Phase-transition effects in BaTiO3from tight-binding energy bands

The behavior of the complex electronic dielectric constant $\epsilon$ in cubic and tatragonal BaTi${\mathrm{O}}_3$ crystals has been deduced from tight-binding energy bands, revealing an anisotropy in agreement with experimental data. The single- and two-oscillator parameters obtained from calculated $\epsilon$ functions are used in the interpretation of the optical properties and polarization-induced effects. The upper oscillator is found polarization dependent. The phase transition induces a remarkable decrease of interband transition strengths along the polar axis and an upward shift of transition energies. The ionic charges, deduced from band results through a population analysis, allow one to estimate the spontaneous polarization in the tetragonal phase. The electro-optic effects are investigated from calculated birefringency, polarization, and oscillator parameters. Optical and electro-optic properties are due to transitions in a relatively wide spectral region.