Domain switching contribution to the ferroelectric, fatigue and piezoelectric properties of lead-free Bi_0.5(Na_0.85K_0.15)_0.5TiO₃ films

Abstract

Lead-free Bi_0.5(Na_0.85K_0.15)_0.5TiO₃ films have been prepared via a solution-gelation technique. The microstructure, domain structure, ferroelectric, fatigue and piezoelectric properties were investigated systematically. This shows that the films have a single-phase perovskite structure and show outstanding ferroelectric, fatigue and piezoelectric properties at room temperature. The maximum piezoelectric coefficient value of the films reaches approximately 158.94 pm V⁻¹, which is comparable to that of polycrystalline lead-based films. Thus good ferroelectric, fatigue and piezoelectric properties are attributed to the well-defined electrical domain structure and its switching for Bi_0.5(Na_0.85K_0.15)_0.5TiO₃ films. The present results suggest that Bi_0.5(Na_0.85K_0.15)_0.5TiO₃ films can be used as a candidate for lead-free films in piezoelectric micro-electro-mechanical systems.

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1. Introduction

Piezoelectric materials are widely used in sensors, actuators, transducers, and other electronic devices.¹ As one kind of most outstanding piezoelectric material, Pb(Zr,Ti)O₃ (PZT)-based materials have been paid attention due to their excellent ferroelectric and piezoelectric properties. However, it is worth noting that waste products containing more than 60 wt% lead (Pb) cause critical environmental problems and the high volatility of toxic lead fumes during processing makes PZT compounds dangerous to handle. To resolve these problems, considerable ideas have been directed toward the development of lead-free ferroelectric materials.² Bismuth sodium titanate Bi_0.5Na_0.5TiO₃ (BNT) is considered to be an excellent candidate to replace lead-based piezoelectric materials. BNT is of perovskite structure with high Curie temperature (T_c = 320 °C).³ However, the draw-backs including smaller remnant polarization, piezoelectric properties and relatively poor fatigue performance are not neglected for BNT materials,^4,5 which limits their further application in ferroelectric storage devices, sensors, actuators and transducers.

A morphotropic phase boundary is known to play a very important role in piezoelectric materials such as PZT materials in which both rhombohedral and tetragonal ferroelectric phases exist simultaneously.⁶ Because more possible polarization variants are available, the ferroelectric, fatigue and piezoelectric properties are enhanced at morphotropic phase boundary bulk compositions.^7–9 Among BNT-based solid solutions, Bi_0.5Na_0.5TiO₃–Bi_0.5K_0.5TiO₃ (BNT–BKT) systems have been received a great deal of attention due to its piezoelectric properties in the rhombohedral–tetragonal morphotropic phase boundary at optimal composition of Bi_0.5(Na_0.85K_0.15)_0.5TiO₃ (BKNT15).^10–12 With the development of micro-electro-mechanical, film devices strongly need to be developed. Therefore, this work aims to prepare single-phase BKNT15 films and investigate crystal structure, domain structure, ferroelectric, piezoelectric, leakage current, and fatigue properties. Well-defined domain structure and its switching have been obtained, which contributes to the outstanding ferroelectric, piezoelectric, fatigue and leakage properties. The origins of the well ferroelectric, piezoelectric, leakage and fatigue properties are discussed in detail. The results provide an instructive route to obtain alternative lead-free films with well ferroelectric, piezoelectric and fatigue properties.

