Thermal stability of the electromechanical properties in acceptor-doped and composite-hardened (Na_1/2Bi_1/2)TiO₃-BaTiO₃ferroelectrics

Lead-free relaxor ferroelectrics are promising candidates for next-generation piezoelectric high-power devices, such as ultrasonic motors, transformers, and therapeutic ultrasonics. These applications require hard ferroelectrics with a broad operating temperature range. Recently, acceptor Zn2+ doping and composite formation with ZnO were proposed to induce hardening in Na1/2Bi1/2TiO3-BaTiO3 and simultaneously increase the depolarization temperature. Here, these two strategies are compared by studying the temperature dependence of electromechanical properties, ferroelectric loops, and nonlinear polarization harmonics. In the modified compositions, depolarization is associated with the shift of the ferroelectric-to-relaxor transition to higher temperatures, while the depolarization onset remains unchanged. This leads to broadening rather than translation of the depolarization region, accompanied by decoupling of the piezoelectric d 33 and d 31 coefficients. The temperature-dependent electromechanical response is stable for composites, while the Zn2+-doped samples exhibit strong temperature dependence akin to acceptor-doped Pb(Zr,Ti)O3. The thermal evolution of electromechanical coefficients is not related to the thermally induced decrease of the coercive/internal bias fields but instead to the ratio of irreversible-to-reversible nonlinear dynamics arising from displacements of domain walls or similar interfaces. The results demonstrate that mechanical stress-based hardening in the composites exhibits superior thermal stability, which can considerably improve the operational range of lead-free piezoelectric materials.