Some new results of the vibronic theory of ferroelectricity

Abstract
Some results of a generalized (lattice instability at an arbitrary q, various types of electron-phonon interaction included, corrections beyond RPA, etc.) version of the vibronic theory of ferroelectric phase transitions in dielectrics and semiconductors are presented. The spontaneous birefringence of BaTiO3-type systems in this framework is explained, the vibronic renormalization of the carriers plasma frequency and some other new features are mentioned.