Comparison of electronic structure and optical properties of BaTiO3 in the cubic and tetragonal phases

The electronic structure and the linear optical properties of BaTiO3 in the cubic perovskite and tetragonal phases are studied by means of first-principles local density calculations. Charge density analysis gives ionic formulas of Ba+1.39Ti+2.79 [O-1.39]3 and Ba+1.78Ti+2.80 [O-1.52]3 for the cubic and the tetragonal phases respectively. Subtle differences in the electronic structure between these two phases are revealed, especially in the splitting of the conduction band minimum due to symmetry lowering in the tetragonal phase. This contributes to anisotropy in the static dielectric constant at low frequency. The calculated optical properties for the cubic phase are in good agreement with the vacuum ultraviolet data on polycrystalline samples for photon energies up to 40 eV.