Shvalya V. Influence of isovalent dopants on critical behavior and dynamic thermal properties of Sn(Pb)2P2S(Se)6 ferroelectric crystals

The thesis is devoted to the comprehensive study of the influence of cationic and anionic atoms substitution on the thermodynamic properties of phosphorus-containing Sn(Pb)2P2S(Se)6 ferroelectric crystals in a wide temperature range, and, especially, in the vicinity of the structural phase transitions by means of photopyroelectric calorimetry technique. Additionally, the photopyroelectric technique has been tested in a "front configuration" in order to directly measure the thermal effusivity of different solid state samples starting with poor thermal conductors as polymers to highly conductive materials (metals). The analysis of the thermal conductivity and thermal diffusivity curves obtained in this study shows a certain correlation between macro parameters of the system and the changes that occur on its microlevel (the change of properties in electronic subsystems, as a result of the introduction of various impurities). Detailed analysis concerning the impact of cationic substitution on the critical behavior has been carried out for different types of the structural phase transitions: from the ferroelectric into the paraelectric one for (PbySn1-y)2P2S6 solid solutions and from the ferroelectric into the incommensurate state for (PbySn1-y)2P2Se6 crystals.