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

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.