Ferroelectricity in (PbySn1−y)2P2S6 mixed crystals and random field BEG model

Abstract

In Sn2P2S6 ferroelectric under compression, the second-order phase transition line is observed down to the tricritical point as the transition temperature decreases to 250 K. A tricriticality at a similar temperature is also revealed in the mixed crystals on S→Se substitution. For the mixed crystals with Sn→Pb substitution, ultrasound, hypersound, and low-frequency dielectric studies also show heterophase features appearing on decrease of the ferroelectric transition temperature below a so-called “waterline temperature” near 250 K. Such behavior agrees with the Blume–Emery–Griffiths (BEG) model appropriate for the ferroelectric system under investigation with a three-well local potential for the order parameter fluctuations. The BEG model modified by the random-field phase diagram in the vicinity of the tricritical point qualitatively explains the influence of the cationic substitution on the (PbySn1−y)2P2S6 ferroelectric properties.