Critical phenomena in ferroelectric-semiconductor crystals Sn2P2S6: Dielectric investigation

The influence of the external electric field on the phase transition of the uniaxial ferroelectric Sn2P2S6 is investigated. It is shown that direct longitudinal electric field suppresses the temperature anomaly of dielectric susceptibility at the second order phase transition (PT), satisfying regularity expected for tricritical point (TCP). The long term crystal exposure in paraphase near the PT temperature (T-0) 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 PT to the Lifshitz point (LP) on the phase diagram and is explained on the base of the assumption that me coordinates of LP depend on the nonequilibrity degree of electronic subsystem in investigated ferroelectric-semiconductors. At T - T-0 < 10 K on the temperature dependence of Sn2P2S6 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 \ln tau\(1/3), where tau = (T - T-0)/T-0. It is assumed, that the observed critical behaviour of uniaxial ferroelectric is caused by evidence of order parameter fluctuations correlation in the vicinity of tricritical Lifshitz point in which the line of TCP and the line of LP merge on the phase diagram.