Orthorhombic-pseudocubic phase transition and piezoelectric properties of (Na_0.5K_0.5)(Nb_1−xSb_x)-SrZrO₃ ceramics

0.96(Na_0.5K_0.5)(Nb_1−xSb_x)-0.04SrZrO₃ ceramics with 0.0≤x≤0.06 were well sintered at 1060°C for 6 hours without a secondary phase. Orthorhombic-tetragonal transition temperature (T_{O -T}) and Curie temperature (T_C) decreased with the addition of Sb₂O₅. The decrease in T_C was considerable compared to that in T_{O -T}, and thus the tetragonal phase zone disappeared when x exceeded 0.03. Therefore, a broad peak for orthorhombic-pseudocubic transition as opposed to that for orthorhombic-tetragonal transition appeared at 115°C-78.2°C for specimens with 0.04≤x≤0.06. An orthorhombic structure was observed for specimens with x≤0.03. However, the polymorphic phase boundary structure containing orthorhombic and pseudocubic structures was formed for the specimens 0.04≤x≤0.06. Furthermore, a specimen with x=0.055 exhibited a large piezoelectric strain constant of 325 pC/N, indicating that the coexistence of orthorhombic and pseudocubic structures could improve the piezoelectric properties of (Na_0.5K_0.5)NbO₃-based lead-free piezoelectric ceramics.