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 °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 P 4 2 / mnm ordered nanoregions disseminated within a P 4/ mbm disordered matrix. The order is still visible in the temperature domain of the cubic phase.