When x is below 0.011, Liþ takes precedence over A-sites to compensate the defects caused by sintering process [29], which will result in lattice shrinkage. When BLT content exceeds 0.011, Liþ (r ¼ 0.76 Å, CN ¼ 6) will enter B-site [30]. The radius of complex cations (Li0.5Ta0.5)3þ (0.7 Å) is larger than that of Ti4þ, resulting in lattice expansion. Fig. 1c and d provide the surface morphology of selected BNBT0.065-xBLT ceramics (x ¼ 0.005 and 0.011), which all exhibit nonporous microstructures with spherical grains. The average grain size for compositions with x ¼ 0.005 and 0.011 are 1.24 and 1.19 mm, respectively, which suggests that the variations of BLT contents show no obvious effect on the grain size. Table 1 presents the measured density, theoretical density and relative density of BNBT0.065-xBLT compositions. The ceramic density was measured using Archimedes method and the theoretical density was calculated based on XRD analysis. For all the compositions, the relative density is above 95%, indicating high densification and good sintering properties of the samples. Fig. 2a and b give the temperature dependent permittivity and loss curves of unpoled BNBT0.065-xBLT ceramics to further elucidate the phase structures. There are two broad frequencydependent permittivity anomalies for BNBT0.065e0.008BLT ceramic as shown in Fig. 2a, implying the relaxor behavior. The anomalies at low temperature defined as Tm1 exhibit strong frequency-dependent diffusion, which is attributed to the thermal evolution of discrete PNRs [16] and previously regarded as rhombohedral-tetragonal phase transition [31]. The peaks with weak frequency-dependent dispersion corresponding to maximum εr is marked as Tm2, which was previously regarded as Curie temperature Tc representing tetragonal-cubic phase transition [32]. Recently, it has been proposed that no structural phase transition occurs at Tm2, which is related to a rhombohedral-tetragonal PNRs relaxation [16]. It is also noted that Tm2 moves towards high temperature, while Tm1 gradually moves towards low temperature as increasing BLT content (Fig. 2b). Fig. 2c shows electric fielddependent dielectric permittivity εr of BNBT0.065-xBLT

Fig. 2. (a) Temperature dependent dielectric permittivity and loss of unpoled BNBT0.065e0.008BLT ceramics measured at different frequencies; (b) Temperature dependent dielectric permittivity and loss of unpoled BNBT0.065-xBLT (x ¼ 0e0.015) ceramics measured at 1 kHz; (c) Electric field dependent dielectric permittivity of BNBT0.065-xBLT (x ¼ 0e0.015) ceramics measured at 400 Hz.

Please cite this article as: Y. Gong et al., Composition-dependent phase evolution and enhanced electrostrain properties of (Bi0.5Na0.5) TiO3eBaTiO3eBi(Li0.5Ta0.5)O3 lead-free ceramics, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152822

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Fig. 3. (a) P-E hysteresis loops of BNBT0.065-xBLT ceramics measured under 5 Hz at room temperature; (b) Coercive field Ec and remnant polarization Pr as a function of BLT content; Bipolar (c) and unipolar (d) strain curves of BNBT0.065-xBLT ceramics measured under 5 Hz at room temperature; (e) Impedance and phase angle spectra of BNBT0.065e0.008BLT; (f) Piezoelectric constant d33 and planar electromechanical coupling factor kp as a function of BLT content.

Fig. 4. (a) Room temperature XRD patterns of unpoled BNBTy-0.008BLT ceramics; (b) Cell parameter a of all ceramics; (ced) Surface images of BNBTy-0.008BLT ceramics: (c) 0.063, (d) 0.069.

Please cite this article as: Y. Gong et al., Composition-dependent phase evolution and enhanced electrostrain properties of (Bi0.5Na0.5) TiO3eBaTiO3eBi(Li0.5Ta0.5)O3 lead-free ceramics, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152822

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the permittivity varies slightly first and then decreases drastically under elevating external field (step 1 or 3), suggesting that the relaxor phase is induced to form long-range ferroelectric phase. However, during the unloading process (step 2 and 4, decrease of applied electric field from ± 60 to 0 V), feature phase transition peaks are detected at point a and b, indicating that the fieldinduced ferroelectric phase transforms back to relaxor phase [33], which displays a transition from nonergodic to ergodic relaxor. With further increasing BLT content to 0.015, there is only minute hysteresis behavioral change in dielectric curve, which characterizes predominantly ergodic relaxor. Fig. 3 presents the composition-dependent ferroelectric and piezoelectric performances of BNBT0.065-xBLT ceramics at room temperature. Although the XRD pattern of unpoled BNBT0.065 at MPB region (Fig. 1a) shows no visible rhombohedral and tetragonal phases, the typical saturated ferroelectric hysteresis loop given in Fig. 3a suggests an irreversible field-induced phase transition from

