Orthorhombic to tetragonal structural phase transition in Na _0.5K _0.5NbO ₃-based ceramics

We report the temperature-dependent x-ray powder diffraction of the Na _0.5K _0.5NbO ₃ (NKN)-based piezoelectric ceramics in the range between 320 and 700 K. In the case of NKN we have detected the structural phase transition region from the orthorhombic phase to the tetragonal phase over a broad temperature range from 496 to 458 K. The region is enlarged and shifted to lower temperature by introducing trace impurity or second component. It is considered that the coexistence of orthorhombic and tetragonal phases in NKN-based ceramics observed near room temperature are essentially due to the shift and enlargement of the phase coexistence region induced by lattice chemical pressure.