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

In SnPS 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 SSe substitution. For the mixed crystals with SnPb 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 (PbSn)PS ferroelectric properties. [ABSTRACT FROM AUTHOR]