The vibronic theory of ferroelectricity

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Abstract
A review of the foundation, developments and latest results of the so-called vibronic theory of ferroelectricity—a new approach to the microtheory of spontaneous polarization and ferroelectric phase transitions in crystals incorporating the interactions of electrons with nuclear displacements (vibronic interactions), is presented.The origin of dipolar instability as due to the pseudo Jahn-Teller effect is demonstrated for a two level system and octahedral [TiO6]8- cluster and generalized for perovskite (BaTiO3) and rock salt (GeTe) type crystals; simple criterion of spontaneous polarization occurrence being derived.The ferroelectric phase transitions are considered in two approaches depending on whether the lattice stabilization at high temperatures is due to the vibronic anharmonicity or to the electron population of the conductivity band (for ferro-electrics-semiconductors). The recent more rigorous treatments, by means of diagram and Green function methods allowing for a better account of the optical mode dispersion, are also reviewed. The Curie temperature, polarization value, order-disorder effects, temperature dependence of band structure, influence of magnetic fields etc., and their dependence on the composition and crystal structure parameters are discussed.