COVALENCY CONTRIBUTIONS TO THE ELECTRONIC POLARIZABILITY IN DIELECTRIC COMPOUNDS

The electron covalency contributions to the polarizability in oxide dielectric compounds are studied in the electron-lattice coupling model by means of the exact diagonalization technique. These contributions originate from the charge transfer between the anion and the cation sites which produces an electric dipole moment extended over the anion-cation bond distance. When a strong on-site Coulomb repulsion between electrons is included, the electronic system is found to undergo a transition between the ionic and Mott insulating phases, and the covalency contributions to the polarizability are largely enhanced near the phase boundary. It is suggested that the present results may explain the high indices of refraction observed in the titanium oxides and the birefringence in the ferroelectric compounds.