Critical exponents of phase transition in ferroelectric Sn2P2S6: comparison of optical and dilatometric data

Abstract

Dilatometric, birefringent, electrooptic and gyrotropic properties of semiconductor-ferroelectric Sn2P2S6 have been experimentally studied and theoretically analysed. It has been shown that the spontaneous mechanical strains, optical birefringence increment and optical activity are governed by different critical exponents due to the temperature-dependent rotation of spontaneous polarisation vector permitted by the symmetry of low-temperature phase in Sn2P2S6 crystals. The critical exponent for the order parameter is estimated to be close to , thus corresponding to the vicinity of the tricritical point. Based on the results obtained for the temperature dependences of birefringence under different hydrostatic pressures, it has been demonstrated that the ferroelectric phase transition in Sn2P2S6 becomes of the first order under high pressures.