Reinvestigation of phase transitions in Na0.5Bi0.5TiO3 by TEM.: Part II:: Second order orthorhombic to tetragonal phase transition

NBT presents an orthorhombic to tetragonal second order phase transition that occurs near 320 degrees C. It corresponds to the so-called antiferroelectric-paraelectric phase transition. A model is presented in which the diffuse phase transition is achieved by the progressive canceling of the antiphase octahedra tilting prevailing within a(-)b(+)a(-) orthorhombic structure, whereas the in-phase tilting is maintained. This transformation gives finally rise to the a(0)a(0)c(+) octahedra tilting system of the tetragonal phase. Electron diffraction experiments show that a long-range ordering occurs within the tetragonal phase. It probably develops as P4(2)/mnm ordered nanoregions disseminated within a P4/mbm disordered matrix. The order is still visible in the temperature domain of the cubic phase.