Medulych M. Lattice dynamics and effects of electron-phonon interaction in ferroelectrics of Sn2P2S6 type

The dissertation is devoted to the study of the lattice dynamics anharmonicity and electron-phonon interaction manifestations in the Raman spectra, optical and dielectric properties of Sn2P2S6 type crystals, based on the analysis of experimental data within the framework of small-radius polaron models and polaronic excitons, the stereoactivity of electron lone pair and valence fluctuations for cations in crystalline structures, the description of phase diagrams and the spectrum of spontaneous polarization fluctuations in the model of quantum anharmonic oscillators with a three-well potential. It has been found out that due to the disproportionation of charges of phosphorus cations, the valency fluctuations, along with the stereoactivity of the lone electronic pair of tin Sn2+ cations, play an important role in the formation of the ferroelectric state for Sn2P2S(Se)6 crystals and determine the properties of the quantum paraelectric state in the presence of weakly stereoactive cations Pb2+ in crystals Pb2P2S(Se)6. It has been established that for ferroelectric Sn2P2S6 observed by Raman scattering temperature and pressure transformations of the phonon spectrum agree with the predicted by model of anharmonic quantum oscillators with a three-well local potential temperature and pressure dependences of pseudospin fluctuations spectra. For layered crystals of CuInP2S(Se)6 ferroelectrics, the manifestation of essential phonon anharmonicity in the temperature dependence of the Raman scattering spectra are preconditioned by the complicated form of the multi-valued potential for the fluctuations of spontaneous polarization.