The authors have calculated the phonon dispersion curves of barium titanate (BaTiO3) in the cubic phase. A nonlinear shell model is used, in which the fourth-order anharmonic core-shell interaction of the oxygen ion is considered within a self-consistent phonon approximation. The temperature dependence of the ferroelectric soft mode is reproduced by their calculations, leading to the linear and nonlinear contributions of the O polarisability. The values of the relevant model parameters are critically analysed.

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The microscopic mechanism of refractive indices, birefringence and spontaneous polarization of BaTiO 3 and KNbO 3 in the tetragonal phase is discussed by using a microscopic model. In this model, we have taken account of a quantum method based upon the orbital approximation and the dipole±dipole interaction due to the local field acting on the constituent ions. It is found that the electronic polarizabilities play a major role in these calculations and that the obtained results of the refractive indices, birefringence dn and spontaneous polarization are in good agreement with the experimental data.

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The microscopic mechanisms of refractive indices, birefringence, spontaneous polarization and linear electro-optic effects are examined for KNbO3 using a microscopic model which takes into account a quantum method based upon the orbital approximation and the dipole-dipole interaction due to the local field acting on the constituent ions. It is found that the electronic polarizabilities play a major role in these calculations and that the birefringence n and the linear electro-optic coefficients are in good agreement with the experimental data.