Abstract
Abstract An influence of the replacement of tin atoms by lead in cation sublattice on the phase transition (PT) character has been ascertained in studies of the temperature dependences of heat capacity and low-frequency dielectric constant of mixed crystals based on Sn2P2S6 proper ferroelectric. Such replacement shifts the second-order phase transition from the Lifshitz point (LP) and tricritical point (TCP), while the replacement of sulphur by selenium in anion sublattice affects the PT character in opposite way like hydrostatic compression. In the latter case, concentrational and baric paths in the phase diagram pass near the tricritical Lifshitz point where the LP and TCP lines converge. The LP line position and the existence area of incommensurate phase are determined at T = OK on the T-x-y diagram of (PbySn1-y)2P2(SexS1-x)6.
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