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rhombohedral R3c polar phase to long-range ferroelectric state [34]. As increasing the BLT content to 0.008, the composition maintains the hysteresis behavior of pure BNBT0.065 as a wellshaped P-E loop is observed. When x equals to 0.011, the sample still exhibits a ferroelectric P-E loop with reduced remanent polarization Pr and coercive field Ec. With further increasing BLT content to 0.013, the sample shows antiferroelectric-like pinched hysteresis loops and both Ec and Pr show a dramatic reduction (Fig. 3b), which can be attributed to the existence of a relatively high proportion of ergodic relaxor state at zero field [9]. For x ¼ 0.015, its P-E loop becomes much slimmer, suggesting the predomination of ergodic relaxor state. These results suggest that the field-induced stable ferroelectric phase in BNBT0.065 can be destroyed by increasing BLT content in this system. The bipolar strain S-E curves of BNBT0.065-xBLT ceramics are presented in Fig. 3c. The butterfly-shaped S-E curves with obvious negative strains Sneg are detected for compositions with x  0.011.

Table 2 Measured density, theoretical density and relative density of BNBTy-0.008BLT ceramics. Composition

Measured density (g/cm3)

Theoretical density (g/cm3)

Relative density (%)

y ¼ 0.063 y ¼ 0.065 y ¼ 0.067 y ¼ 0.069 y ¼ 0.071 y ¼ 0.073

5.788 5.747 5.799 5.842 5.727 5.707

5.961 5.960 5.960 5.953 5.953 5.941

97.10 96.43 97.30 98.14 96.20 96.06

Fig. 5. Temperature dependence of dielectric permittivity εr and dielectric loss tand of poled BNBTy-0.008BLT ceramics: (a) y ¼ 0.063; (b) y ¼ 0.065; (c) y ¼ 0.067; (d) y ¼ 0.069; (e) y ¼ 0.071; (f) y ¼ 0.073.

Please cite this article as: Y. Gong et al., Composition-dependent phase evolution and enhanced electrostrain properties of (Bi0.5Na0.5) TiO3eBaTiO3eBi(Li0.5Ta0.5)O3 lead-free ceramics, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152822

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Normally, Sneg corresponds to the ferroelectric domain backswitching and represents ferroelectricity contribution [35]. For compositions with x  0.013, sprout-shaped loops are observed with no Sneg, indicating the emergence of ergodic phases [9]. Fig. 3d gives the unipolar strain curves, which provides more intuitive variation trend of field-induced strain. Linear strain curves with weak hysteresis behavior are obtained from the compositions with x  0.011, which are mainly attributed to the intrinsic piezoresponse [17,18]. These compositions are poled well with high phase angle q (Fig. 3e) [36]. As given in Fig. 3f, their d33 and kp are above 120 pC/N and 0.25, respectively. And the highest d33 ~146 pC/ N and kp~0.28 are achieved for x ¼ 0.008 and 0.005, respectively, indicating that small amount of BLT doping can facilitate domain orientation and enhance piezo-response. For compositions with x  0.013, the unipolar strain curves show larger strain response with strong hysteresis behavior, and a maximum strain ~0.35% under 60 kV/cm is obtained from the composition x ¼ 0.013 despite of its small piezo-response (d33 ¼ 10 pC/N). The composition x ¼ 0.013 shows the highest strain response in both bipolar and unipolar strain tests, which is suggested to be associated with the field-induced reversible relaxor to ferroelectric phase transition. This finding indicates that the filed-induced reversible phase transition shows stronger strain response than the intrinsic piezoresponse [8].

3.2. Structure and electrical performance of BNBTy-0.008BLT To further optimize strain response of the BNBT-BLT systems, the Ba content is adjusted from 0.063 to 0.073 around the MPB region.

The BLT content is fixed to 0.008 due to the good piezoelectric property (Fig. 3f). Fig. 4a presents the room temperature XRD patterns of unpoled BNBTy-0.008BLT ceramics, which remain pure perovskite phase without obvious impurities, and no splitting of (2 0 0)pc and (2 1 1)pc peaks is detected with changing Ba content. And only slight changes occur in cell parameter a as shown in Fig. 4b. In addition, Ba substitution also shows no obvious effect on the microstructures as shown in Fig. 4c and d. The average grain size is 1.21 and 1.15 mm for compositions with y ¼ 0.063 and 0.069, respectively. The relative densities for all the compositions are over 96% as shown in Table 2. The temperature dependent permittivity and loss curves of poled BNBTy-0.008BLT ceramics are presented in Fig. 5 to investigate the composition-dependent TF-R changes. Compared with the dielectric curves of unpoled BNBT0.065e0.008BLT ceramic (Fig. 2a), an additional anomaly for poled BNBT0.065e0.008BLT ceramic (Fig. 5b) is detected at 60.7  C, which is clearly identified in both permittivity and loss curves and marked as TF-R, indicating phase transition temperature between ferroelectric and relaxor states and confirming the field-induced stable ferroelectric state for poled ceramic at room temperature. It is noticed that TF-R is reduced from 62.3 to 48.9  C with increasing Ba content from 0.063 to 0.067. When Ba content is further increased to 0.069, TF-R cannot be observed above room temperature, indicating that TF-R shifts below ambient temperature and the ergodic relaxor state exists stably at room temperature. This result imply that appropriate Ba doping can shift the phase transition between field-induced ferroelectric state and relaxor state to room temperature, thus large filed-induced strain due to reversible phase transition can be obtained at this

