Reinvestigation of Phase Transitions in Na_0.5Bi_0.5TiO₃ 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 a0a0c+ 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 P42/mnm ordered nanoregions disseminated within a P4/mbm disordered matrix. The order is still visible in the temperature domain of the cubic phase.