Phase diagram and electric properties of the (Mn, K)-modified Bi_0.5Na_0.5TiO₃–BaTiO₃ lead-free ceramics

In this study, the phase diagram and electric properties were demonstrated for a (Mn, K)-modified Bi_0.5Na_0.5TiO₃ (BNT)-based solid solution. (0.935−x) Bi_0.5Na_0.5TiO₃−xBi_0.5K_0.5TiO₃−0.065BaTiO₃ with 0.5% mol Mn doping was prepared by a conventional solid-state reaction method. A morphotropic phase boundary (MPB) formed between the ferroelectric rhombohedral and tetragonal phases around x of 0.04 with the MPB tolerance factor t of 0.984–0.986. The temperature and composition dependence of the dielectric, piezoelectric, ferroelectric properties along with the strain characteristics were investigated in detail and a phase diagram was presented. Around the MPB region, the maximum values of piezoelectric constant \( d_{33}^{*} \) of 290 pC/N, d ₃₃ of 155 pC/N, dielectric constant \( \varepsilon_{33}^{T} /\varepsilon_{0} \) of 1059 and low dielectric loss tangent tan δ of 0.017 were obtained. In addition, the authors also suggest that the solid solution with composition x of 0.24, exhibiting both high-depolarization temperature T _d of 182 °C, \( d_{33}^{*} \) of 156 pC/N, d ₃₃ of 130 pC/N, will be favorable for high-temperature actuator and sensor applications.