2. Experiment

The lead-free BKNT15 films were prepared using the solution-gelation method. In order to obtain the denser microstructural films with good crystallinity, Pt(100)/Ti/SiO₂/Si wafer is selected as the substrate. The raw materials used for the precursor solutions are bismuth nitrate pentahydrate [Bi(NO₃)₃·5H₂O], sodium nitrate [NaNO₃], potassium nitrate [KNO₃], butyl titanate [CH₃(CH₂)₃O]₄Ti. Bismuth nitrate pentahydrate was added to the ethylene glycol solvent in proportions of 1.1 : 1, with an excess 10% Bi to compensate for its loss during annealing. Sodium nitrate [NaNO₃] and potassium nitrate [KNO₃] were added to the ethylene glycol solvent in proportions of 1.05 : 1, with an excess 5% Na and K respectively to compensate for its loss during annealing. Then stir two solution until the solution is clear transparent at room temperature. [CH₃(CH₂)₃O]₄Ti and C₅H₈O₂ were dissolved in the ethylene glycol methyl ether. Bismuth nitrate, sodium nitrate and potassium nitrate precursor solution was titrated to butyl titanate precursor solution to obtain the sodium bismuth potassium titanate precursor solution. Then the films deposition were carried out by spin coating the above solution on to Pt(100)/Ti/SiO₂/Si wafer at 4000 rpm for 30 s. And the films were dried at 280 °C on the thermostatic oven for 300 s. Then the dried deposited films were annealed at 710 °C in rapid thermal processor system for 5 min under the oxygen atmosphere with flow of 1.5 L min⁻¹. Repeat the above steps 22 times, on the last times, the films were annealed at 710 °C for 20 min. The crystalline phases of films were characterized by X-ray diffraction (XRD, Panalytical-Empyrean) on a D/MAX-RA diffractometer using CuKα radiation. For the fabrication of BKNT15 film capacitors, Au top electrodes with a diameter of 0.2 mm were deposited on the surface of the films using an ion-sputtering method. Their ferroelectric, leakage and fatigue properties were studied by a multiferroic tester system (MultiFerroic100V, Radient Technology, USA) at room temperature, respectively. The domain structure and piezoelectric coefficients d₃₃ were investigated by an piezoresponse force microscopy (PFM, Asylum Research Cypher™). The surface morphologies of the films were investigated by a scanning electron microscope (SEM, Hitachi SU-3500) with an operating voltage of 15 kV.

3. Results and discussion

Fig. 1(a) shows the X-ray diffraction patterns of BNKT15 films, which reveals a pure single-phase structure with no pyrochlore phases. BNKT15 films exhibit a polycrystalline perovskite structure without preferred orientations. The surface morphological features of BKNT15 films are shown in Fig. 1(b). The compact microstructures containing variable grain size with significantly reduced intergranualar porosity were observed in BKNT15 films. Thus relatively dense microstructure is beneficial to its ferroelectric properties. Fig. 1(c) presents the cross-sectional SEM image of BKNT15 films, which shows obvious layered structure. The thickness of BKNT15 films was estimated to be 680 nm. The films thickness is rather uniform throughout the length, as confirmed by several thickness measurements performed at various spots on the films. Similar results have also been observed for the other BKNT15 films.

Fig. 1 (a) Shows the X-ray diffraction patterns of BNKT15 films; (b) SEM image of BKNT15 films; (c) the cross-sectional SEM images of BKNT15 films.

The local polar structures and static ferroelectric domains of BKNT15 films have been imaged by means of piezoresponse force microscopy (PFM), as shown in Fig. 2. Fig. 2(a) shows the atomic force microscopy (AFM) morphologies with 1 × 1 μm² scanning area of BKNT15 films. Fig. 2(b) and (c) show the simultaneously obtained PFM amplitude and phase images of the as-grown BKNT15 films, respectively. For PFM amplitude and phase images, different colors represent different response intensities and local polarization orientations, respectively. From Fig. 2(a), it can be seen that the sample has smooth surface, distinct grain boundaries and uniform grains. The average grain size of BKNT15 films is approximately 130 nm, which is consistent with the results of SEM imaging. The PFM amplitude and phase images show that the as-prepared BKNT15 films have clear domain structures as shown in Fig. 2(b) and (c). The domain walls between two kinds of domains with different polarization are found to be presented within the same grain in all the samples due to the different polarization orientations. The domain structures of BKNT15 films also show that the domains are pinned at the ground boundaries. Combining surface topographies and domain structures, one can be observed that a great deal of domain boundaries coincide with grain boundaries, which suggests that the grain boundaries confine the shape of domain boundaries and affect the domain structures.¹³ It has been proved that the large grains possesses a polydomain configuration. However, only monodomains are found if the grain size becomes smaller.¹⁴ For the present BKNT15 films, the lager domain size and less density of the domain walls are obtained, which originates from the tensile strain enhanced by the combined effects of K and Na ions, the bigger grain size or even the misfit dislocation between the films and the substrate.¹⁵ Therefore, the ferroelectric polarization switching could be effected by the different domain wall densities in small or larger grains, the inbuilt electric field between these domain walls and the change in the strain conditions as the result of ferroelastic domain displacement/creation. At the same time, the less density of the domain walls is expected to obtain a low leakage current in BKNT15 film.¹⁶

Fig. 2 Simultaneously obtained (a) surface topography, (b) PFM amplitude, and (c) PFM phase images of the as-grown BKNT15 films; simultaneously obtained (d) surface topography, (e) PFM amplitude and (f) PFM phase images obtained after the tip poling.

