Reinvestigation of phase transitions in Na0.5Bi0.5TiO3 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 Na(0.5)Bi(0.5)TiO(3) (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 degrees C by the disappearance of the ferroelectric-ferroelastic domains. The intermediate modulated phase is then formed from 230 to 300 degrees 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 degrees 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.