*2.2. Sample Characterization*

The crystallographic structure of the samples was characterized by X-ray diffractometer (XRD, DX−2700B, HAOYUAN INSTRUMENT, Dandong, China) using CuK<sup>α</sup> radiation. The microscopic morphology of the samples was observed by scanning electron microscope (SEM, Quanta FEG 250, FEI, Waltham, MA, USA). The elemental analysis of the samples

was carried out by the energy-dispersive X-ray spectroscopy (EDS) attached to the scanning electron microscope. In the frequency range of 100 Hz–100 kHz, the dielectric constant and loss of the samples as a function of temperature (room temperature~650 ◦C) were detected by an LCR meter (TH2829A, Tonghui Electronic, Changzhou, China) attached to a programmable furnace. At various temperatures (room temperature~650 ◦C), the AC impedance spectrum of the samples as a function of frequency (20 Hz–2 MHz) was measured by using an impedance analyzer (TH2838, Tonghui Electronic) attached to a programmable furnace. The electromechanical resonance spectroscopy of the poled samples was investigated by a broadband LCR digital bridge in the frequency range from 280 to 390 kHz. The planar electromechanical coupling factors *k*p, mechanical quality factors *Q*m, and planar frequency constant *N*<sup>p</sup> were determined according to Onoe's equations [27]. The piezoelectric charge coefficient of the samples was measured using a quasistatic *d*33/*d*<sup>31</sup> m (ZJ−6AN, IACAS, Beijing, China).
