Liubachko V. Thermal transport and phase transitions in ММ'P2(S,Se)6 (M, М' = Cu, Ag, In, Bi, Sn, Pb) phosphorus chalcogenides

The dissertation is devoted to the complex study of the influence of cationic and anionic atoms substitution on thermodynamic properties of (Cu,Ag)+(In,Bi)3+P2(Se,S)6 and Sn(Pb)2P2S(Se)6 phosphorous chalcogenide crystals in a wide temperature range, and, especially, in the vicinity of the structural phase transitions by means of ac photopyroelectric calorimetry in the standard back-detection configuration. For the first time, the temperature dependencies of thermal diffusivity and thermal conductivity of M1+M3+P2(Se,S)6 (M1+ = Cu, Ag; M3+ = In, Bi) layered crystals. Heat capacity of layered crystals have been calculated through the evaluation of the phonon spectra using density-functional perturbation theory, and, through the combination of both variables. The effect of lead and germanium partial substitution in (PbySn1-y-zGez)2P2(SexS1-x)6 solid solutions on temperature dependencies of thermal diffusivity has been studied. A temperature-concentration phase diagram for Sn(Pb)2P2S(Se)6 mixed crystals in the vicinity of the tricritical Lifshitz point within the framework of the combination of Blume-Emery-Griffith – ANNNI model was analyzed.