Polarization mechanism of ferroelectric lattice instability in crystals

It is shown that the intercell dipole-dipole (polarization) interaction lowers the frequencies of the polar transverse-optical (TO) lattice vibrational modes (to a greater extent, the larger the corresponding dipole oscillator strengths) and can be a cause of ferroelectric instability in crystals. A model-independent method for determining the frequencies, eigenvectors, and dipole oscillator strengths of all the polar TO modes for a crystal with a suppressed polarization interaction from the known values of these quantities for the corresponding real crystal is proposed. The results of the corresponding calculations for several compounds with perovskite structure (SrTiO₃, BaTiO₃, KTaO₃, KNbO₃, and KCoF₃) and rutile structure (TiO₂, SnO₂, SiO₂, CoF₂, and NiF₂) are presented, and the nature of the ferroelectric soft mode in oxides with crystal structures of these two types is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 687–693 (April 1997)