Theoretical study of refractive indices, birefringence and spontaneous polarization of BaTiO3 and KNbO3 in the tetragonal phase

The microscopic mechanism of refractive indices, birefringence and spontane ous polarization of BaTiO3 and KNbO3 in the tetragonal phase is discussed b y using a microscopic model. In this model, we have taken account of a quan tum method based upon the orbital approximation and the dipole-dipole inter action due to the local field acting on the constituent ions. It is found t hat the electronic polarizabilities play a major role in these calculations and that the obtained results of the refractive indices, birefringence del ta n and spontaneous polarization are in good agreement with the experiment al data.