Fig. 6. (a) P-E hysteresis loops; (b) I-E curves; (c) bipolar and (d) unipolar strain hysteresis loops of BNBTy-0.008BLT ceramics (y ¼ 0.063e0.073) measured under 60 kV/cm and 5 Hz at room temperature; (e) Impedance spectra and phase angle of BNBT0.067e0.008BLT; (f) Piezoelectric constant d33 and planar electromechanical coupling factor kp of BNBTy0.008BLT ceramics (y ¼ 0.063e0.073).

Please cite this article as: Y. Gong et al., Composition-dependent phase evolution and enhanced electrostrain properties of (Bi0.5Na0.5) TiO3eBaTiO3eBi(Li0.5Ta0.5)O3 lead-free ceramics, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152822

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critical composition. The effect of Ba content on the ferroelectric and piezoelectric performances of BNBTy-0.008BLT ceramics at room temperature is given in Fig. 6. For compositions y ¼ 0.063e0.067, typical saturated P-E loops are observed (Fig. 6a) and their corresponding I-E curves (Fig. 6b) show one pair of current peaks near Ec, corresponding to the ferroelectric domain switching. When BT content is above 0.069, all samples show similar pinched hysteresis loops, and two pairs of current peaks marked as P1 and P2 are detected in the I-E curves, indicating the emergence of relaxor state at zero field. This result suggests that higher Ba content can also destabilize the filedinduced long-range ferroelectric order, which reverses back to relaxor state after removing the external electric field [9,37]. Moreover, it is easy to achieve the phase transition between ergodic relaxor state and long-range ferroelectric state evidenced by the current peak P1 near zero field and their comparable free energy inferred from resemble maximum polarization value [38]. The butterfly-shaped bipolar S-E loops (Fig. 6c) and slim unipolar S-E hoops (Fig. 6d) observed from compositions with y  0.067 correspond to their typical ferroelectric hysteresis behavior given in Fig. 6a, while for compositions with y  0.069, they exhibit the sprout-shaped bipolar S-E loops and strong hysteretic unipolar strain response due to the reversible field-induced phase transition. Maximum strain of 0.43% from bipolar strain

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curve and 0.39% from unipolar strain curve under 60 kV/cm are obtained from composition with y ¼ 0.069. Generally, large driving field is applied to obtain high strain response, therefore normalized strain d33* (Smax/Emax) is introduced for comparison with other BNBT-based systems or lead-free piezoelectrics. Fig. 6f gives Ba content dependence of d33* according to the unipolar strain results. The samples with y  0.067 show large d33 due to their high remnant polarization Pr (Fig. 6a) but relatively small d33* (185e246 pm/V) because of the weak strain response (Fig. 6d). When y increases to 0.069, long-range ferroelectric state is destroyed and the ergodic relaxor state emerges after electric poling, resulting in a sharp drop in d33. Simultaneously, the highest d33* of 654 pm/V is obtained due to the large strain response caused by reversible phase transition between ergodic relaxor state and long-range ferroelectric state, which is higher than that of most reported BNBT-based systems or other lead-free piezoelectrics [39e43]. With further increasing y contents, all samples still show large d33* above 500 pm/V with near-zero d33 due to the coexistence of ergodic relaxor state and long-range correlation. These results validate that TF-R can be moved to low temperature by adjusting Ba content and optimized strain can be obtained at room temperature. To visualize the relationship between strain response and phase evolution in BNBTy-0.008BLT ceramics, Fig. 7 gives the temperature dependent P-E loops, bipolar and unipolar S-E loops. At room

Fig. 7. Temperature dependent P-E hysteresis loops, bipolar and unipolar S-E of BNBTy-0.008BLT ceramics with y ¼ 0.063 (aec) and y ¼ 0.069 (def).