To investigate the tip electric bias dependence of domain switching after poling procedure, the domain images on the poled region of BNKT15 films were measured by PFM. Topographic image of BNKT15 films with 1 × 1 μm² scanning area is shown in Fig. 2(d). Fig. 2(e) and (f) display amplitude and phase images of BNKT15 films after poling procedure, which was poled by 15 V tip electric bias with 1 × 1 μm² area. The contrast of the images represents the relative polarization or charge density of the films. Compared with the morphologies of the as-grown BKNT15 films, Fig. 2(d) has not at all changed, which shows that the grains of the films are not affect by the tip electric bias. However, the domain structures change obviously, as shown in Fig. 2(e) and (f). As shown in the PFM phase images, the grain boundaries are clearly observed and always serve as the domain boundaries as well. The domain structures of BKNT15 films after poling procedure show more homogeneous domains and the domains are pinned at the ground boundaries. And the density of the domain walls in BKNT15 films after poling procedure is less than that of the as-grown BKNT15 films. Besides, the ferroelectric domain structures of BKNT15 films after poling procedure exhibit a fractal growth habit with a domain size similar to the grain size rather than a disordered growth habit as the as-growth BKNT15 films. For the domain switching, on one hand, strong amplitude signal and phase contrast in Fig. 3(e) and (f) suggest complete switching because piezoresponse signal is rather uniform within the grains. On the other hand, the polarization directions of most of domains apparently switches to the applied biases, which indicates that minor domains have not switched after poling. Basing on the chosen three domain switching cases, it suggests that there are following reasons to explain the unswitched domains. Firstly, when a bias electric voltage is applied to the polycrystalline BKNT15 film, the mobile charge carriers move to the grain boundaries, and thus the grain boundaries become the pinning centers that leads to domain wall pinning and impedes domain switching.^17,18 Secondly, the electrical domains are likely to be restricted by the strong strain conditions or some other configuration reasons such as the structural incompatibility-induced local stress across the grain boundary. Thirdly, grain boundary-like trenches are likely to reduce the mismatch of elastic distortions between individual domains, which limits the switching of the domains.¹⁹ Besides, the area of the applied electric bias is much smaller than the area of the domain boundaries since a nano-sized conductive atomic force microscopy (AFM) probe is used as the top electrode. Therefore, it is difficult to drive the bigger domain to switch completely. Based on those discussion above, it can draw a conclusion that domain walls play a key role to clarify polarization switching mechanisms. The structure and the density of the domain walls of the present BKNT15 films also suggest that it undergoes a higher electric bias and is easier to be polarized.

Fig. 3 Simultaneously obtained (a) typical surface topography, (b) PFM amplitude, (c) phase images of the as-grown BKNT films; simultaneously obtained (d) typical surface topography, (e) PFM amplitude, (f) phase images obtained after DC poling; (g) the schematic of the writing electric bias.

