Ultrahigh Electromechanical Coupling and its Thermal Stability in (Na 1/2 B i 1/2 )TiO 3 -Based Lead-Free Single Crystals

: In this work, we report the ultrahigh electromechanical coupling performance of NBT-6BT-KNN lead-free single crystal at room temperature. The thickness mode electromechanical coupling coefficient ( k t ) and the 31 mode electromechanical coupling coefficient ( k 31 ) reach 69.0% and 45.7%, respectively, which are superior to the PZT-5H lead-based ceramics of k t ~ 60% and k 31 ~39%. In addition, the evolution of the crystal structure and domain morphology is revealed by Raman scattering spectra, a polarizing microscope and piezoelectric force microscopy characterization.


Introduction
Piezoelectric materials, which convert mechanical to electrical energy (and vice versa), are crucial in sensors, actuators, transducers, and ultrasonic devices [1][2][3]. Lead zirconium titanate (PZT) based ceramics are currently market-dominating due to their excellent piezoelectric properties [4,5]. However, these PZT family materials contain toxic lead, which is restricted in view of the environment. Great efforts have been made worldwide to seek alternative lead-free piezoelectric materials [6][7][8]. The essential strategy for achieving a high piezoelectric response is to seek lead-free materials displaying a transition region in their composition phase diagrams, known as a morphotropic phase boundary (MPB), where energetically comparable polar states coexist and compete with each other [6,7]. Such critical energy landscapes cause a fragility and instability of the polar state, so that the polarization can be easily rotated and altered by external stress or electric field. Some developments in the search for lead-free piezoelectric materials are BaTiO3-based, (K1/2Na1/2)NbO3-based, and Na1/2Bi1/2TiO3-based systems approached by constructing an MPB [6,9,10]. It was later discovered that an MPB could be also created through external mechanical stress or electrical field, which results in a strong piezoelectric response [11,12]. Electric-field in situ Transmission Electron Microscopy (TEM) has revealed that the overall poling-induced phase transitions in 0.94Na1/2Bi1/2TiO3-0.06BaTiO3 ceramics can be described as R3c/P4bm→P4mm/R3c→ R3c [11]. Ge et al. and Daniels et al. proposed that an E-field induced phase transformation from pseudo cubic to T structure is responsible for the large electrically induced strain [13,14]. Recently, Wang et al. [15] reported a ternary lead-free Na1/2Bi1/2TiO3-BaTiO3-(K1/2Na1/2)NbO3 (NBT-BT-KNN) single crystal that exhibits an unprecedented piezoelectric coefficient of d3 ≈ 840 pC N −1 . The origin of the high piezoelectricity is attributed to a phase boundary constructed by the poling field between multiple weak-polar R3c/P4bm phases and ferroelectric tetragonal phases with P4mm symmetry.
The electromechanical coupling coefficient is an important parameter which characterizes the coupling between the mechanical energy and electrical energy of piezoelectric materials. However, the electromechanical coupling coefficient of this ternary NBT-BT-KNN single crystal has not yet been presented. In this communication, we report the ultrahigh electromechanical coupling performance of NBT-6BT-KNN single crystal. The thickness mode electromechanical coupling coefficient (kt) and the 31-mode electromechanical coupling coefficient (k31) reach 69.0% and 45.7%, respectively, which are superior to the PZT-5H lead-based ceramics of kt ~ 60% and k31~39%. The temperature-dependent kt and k31 are further studied, which reveals a mechanism for temperature-induced phase transition.
