Journal of the American Ceramic Society, Vol.103, No.4, 2509-2519, 2020
Second-order-transition like characteristic contributes to strain temperature stability in (K, Na)NbO3-based materials
High strain and good temperature stability are contradictory properties in (K, Na)NbO3 (KNN)-based materials. Herein, good temperature stability with high strain is obtained in a multiphase coexistent [ie, orthorhombic-tetragonal (O-T) and rhombohedral-orthorhombic-tetragonal (R-O-T)] KNN. A second-order transition-like characteristic should contribute to the temperature stability, in which an intrinsic lattice structure forms a bridge between them. The observed second-order transition-like characteristic is due to the reduced discrepancy among different lattice symmetries and a broadened temperature region for the phase transition. These integrated factors can slow the latent heat in a first-order transition and extend it over a wide temperature region, thereby exhibiting second-order transition-like behavior. Correspondingly, the abrupt increase in strain near the phase transition temperature significantly slows. In addition, the appearance of pure tetragonal symmetry (P4mm) is deferred to a much higher temperature than TO-T, in which the strain will inevitably decrease. As a result, good temperature stability with a high strain response can be realized in multiphase coexistent KNN materials, including d(33)*=448 pm/V, -27.5%<= fluctuation <= 4.2% for O-T, and d(33)*=446 pm/V, -17.5%<= fluctuation <= 7.6% for R-O-T, over the whole temperature range 25 degrees C-190 degrees C.