Dielectric, ferroelectric, and piezoelectric properties in potassium sodium niobate ceramics with rhombohedral-orthorhombic and orthorhombic-tetragonal phase boundaries

A new phase boundary with rhombohedral-orthorhombic and orthorhombic-tetragonal phase boundaries is designed in (K_0.48Na _0.52)NbO₃ by adding Bi_0.5(Na _0.7K_0.2Li_0.1)_0.5ZrO₃ (BNKLZ), where Zr⁴⁺ and (BNKL)²⁺ are respectively used to improve the temperature of a rhombohedral phase and to decrease the temperature of an orthorhombic-tetragonal phase coexistence. These ceramics endure several continuous phase transitions with increasing BNKLZ content, i.e., an orthorhombic phase (0≤x<0.03), orthorhombic-tetragonal phases (x=0.03), orthorhombic-tetragonal and rhombohedral-orthorhombic (O-T and R-O) phase existence (0.03<x≤0.05), a rhombohedral phase (0.05<x≤0.07). The ceramics with O-T and R-O have a better piezoelectric behavior as compared with other phases because of more polarization states, enhanced ε_r and P_r, and a dense microstructure. Moreover, piezoelectric properties could be further optimized by modifying their sintering and poling temperatures. As a result, the construction of O-T and R-O phase coexistence benefits the improvement of piezoelectric properties in KNN-based ceramics.