For the macroscopic electrical characterization, the as-grown single crystals were sliced into wafers with a thickness of ~0.40 mm, perpendicular to their pseudo cubic <100> directions. Based on the IEEE standards, the [001]-oriented crystal planes with a size of 4 × 4 × 0.4 mm 3 were cut to obtain thickness mode samples. The 31 mode samples were cut into rectangular parallelepiped shapes along with crystallographic directions of [100]/[010]/[001] with a size of 12.3 ×2.4 × 0.4 mm 3 . Then, the main faces of the samples were sputtered with gold electrodes. The crystal samples were poled in silicon oil at room temperature for 15 min with a dc electric field of 25 kV cm −1 . Dielectric/loss-temperature and impedance-phase angle measurements were conducted using a computer-controlled Novel control CmbH Concept 40 broadband dielectric spectrometer. Raman measurements were conducted at room temperature using a Raman spectrometer (LabRAM HR800, Horiba JobinYvon, Paris, France). The domain structure was observed using an Olympus (Tokyo, Japan) U-CMAD3 polarizing microscope (PLM) equipped with a Linkam THMS600E (Linkam Scientific Instruments Ltd.,Tadworth, Surrey, U.K). The PLM is used in transmission geometry, and the principle of setup in our case was as follows: we defined the "θ" as the angle between the polarizer/analyzer (P/A) pair and the pseudo cubic <100> direction. For the domains of the tetragonal phase, the optical extinction is achieved when the P/A is set along the <100> direction, namely at θ = 0 o with respect to the <100> direction. For the domains of the rhombohedral phase, they exhibit optical extinction when the P/A is set along the <110> direction, namely at θ = 45 o with respect to the <100> direction. The PFM experiments were carried out using a commercial atomic force microscope MFP-3D (Asylum Research, Goleta, USA). An ac modulation voltage of 7V (peak to peak) with a frequency of 10 kHz was applied between the conductive tip and the bottom gold electrode. Figure 1a,b shows the thickness mode and 31 mode impedance and phase angle of the poled NBT-6BT-KNN crystal along the <001> direction at room temperature, respectively. It is clear from the figure that the phase angle of NBT-6BT-KNN crystal in the two modes is close to 80°, indicating the completely polarized state. The electromechanical coupling coefficients kt and k31 are calculated by the resonance-antiresonance method (as shown in Formula 1-1, 1-2), which are 69.0% and 45.7%, respectively, superior to those of kt ~ 60% and k31-39% of PZT-5H lead-based ceramics [16]. We have also compared our own results in this work to the results from full sets of electromechanical constants of other lead-free ferroelectric (piezoelectric) ceramic/single crystals; details can be seen in Table 1 [

Results and Discussion
Figure 1c,d shows the temperature-dependent behavior of kt and k31, respectively. It is clear from the figures that kt and k31 exhibit good temperature stability within this temperature range between −150 and 40 °C, indicating an excellent cryogenic electromechanical coupling property and a promising candidate for advanced electromechanical transducer applications below room temperature. When the temperature reaches above 45 °C, the value of kt and k31 degenerates sharply to zero. We also measured the temperature-dependent dielectric constant and dielectric loss curves of <001> poled oriented NBT-6BT-KNN crystal at different frequencies, as shown in Figure 2. In general, two dielectric anomalies can be observed during the heating process. The temperature corresponding to the maximum dielectric constant is considered to be the phase transition temperature from the tetragonal phase to the tetragonal + cubic two phase region [10], and the dielectric anomaly at low temperature around 150 ℃ range shows strong relaxation characteristics of dielectric broadening and frequency dispersion, which are closely related to the polar nanoscale (PNRs) transition from the R3c to the tetragonal P4bm phase according to the TEM results of NBT-based ceramics. These PNRs in the R3c or P4bm phase have a strong relaxation and show a weak/non-polar state [22]. It should be noted that a discontinuous variation in dielectric/loss curves can be only observed in poled crystal and the anomaly also appears exactly at 45 ℃, defined as depolarization temperature (Td). This is well consistent with the degeneration temperature of kt and k31. The reason for this observation is most likely that the electric field induces the transformation from the weak/non-polar pseudocubic phases (R3c/P4bm) to the polar tetragonal phase (P4mm) and heating causes the tetragonal polar phase to transform back to the non-polar phases [15].  In order to verify this assumption, we observe the domain evolution of <001> oriented NBT-6BT-KNN crystals under different electric fields and temperatures using a polarizing microscope, as shown in Figure 3. At zero electric field (E = 0 kV/cm), NBT-6BT-KNN crystals show complete light extinction, indicating the initial pseudocubic non-polar state. This is consistent with the XRD result of as-grown NBT-6BT-KNN crystal powder (Figure 4). The diffraction pattern shows no profile shape asymmetry or splitting at any reflections, revealing that the sample is an initial cubic or pseudo cubic structure. When the electric field increases to 25 kV/cm, obvious macroscopic ferroelectric domains are present for P/A: 45° (θ = 45°), indicating that the electric field induces the phase transition from a non-polar pseudocubic phase to a ferroelectric polar phase [15,23]. The red dotted line and arrows in Figure 3b show the single domain area and crystal cracks, respectively. When the electric field is removed, the ferroelectric domains are still maintained (Figure 3c), revealing that the polar phase is stable after removing the electric field. In addition, as shown in the inset of Figure 3c, the optical extinction of these domains for P/A: 0° (θ = 0°) indicates that the polar phase belongs to the P4mm symmetry. When the temperature is increased above Td (T = 50 °C), the ferroelectric domain disappears completely, as presented in Figure 3d, which confirms the temperature induced a transition back to the non-polar phases.  