Electron-phonon interaction, microscopic mechanism and properties of ferroelectric phase transitions

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Abstract
The vibronic theory of ferroelectric phase transitions is reviewed and the essential contribution of electron-phonon interaction into the microscopic origin of the ferroelectricity is demonstrated. In the narrow-gap (semiconducting) systems the soft mode is caused by interband vibronic interaction and the temperature dependencies are essentially determined by band occupation numbers. In a general case the phonon anharmonicity must also be taken into account. In the large-gap dielectric systems (BaTiO3) this mechanism leads to a negative value of the square of an initial anomalously small frequency. The thermal jumps of electrons are now absent and the temperature dependencies (and the stabilization of the crystal) are essentially determined by the phonon anharmonicity. A theory for hydrogen-containing ferroelectrics with proton-proton, proton-phonon and phonon-phonon interactions taken into account is also developed. The main properties and characteristics of phase transitions in the mentioned scheme are considered. The semiempirical test of the theory leads to reasonable results. Semiconducting properties of ferroelectrics are analysed: the influence of nonequilibrium carriers and impurities on Tc, and the dependence of the forbidden gap on temperature in BaTiO3.