Critical behaviour of uniaxial Sn₂P₂S(Se)₆ferroelectrics

Critical behaviour of proper uniaxial Sn2P2(SexS1-x)6 ferroelectrics whose phase diagram contains a Lifshitz point and a “virtual” tricritical point is studied by means of measured temperature dependences of birefringence and heat capacity. An anomalous heat capacity change in paraphase is described by an exponential function δC p ∼ τ−α. Far from the phase transition temperature, α = 0.5 and this corresponds to the first fluctuational correction. In the vicinity of the phase transition to ferroelectric phase for Sn2P2S6, α = 3/2, which corresponds to dipoled effects while in the vicinity of phase transition (PT) to incommensurate phase for Sn2P2Se6, α = −0.02, which satisfies the Heisenberg double-component model. The crossover from tricritical behaviour to critical one is displayed in ferroelectric and incommensurate phases. In the vicinity of Lifshiftz point, β = 0.2 for a compound with x = 0.29 within a wide temperature range.