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Abstract
The physical origin of the ferroelectricity and refractive indices in tetragonal, orthorhombic and rhombohedral phases of KNbO3 are discussed quantitatively from the dipole-dipole interaction caused by the strong local field acting on the constituent ions. In this microscopic model, all the ions, except the barium, are assumed to have the spontaneous shifts. The results of calculations point out the important role of the lattice deformations and the electronic polarizabilities of the constituent ions. It is found that the ferroelectricity and the refractive indices are in good agreement with the experimental data.