Orthorhombic‐pseudocubic phase transition and piezoelectric properties of (Na 0.5 K 0.5 )(Nb 1− x Sb x )‐SrZrO 3 ceramics

0.96(Na 0.5 K 0.5 )(Nb 1− x Sb x )‐0.04SrZrO 3 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 Sb 2 O 5 . 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 0.5 K 0.5 )NbO 3 ‐based lead‐free piezoelectric ceramics.