In order to further study the domain switching behavior of BKNT15 films, one writes a 1.2 × 1.2 μm² negatively polarized domain embedded in a 3 × 3 μm² equivalent positively polarized domain. Fig. 3(a)–(c) show the simultaneously obtained typical surface topography, PFM amplitude and phase images of the as-grown BKNT films, respectively. Fig. 3(d)–(f) show the simultaneously obtained typical surface topography, PFM amplitude and phase images obtained after direct current (DC) poling with an applied electric bias. Fig. 3(g) gives the schematic of the writing electric bias, which presents information of the writing voltage, and the dark and bright areas are applied −15 V and +15 V DC biases, respectively. As shown in the PFM images, the piezoresponse signal is not uniform within the same grains and differs from grain to grain because of different polarization orientations. At the same time, the domain walls between two kinds of domains with different polarization are found to be present within the same grains in all the samples. Fig. 3(b) and (c) are the domain images of the as-growth BKNT15 films with random orientation on Pt(100)/Ti/SiO₂/Si substrate. No pattern was observed without applied electric bias. While −15 V and +15 V DC biases were applied to write domain, which obtains regular squares. The center square area was poled by −15 V DC electric biases with 1.2 × 1.2 μm² area, and the other areas were scanned with +15 DC electric bias on 3 × 3 μm² area. Compare with the as-grown BKNT15 films before poling, surface topography of those does not change, as shown in Fig. 3(d). However, the amplitude and phase PFM images of BKNT15 film after poling change obviously. Fig. 3(e) shows that the squares patterns with bright contrast are observed at the negative-voltage applied region. Thus clearly exhibited square patterns are caused by the electrostriction, which also suggests that BKNT15 films are respected to obtain well piezoelectric effect. At the same time, Fig. 3(f) shows the domain images of BKNT15 films after the domain switching induced by the external DC electric bias. The square is patterned by applying the bias to −15 V DC biases marked by the square in Fig. 3(f). Most of domains in this square switch to the same polarized state. And the amplitude and phase images in the center region appear regular squares. Thus polarization directions in the squares patterns of most of domains apparently switch to the applied electric biases, which indicates that only minor amount of domains have not been switched after poling. It may be attributed to the back-switching of domain due to the presence of internal field at the bottom electrode interface and the effect of grain boundaries on the domain switching.^13,20 Meanwhile, the domains perpendicular to the direction of the applied electric bias were hardly switched. Therefore, the squares patterns are partially formed.

On the contrary, in the positive-voltage region where +15 V DC electric biases are applied, most of domain polarization direction of the grain boundaries interior reaches agreement with each other, corresponding to the violet and yellow phase tone with the clear change of the phase contrasts. However, not all the ferroelectric domains are switched but only partial ferroelectric domains are switched to other polarization state, which is attributed to the fact that the poling is performed close to the coercive field and the positive electric bias is not sufficient to switch all of the domains. Therefore, the irregular domain is patterned in the positive-voltage region, whereas the regular squares patterns clearly appears in the negative-voltage region. That is to say, the polarization state under −15 V poling have been changed, along the polarization of the dominant polarized domains, however, it was much harder than polarization switching under +15 V poling. These results implies an unsymmetric phase hysteresis characteristic possessed by our samples. Positive ferroelectric coercive force is predicted to be larger than the negative one, which finally indeed is confirmed by our experimental results as follows.

The PFM phase signal versus the electric bias applied on the tip was shown in Fig. 4, which is the local electromechanical hysteresis loop related to the process of a single domain reversal under the tip. This approach is referred to as piezoresponse force spectroscopy (PFS). Because of the extremely small size of the volume excited during PFS measurements (∼10³ to 10⁵ nm³), PFS offers a unique possibility of studying the role of individual nano- and atomic-scale defects in switching.^21,22 A set of raw data of the local electromechanical hysteresis loops were gained for each location chosen arbitrarily and randomly from the present films. Consequently, it always obtain the closed hysteresis loops, when the maximum bias ±20 V and ±15 V were applied on the tip, which indicated a higher ferroelectric coercive force and not all the domains could be perfectly reversed by switching bias. As can be seen, the positive saturate bias, around 3 V, is larger than the negative one, around −1 V, corresponding to our prediction of the unsymmetric coercive force. Besides, the unknown but absolutely subsistent impurities would also be one of the non-negligible reasons. Under a large DC field, all the domains are supposed to be poled in one direction. However, the impurities are unable or very difficult to be reoriented by external field due to their immobile nature. Therefore, the uncomplete ferroelectric domain switching could be ascribed to local stress across the grain boundary, domain wall densities and displacement/creation, domain states (monodomain or polydomains) in different grains and impurities. Meanwhile, in positive poling regions, since the domains are restricted by a higher ferroelectric coercive force, +15 V was not sufficient to reverse all the ferroelectric domains.

Fig. 4 The phase versus electrical bias hysteresis loops at arbitrary and random point of BKNT15 films.

Fig. 5(a) presents the polarization (P)–electric field (E) hysteresis loops of BKNT15 films measured at room temperature. As can be seen, the hysteresis loops are up to standard at high electric field. The observed P_r and P_s increase with increasing applied electric field, and become almost saturated at a value of 38 μC cm⁻² and 76 μC cm⁻², respectively, under an alternating current electric field of amplitude 1200 kV cm⁻¹. It shows that BKNT15 films obtain the outstanding ferroelectric properties, which is superior to those reported bulk materials or other similar perovskite systems.^23–25 Thus excellent ferroelectric properties of BKNT15 films are attributed to the well-defined domain structure and domain switching. To be specific, the excellent ferroelectricity in BKNT15 films are related to the domain wall density and the internal stress derived from the combined effects of K and Na ions, decreased oxygen vacancies and the formation of a dense surface microstructure. Besides, the excellent ferroelectric properties are associate with rhombohedral–tetragonal morphotropic phase boundary composition of BNT–BKT system and good leakage performance.

