Bilanych R. Anharmonicity of the lattice dynamics and peculiarities of the dipole ordering in chalcogenide crystals

The thesis is dedicated to the investigation of the lattice dynamics anharmonicity and peculiarities of the dipole ordering in (PbySn1-y)2P2S6 and Sn2P2(SexS1-x)6 ferroelectric mixed crystals. The elastic properties of (PbySn1-y)2P2S6 solid solutions were studied by using Brillouin scattering technique at room temperature. All findings indicate weakening of stiffness of chemical bonds and increasing of anharmonicity with substitution of tin by lead in the investigated crystals at room temperature. By hypersound investigations and by analyzing of temperature dependence of the longitudinal ultrasound and low frequency dielectric properties for the (PbySn1-y)2P2S6 mixed crystals, it have been found that the tricritical point could be reached at the phase transition temperature lowering below 250 K while lead concentration increases more than y = 0.20. It was established that the (PbySn1-y)2P2S6 mixed crystals represent disordered ferroelectric system that could be described by Blume-Emery-Griffits model with random fields. For the Sn2P2(SexS1-x)6 mixed crystals by using of the obtained temperature dependence of hypersound velocity and attenuation, together with analysis of ultrasound, dielectric and thermal diffusion data, the manifestations of nonequilibrium effects in the vicinity of a second-order phase transition were studied. Such nonequilibrium effects at the second order phase transitions are described by Kibble-Zurek model, which establishes a relationship between the concentration of topological defects and the rate of cooling through the continuous phase transition.