Critical phenomena in ferroelectric crystals Sn2P2S6: dielectric investigation

Abstract

The influence of the external electric field on the phase transition of the uniaxial femoelectric Sn2P2S6 is investigated. It is shown that direct longitudinal electric field suppresses the temperature anomaly of dielectric susceptibility at the second order phase transition (F'T), satisfying regularity expected for tricritical point (TCP). The long term crystal exposure in paraphase near the PT temperature (To) results in the appearence of an intermediate state, that has dielectric properties, typical for an incommensurate phase. This effect is the consequence of proximity of the ferroelectric F" to the Lifshitz point (LP) on the phase diagram and is explained on the base of the assumption that the coordinates of LP depend on the nonequilibrity degree of electronic subsystem in investigated ferroelectric-semiconductors. At T -To < 10 K on the temperature dependence of Sn#zS6 dielectric susceptibility measured in slow cooling mode deviation from the Curie-Weiss law was observed which can be described by logarithmic multiplicative correction of the type IlmlL"w, here T = (T - TOYTOI.t is assumed, that the observed critical behaviour of uniaxial ferroelechic is caused by evidence of order parameter fluctuations correlation in the vicinity of tricritical Lifshitz point in which tbe line of TCP and the line of LP merge on the phase diagram.