To improve the piezoelectric properties of Bi_0.5Na_0.5TiO₃-based ceramics, a new perovskite-type leadfree piezoelectric (1 – 𝑥 – 𝑦)Bi_0.5Na_0.5TiO₃−𝑥Bi_0.5K_0.5TiO₃−𝑦BiMnO₃ system has been fabricated by a conventional solid–state reaction method and their microstructure, dielectric and piezoelectric properties have been investigated. The results of X-ray diffraction (XRD) analysis reveal that the addition of small amounts of BiMnO₃ did not cause a remarkable change in crystal structure, but resulted in an evident evolution inmicrostructure. An obvious secondary phase was observed in samples with high Bi_0.5K_0.5TiO₃ content. It is found from dielectric constant curves that low-temperature hump disappeared with increasing y and it appeared again with increasing x. The piezoelectric properties significantly increase with increasing Bi_0.5K_0.5TiO₃ and BiMnO₃ content. The piezoelectric constant and electromechanical coupling factor attain maximum values of 𝑑₃₃ = 182 pC/N at 𝑥 = 0.21(𝑦 = 0.01) and 𝑘_p = 0.333 at 𝑥 = 0.18 (𝑦 = 0.01), respectively.

The correlation between permittivity frequency dispersion and depoling process upon heating was investigated in Zr⁴⁺-modified 0.75BiFeO₃–0.25BaTiO₃ (BF–BZT) ceramics. The temperature-dependent permittivity 𝜀_r(𝑇) and the piezoelectric coefficient 𝑑₃₃ for poled samples were measured under heating conditions to clarify the depolarization mechanism. The results indicate that the poling temperature plays a crucial role in the domains' alignment process, as expected. The temperature-dependent permittivity frequency dispersion and depolarization behaviours may have same origin. The aligned domains' break up into random state/nanodomains at depoling temperature (𝑇 _d), which causes strong frequency dependence of the permittivity, simultaneously, induces the loss of piezoelectricity. It suggests that the temperature-dependent permittivity measurements method is a simple way to determine the depolarization temperature.

It is well known that domains and crystal structure control the physical properties of ferroelectrics. The ex-situelectric field-dependent structural study, carried out in unpoled/poled crushed powder and bulk samples for (Li$_{0.5}$Nd$_{0.5}$)$^{2+}$ modified 0.95Bi$_{0.5}$Na$_{0.5}$TiO$_3$−0.05BaTiO$_3$ solid solution, established a correlation between domain configuration andcrystal structure variation. Under applying electric field, the smeared ferroelectric phase structure due to coherence diffractioneffect of nanodomains reappeared due to obsolescent coherence effect associated with the field-induced ordered nanodomains.The macroscopic characterizing techniques of domain configuration such as dielectric constant spectroscopy and X-raydiffraction measurement can provide a basis for understanding the correlation between domains configuration and crystalstructure in ferroelectric ceramics.

The complex ions (Mn$_{1/3}$Nb$_{2/3}$)$^{4+}$ doped 0.82BNT–0.18BKT (BNKT-xMN) ceramics were prepared by conventional solid-state sintering. The effects of the MN content on the structural and electrical properties of the BNKT-$x$MN ceramics were investigated. The grain size decreases sharply after doping MN. With the increase of the MN content, the phase structure changes from the rhombohedral and tetragonal phase to the tetragonal phase, then to the pseudo-cubic phase. The ferroelectric phase transforms to the relaxor phase. At critical phase (x = 0.03), the maximum positive bipolar strain and unipolar strain are 0.38 and 0.386%, respectively. The corresponding $d^*$$_{33}$ and $d_{33}$ are 767 pm V$^{–1}$ and 158 pC N$^{–1}$, respectively. Meanwhile, the dielectric constant gradually decreases with the increase of the MN content, which flattens the permittivity curves. The large piezoelectric responses are closely associated with the reversible relaxor ferroelectric phase transformation.

In this study, Ho$^{3+}$ doped 0.825K$_{0.5}$Na$_{0.5}$NbO$_3$-0.175Sr(Yb$_{0.5}$Nb$_{0.5}$)O$_3$ luminescence transparent ceramics were prepared via the traditional solid-state sintering method. The structure and optical properties of the ceramics before and after polarization were studied at 40 kV cm$^{-1}$ for 0.5 h. With the increase of Ho content, the phase structure of the ceramics changed from a pseudo-cubic phase to the tripartite and the orthorhombic phases, and the light transmittance decreased. The ceramics demonstrated an up-conversion luminescence characteristic under the excitation of a 980 nm laser, and the emission wavelengths were 550 and 670 nm. The best up-conversion luminescence performance was obtained when the Ho content was 0.1%. Moreover, the polarization markedly enhanced the luminescence performance of the 0.825K$_{0.5}$Na$_{0.5}$NbO$_3$-0.175Sr(Yb$_{0.5}$Nb$_{0.5}$)O$_3$-0.1%Ho ceramics due to the increased possibility of energy-level radiative transition of rare-earth Ho$^{3+}$ ions and reduction of the $E$$_g$ value of the ceramic.