Please cite this article as: Y. Gong et al., Composition-dependent phase evolution and enhanced electrostrain properties of (Bi0.5Na0.5) TiO3eBaTiO3eBi(Li0.5Ta0.5)O3 lead-free ceramics, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152822

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temperature, BNBT0.063e0.008BLT shows typical P-E loop (Fig. 7a) and butterfly-shaped S-E loop (Fig. 7b) due to the field-induced ferroelectric state. When the measuring temperature increases to 60  C, which is near the phase transition temperature TF-R indicated in Fig. 5a, the sample exhibits a pinched P-E loop and a sproutshaped S-E loop due to the reversible transition between relaxor phase and ferroelectric state. The best field-induced strain ~0.34% is also observed at 60  C. With further increasing the temperature to 100  C, slim P-E loop with weak hysteresis corresponding to the predominant ergodic relaxor phase is observed, and reduced strain response is detected simultaneously. As for BNBT0.069e0.008BLT, it exhibits pinched P-E loop and sprout-shaped S-E loop with maximum strain response at room temperature (Fig. 7d and e). When temperature increases, both its ferroelectric hysteresis behavior and strain (bipolar and unipolar) response are continuously degenerated and behave like normal dielectric materials at 100  C due to the nearly stabilized relaxor state. This finding reveals that high electrostrain response is exhibited at TF-R regions due to field-induced reversible phase transition. Consequently, maximus filed-induced strain can be obtained in BNBTy-xBLT system at room temperature by shifting TF-R to room temperature through adjusting the Ba and BLT doping level. Piezoresponse force microscopy (PFM) is a powerful tool to visualize domain structure in microscopic scale. To better understand the interplay between compositions and strain response, the composition dependent domain structures of unpoled BNBTy0.008BLT (y ¼ 0.063 and 0.069) ceramics are given in Fig. 8. No additional information is provided in topography. However, obvious differences are observed in out-of-plane (OP) amplitude

and phase images between the two samples. For composition with y ¼ 0.063, there are two kinds of PFM responses (Fig. 8c). Most of them are stripe-like domains with strong amplitude response (Fig. 8b). Small amount of irregular nanodomains with weak amplitude response, indicating PNRs, are randomly distributed between stripe-like domains, which is a unique characteristic of relaxor ferroelectrics [44]. This observed domain structures are similar to that of unpoled BNT-6BT ceramic at MPB region reported recently by Zhao et al. [45], which exhibits excellent ferroelectric and piezoelectric performances after electric poling owing to the complete switching of the domains. As for y ¼ 0.069, it also shows complex domains consisting of stripe-like domains and PNRs. Compared with BNBT0.063e0.008BLT, by increasing Ba content, BNBT0.069e0.008BLT exhibits more PRNs than stripe domains, indicating the domination of relaxor state. It is believed that domain morphology has great influences on ferroelectric and piezoelectric properties. The relationship between domain wall energy g and domain size D can be described as follows [46]:

pffiffiffi Df g

(2)

Small domains have lower domain wall energies, which lead to the improved flexibility and easier response to external stimuli. Therefore, PNRs can be easily induced into ferroelectric state under external voltage due to the high flexibility, while the long-range ferroelectric order is unstable and easy to relax back to PNRs after removing the external field. The evolution between PNRs and long-range ferroelectric domains is generally accompanied by large strain response. This observed microphenomenon is consistent with the macro-performance shown in Fig. 6. 4. Conclusions In summary, the BNBTy-xBLT solid-solution systems were successfully fabricated through solid-state reaction. By adjusting BLT and Ba contents, the TF-R is shifted to room temperature regions or even lower. The BNBT0.0069e0.008BLT composition shows a maximum unipolar strain of 0.39% (equivalent normalized strain d33*~654 pm/V, 60 kV/cm) at room temperature, which is due to the reversible relaxor-ferroelectric phase transition induced by electric field. Moreover, the domain structure analysis further confirms the origin of the giant strain response. This study provides an alternative approach to further optimize the electrostrain properties of BNT-based ferroelectrics. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21577143, 51502325, 51872311 and 51902331), the Natural Science Foundation of Fujian Province (Grant No. 2017J05031 and 2018I0021), the Natural Science Foundation of Shanghai (Grant No. 19ZR1464900) and the Frontier Science Key Project of the Chinese Academy of Sciences (QYZDB-SSWJSC027). References

Fig. 8. Composition dependent topography, out-of-plane (OP) amplitude and phase PFM images in a 5  5 mm2 area of unpoled BNBTy-0.008BLT ceramics: (aec) y ¼ 0.063; (def) y ¼ 0.069.

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Please cite this article as: Y. Gong et al., Composition-dependent phase evolution and enhanced electrostrain properties of (Bi0.5Na0.5) TiO3eBaTiO3eBi(Li0.5Ta0.5)O3 lead-free ceramics, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152822