Grabar A. Energy spectra and photoinduced phenomena sn the ferroelectrics-semiconductors of the Sn2P2S6 type.

The thesis is devoted to the studies of the elementary excitation spectra (electrons and phonons), photoinduced phenomena, optical non-linearity, as well as the peculiarities of the phase transitions and domain structure of the Sn2P2S6 type ferroelectrics - semiconductors. Special attention was paid to the study of the possibilities of its applications as a photorefractive material. It is established that the phase transition in these crystals occurs with the simultaneous displacement of the ferroactive tin ions from center-symmetrical sites in paraelectric phase and its ordering in the local two-well potential. The electronic structure, as well as photoinduced changes of the optical and electric parameters are studied. The electro-optic effect and optical non-linearity of the Sn2P2S6 are investigated. The photorefractive properties of the studied crystals are examined, and a pronounced enhancement of these parameters were achieved by using technological methods. Using these crystals, a set of the practical schemes of the dynamic holography were realized, partially, image dynamic filtering and correlation, laser beam wavefront conversion etc. New methods of the unipolarity space distribution and domain switching based on the photorefractive properties of Sn2P2S6 are proposed. It is shown that the domain structure in these crystals is formed by charged domain walls.