Fedyo K. Electronic energi spectra and semiconducting properties of ferroelectric Sn2P2S6

The thesis is dedicated to the investigation of chemical bonds nature, electron energy spectra and the ferroelectrics phase transition mechanism in Sn2P2S6 crystals. The influence of local energy states of vacancies and Ge, In, Te impurities on these crystals optical, photorefractive and electrophysical properties is studied. It was found that at ferroelectric phase transition a redistribution of electron density occurs due to the stereoactivity of tin cations electron lone pair, the spontaneous polarization appearance is accompanied by increased covalency of chemical bonds. The energy diagram for the Sn2P2S6 crystals was proposed with account of tin vacancies acceptors states, sulfur vacancies donor states, small electron polarons at Sn+ cations and small hole polarons at S- anions. It was found that Ge impurity increases the stereoactivity in Sn2+ cation sublattice, In impurity creates donor centers for effective compensation of acceptors states, and Te impurity modifies the energy spectrum at the top of valence band of Sn2P2S6 crystals.