LATTICE-DYNAMICS AND FERROELECTRIC INSTABILITY OF BARIUM-TITANATE

Using first-principles calculations, we investigate the lattice dynami cs of cubic and rhombohedral BaTiO3 and rye discuss the origin of the structural instability of this ferroelectric material. First, we repor t results on the Born effective charges and the dielectric tensor and we emphasize the important reduction of these quantities in the ferroe lectric phase, Then, the phonon frequencies at the Gamma point are cal culated. We point out the similarity of theoretical eigenvectors in th e cubic and rhombohedral phases. We examine the interaction of the vib ration modes with the electric field and in particular the giant LO-TO splitting of the ferroelectric mode. Finally, separating the dipole-d ipole interaction from the remaining short-range forces, we quantify t he balance of forces leading to an unstable phonon in the cubic phase and we demonstrate its sensitivity to tiny effective charge changes. W ithin our decomposition, the stabilization of the unstable mode in the rhombohedral phase is produced by a reduction of the Born effective c harges, while its stabilization under isotropic pressure is associated with a modification of the short-range forces.