Fig. 5 (a) The polarization (P)–electric field (E) hysteresis loops of BKNT15 films; (b) amplitude–voltage butterfly loops and d₃₃–V piezoelectric hysteresis loops of BKNT15 films.

Fig. 5(b) displays the applied voltage dependence of the piezoelectric response for BKNT15 films. The displacement was achieved by keeping the PFM tip fixed above the interesting point and applying a DC voltage from −20 V to 20 V while recording the piezoresponse signal. As can be seen, a typical well-shaped displacement–voltage (D–V) ‘‘butterfly’’ curve is obtained with a maximum displacement of 2.85 nm appearing at 20 V, which shows a strain as high as 3.56‰ ratio for BKNT15 films. The piezoelectric coefficient d₃₃–voltage (d₃₃–V) loops can be indirectly calculated from the corresponding D–V ‘‘butterfly’’ curves via the following modified equation of converse piezoelectric effect: d₃₃ = (D − D₁)/(V − V₁), where D₁ and V₁ are the piezoelectric deformation and the applied voltage at the intersection, respectively.²⁶ The piezoelectric hysteresis loop (d₃₃–V) unambiguously further confirms the ferroelectric behavior of BKNT15 films. The piezoelectric coefficient d₃₃ of BKNT15 films derived from the slope of the displacement versus voltage curve reaches 158.94 pm V⁻¹. The value of piezoelectric coefficient d₃₃ (158.94 pm V⁻¹) of BNKT15 film is much larger than those of other lead-free piezoelectric films, even also larger than polycrystalline PZT (about 50–90 pm V⁻¹) films.^27–29 Therefore, environment friendly BNKT15 films are considered as a promising alternate film materials replacing PZT in piezoelectric devices.

The mechanisms concerning the dependence of the high piezoelectric performance for BKNT15 films are attributed to the domain structure and its switching. In fact, peizoelectric coefficient d₃₃ in perovskite ferroelectrics can be expressed via the following equation.³⁰

d₃₃ = 2Q_effεP (1)

where Q_eff is the effective electrostrictive coefficient, P and ε are spontaneous polarization and permittivity, respectively. Thus P and Q_eff play important roles in determining d₃₃. As discussed above, BKNT15 films show larger spontaneous polarization and larger P_r and P_s are obtained due to the well-defined domain switching. Besides, when applied a electric bias on the films, obvious electrostrictive behaviors are observed, as shown in Fig. 3(e). Thus larger effective electrostrictive coefficient Q_eff is beneficial to the large value of d₃₃. Besides, the electric field will be quite easy to tilt the polar vector of domain due to the reduction in direct coupling of the domain walls to the grain boundaries,³¹ which give rise to very strong piezoelectric activity.³²

Fig. 6(a) shows the characteristic of the leakage current density (J) versus applied electric field (E) for BKNT15 films at room temperature. The leakage current density for BKNT15 films is 4.0 × 10⁻⁶ A cm⁻² at an applied field of 1000 kV cm⁻¹, which obtain relatively excellent leakage current performance. The excellent leakage properties of BKNT15 films is believed to be attributed to the decreased density of the domain walls. Moreover, thus good leakage current properties are related to morphotropic phase boundary composition of BNT–BKT system films and the leakage conduction mechanism. Fig. 6(b) plots the curves of ln(J) versus ln(E), which can definitely determine the nature of the conduction mechanism. For the present films, the fitting slope value is 1.12, approximating to 1 in all electric field, which indicates that the leakage current conduction mechanism agrees well with the ohmic mechanism expressed by the equation:³³

J = qμN_eE (2)

where J is the leakage current density, μ is the ion mobility, and N_e is the density of thermally stimulated electrons. For BKNT15 films, the leakage current conduction mechanism is the sole ohmic mechanism at a wide range of the applied electric fields, which obtain outstanding leakage properties. In other words, the single type of ohmic mechanism in BKNT15 films is beneficial to obtain excellent leakage properties.