Macroscopic Raman spectra contains local information via local phonon signals. Previous studies of NBT-based systems reveal that the changes in these Raman modes can reflect the crystal phase transition sensitively. Figure 5a,b shows the room temperature Raman spectra of unpoled (E = 0 kV/cm) and poled (E = 25 kV/cm) NBT-6BT-KNN crystals, respectively. The Raman spectra have been deconvoluted into six vibrational modes and the assignment of these modes is marked. The bands around 300 cm −1 are assigned to the Ti-O lattice vibration. The bands (around 527 cm −1 and 610 cm −1 ) between 400 and 700 cm -1 are assigned to the TiO6 octahedron vibration closely related to crystal symmetry [24,25]. After applying an electric field, two changes are clearly noticed. The relative intensity of 527 cm −1 and 610 cm −1 related to TiO6 octahedron vibration changes significantly, and the vibration around the 610 cm −1 band is significantly weakened compared to the mode around 527 cm −1 . The behavior in these high frequency modes suggests a reduced degree of disorder [26], which further supports the electric-field-induced phase transition from pseudocubic a state to a ferroelectric tetragonal state. The electric-field-induced TiO6 octahedron dynamics, in turn, affect the Ti-O band. The poled crystal exhibits relatively more symmetry around the 300 cm −1 band, as compared to that of the unpoled crystal, which is also consistent with the reduced degree of disorder. When the poled crystals are heated to a temperature above Td (T = 50°C), the Raman spectra are then measured and shown in Figure 5c. It is clear that the Raman scattering profile almost completely evolves back to the initial state without poling, indicating that the temperature induced phase transitions from the polar tetragonal state back to the initial nonpolar pseudocubic state. These results are exactly consistent with the domain evolutions in Figure 3. The sharply degenerated kt and k31 at Td for poled NBT-6BT-KNN crystals can now be well understood by the electric filed/temperature induced phase transition between the non-polar and polar states. When the crystals are poled at 25 kV /cm, the phase transitions occur from the pseudocubic non-polar phase to the tetragonal polar phase. However, when the crystals are heated to a temperature reaching above Td, the ferroelectric polar phases are unstable and transform back to a pseudocubic non-polar phase, accompanied by a sharp degenerated kt and k31 and a discontinuous variation in dielectric/loss curves at exactly Td.
The piezoelectric and electromechanical coupling coefficients are strongly dependent on microstructural changes of nanodomains [27,28]. Piezoelectric force microscopy (PFM) is a very good experimental approach to detect ferroelectric nanoscale domains. In order to trace the origin of the ultrahigh electromechanical coupling coefficients kt and k31 in NBT-6BT-KNN crystals, we used PFM to test the microscopic domain structure of NBT-6BT and NBT-6BT-KNN crystals. Figure 6 shows the PFM images of NBT-6BT-KNN and NBT-6BT crystals along the <001> direction. It can be seen that nano-regions of NBT-6BT-KNN exhibit a more dense and uniform distribution than that of NBT-6BT. In particular, the polar-regions became notably narrower with sizes, on average, of ~50 nm. As reflected in the amplify images (Figure 6c-d), the boundaries between regions became much smoother. These results clearly show that the addition of KNN to NBT-BT refines the size of the polar nanoregions and enhances their self-organization. Since the domain size is proportional to the square root of the domain wall energy, the smaller domain size makes it easily reoriented under external excitations, e.g., applied electric field or mechanical force [29,30]. Thus, the piezoelectric properties significantly improve with a decreasing domain size. It should be noted that these results have been found in BaTiO3 ferroelectric single crystal [31][32][33][34]. Therefore, such refinement of ferroelectric domains in NBT-6BT-KNN crystal would lead to an increased domain-wall density and contribute to its ultrahigh value of electromechanical coupling performance. Conclusions 0.93(Na1/2Bi1/2)TiO3-0.06BaTiO3-0.01K1/2Na1/2NbO3 (NBT-6BT-KNN) single crystals show ultrahigh electromechanical coupling coefficients kt and k31 reaching 69.0% and 45.7% respectively, which are superior to the PZT-5H lead-based ceramics of kt ~ 60% and k31 ~ 39%. kt and k31 vary little and exhibit a good temperature stability within this temperature range from −150 to 40 °C. When the temperature reaches above 45 °C, the value of kt and k31 degenerates sharply to zero, which suggests a temperature induced tetragonal polar to pseudocubic non-polar phase transition verified by dielectric, polarizing microscope and Raman scattering measurements. The refinement of the ferroelectric domains and high domain-wall density was observed in the PFM results, which contributes to the ultrahigh electromechanical coupling of NBT-6BT-KNN crystals.
Author Contributions: For research articles with several authors, a short paragraph specifying their individual contributions must be provided. Chao Chen gave the onceptualization and wrote original draft preparation.; Li Yang did data curation,; Xingan Jiang, Xiaokun Huang, Xiaoyi Gao, Na Tu, Kaizheng Shu joined formal analysis,.; Xiangping Jiang, Shujun Zhang, Haosu Luo supervised this work,. All authors have read and agreed to the published version of the manuscript." please turn to the CRediT taxonomy for the term explanation. Authorship must be limited to those who have contributed substantially to the work reported.
Funding: This work is financially supported by the National Natural Science Foundation of China (51862016, 51762024, 51602135), the Foundation of Jiangxi Provincial Education Department (GJJ190712). The author (Chao Chen) wishes to acknowledge the support from the Jiangxi Voyage Project and China Scholarship Council. The author also thanks Kunyu Zhao for performing the PFM measurements.

Conflicts of Interest
The authors declare no conflict of interest