Ferroelectric Phase Transitions in Systems with Broad Electronic Bands Caused by Interband Electron‐Phonon Interaction

The possibility of arising of ferroelectric phase transitions and its properties in broad band systems (semimetals, semiconductors) are studied within the frames of the vibronic theory. The quasi‐one‐dimensional, quasi‐two‐dimensional and isotropic three‐dimensional cases are considered in the mean field approximation. The chain and layer structures reveal in this approximation the greatest tendency to ferroelectric phase transitions. The spontaneous lattice distortion (polarization), dielectric susceptibility, forbidden gap, etc. are calculated. The possibility of arising of the ferroelectric semimetal‐dielectric phase transition in all cases is studied. In the case of the isotropic three‐dimensional electronic spectra of the active bands the interband electron‐phonon interaction must be strong.