Fig. 6 (a) The characteristic of the leakage current density (J) versus applied electric field (E) for BKNT15 films; (b) the curves of ln(J) versus ln(E) for BKNT15 films.

The fatigue behaviors of BKNT15 films were studied by observing the switchable polarization as a function of the number of reversal cycles. Fig. 7(a) plots the degradation of switchable polarization P*−Pˆ of BKNT15 films against the number of switching cycles. P* represents the switching polarization between two pulses with opposite polarity, and Pˆ represents the non-switching polarization between two pulses with the same polarity. For BKNT15 films, the fatigue behaviors were measured with a frequency of 1 MHz and driving electric fields of ∼710 kV cm⁻¹. It shows that the polarization value slowly drops with increasing switching cycles. The switchable polarization P* of BKNT15 films decreases by approximately 6.63% from 62.14 μC cm⁻² to 58.02 μC cm⁻² after 10¹⁰ switching cycles. Moreover, non-switching polarization Pˆ of BKNT15 films also decreased by approximately 6.63% from 31.79 μC cm⁻² to 29.68 μC cm⁻². Thus results indicate that BKNT15 films exhibit excellent anti-fatigue characteristics, which exhibits the best anti-fatigue properties and no significant lowering of polarization since 10¹⁰ cycles. Fig. 7(b) further presents the comparison of the P–E hysteresis loops before and after being subjected to 10¹⁰ switching cycles. As can be seen, the saturation polarization P_s is 76.5 μC cm⁻² at applied electric field of 1200 kV cm⁻¹ before switching. In comparison, P_s after switching of BKNT15 films is 71.7 μC cm⁻² at the same electric field of 1200 kV cm⁻¹. The polarization only slightly changes after being subjected to 1 × 10¹⁰ switching cycles. The lead-free perovskite BKNT15 films obtain outstanding fatigue behaviors and much better anti-fatigue characteristics. Thus well ferroelectric fatigue resistance is beneficial from the domain structure and it switching, which depresses the pinning effect of the domain walls.

Fig. 7 (a) Degradation of polarization P*–Pˆ of BKNT15 films against the number of switching cycles, (b) the comparison of the P–E hysteresis loops before and after being subjected to 10¹⁰ switching cycles.

Particularly, the physical mechanism for the polarization fatigue in ferroelectric films is mainly the pinning effect of domain walls derived from structural defects such as oxygen vacancies and porosity.^34,35 It is well known that the reversal of ferroelectric domains does not change the surrounding stress conditions. Therefore, the complex multistep switching process of polarization reversal results in domain wall pinning by the incorporation of mobile charge carriers into non-neutral domain walls.^36,37 However, other ferroelastic domain reorientation induces a mechanical stress field near the grain boundary and inside grains since it demands coordinate accommodation by adjacent.^38,39 More ferroelastic domain switching can depress domain wall pinning and enhance fatigue resistance in the films. The present BKNT15 films obtain the lower defect concentration including space charges, oxygen vacancies, which depresses the pinning effect of the domain walls⁴⁰ and further results in the well fatigue resistance. Besides, more ferroelastic domain switching in BKNT15 films depresses the pinning domain walls, which improves the ferroelectric fatigue resistance. Thus results implies that the outstanding piezoelectric properties exist still after electrical recycling for many times since piezoelectric coefficient proportional to ferroelectric polarization. The environmentally friendly BKNT15 films with excellent fatigue resistance can be candidate as piezoelectric memory.

4. Conclusions

In summary, BKNT15 films were prepared by chemical solution deposition. The microstructure, domain structure, ferroelectric, piezoelectric and fatigue properties for BKNT15 films were systematically investigated. Well-defined domain structure and strong ferroelectric, piezoelectric properties have been observed in the fabricated BKNT15 film. The excellent ferroelectric, leakage, anti-fatigue and piezoelectric properties were obtained due to the well-defined domain structure and its switching. The present BNKT15 films can been used as candidates of the environmentally friendly piezoelectric memory devices due to their larger piezoelectric coefficient with value of 158.94 pm V⁻¹ and well fatigue resistance.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 11264026, 11564028), and Inner Mongolia Science Foundation for Distinguished Young Scholars (Grant No. 2014JQ01).

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