Reinvestigation of Phase Transitions in Na_0.5Bi_0.5TiO₃ by TEM. Part I: First Order Rhombohedral to Orthorhombic Phase Transition

This study shows for the first time that the rhombohedral to tetragonal phase transition in Na0.5Bi0.5TiO3 (NBT) is a two step phase transition. The transformation begins by a first order phase transition involving the reconstructive transformation of the rhombohedral phase into an orthorhombic one, through the formation of an intermediate modulated phase. This phase transition begins slightly over 200 °C by the disappearance of the ferroelectric−ferroelastic domains. The intermediate modulated phase is then formed from 230 to 300 °C, the temperature at which it disappears. The modulated phase corresponds to an intergrowth of rhombohedral perovskite blocks in which Pnma orthorhombic sheets are formed by a microtwinning process of the rhombohedral phase. The intermediate orthorhombic phase is then formed at 300 °C and immediately turns to the tetragonal one. A model is presented explaining the formation of the modulated phase and the origin of the antiferroelectric and relaxor behaviors of NBT.