Beley L. Thermodynamic properties of ferroelectric crystals CuInP2(SexS1-x)6

The thesis is dedicated to the investigation of the temperature-concentration phase diagram for CuInP2(SexS1-x)6 layered ferrielectric crystals, as well as to the study of their vibration spectra and thermodynamic properties. By means of X-ray diffraction methods, dielectric and optical (birefringence, Raman scattering, optical absorption) spectroscopy, the information about the structure of crystals under study and their phase transitions was obtained. The symmetry analysis of lattice dynamics of CuInP2S(Sе)6 crystals was carried out, which allowed us to explain temperature and concentration variations of Raman spectra. The existence of a new type of dipole glassy state in CuInP2(SexS1-x)6 solid solutions at low temperature is shown. The sequence of two phase transitions of the second and the first order in CuInP2S6 crystals, as well as three second order phase transitions in CuInP2Sе6 crystals was revealed. Anomalies of dielectric permittivity, heat capacity, spontaneous polarization, induced by these transitions are described in a mean-field approach within a model based on structural and dipole order parameters.