Theory of optical anisotropy and spontaneous polarization in the tetragonal phase of BaTiO3

The microscopic mechanism of optical anisotropy in the tetragonal phase of BaTiO₃ is discussed taking account of the dipole-dipole effect due to the ionic and electronic polarizations of the crystal and the spontaneous Kerr effect. The last effect is a cause of the strong local field acting on the constituent ions. For the spontaneous polarization we use only the first effect. In this molecular model, only the Ti ions situated at the center of the lattice are assumed to have the spontaneous shift. It is found that the spontaneous polarization and the birefringence are in good agreement with the experimental data.