Dynamic microscopic structures and dielectric response in the cubic-to-tetragonal phase transition for BaTiO₃ studied by first-principles molecular dynamics simulation

The dynamic structures of the cubic and tetragonal phase in BaTiO ₃ and its dielectric response above the cubic-to-tetragonal phase transition temperature (T_p) are studied by first-principles molecular dynamics (MD) simulation. It is shown that the phase transition is due to the condensation of one of the transverse correlations of the off-center displacements. Calculation of the phonon properties for both the cubic and the tetragonal phase shows a saturation of the soft mode frequency at ∼60 cm^-1 near T_p and advocates its order-disorder type mechanism. Our calculation also leads directly to a two mode feature of the dielectric function above T_p [H. Presting, J. A. Sanjurjo, and H. Vogt, Phys. Rev. B 28, 6097 (1983)] and the two modes are attributed to the coexistence of renormalized and normal soft mode excitation as a result of strong coupling between the soft mode and the off-center displacements.