On the tricritical behaviour of the Sn2P2S6 crystals

Based on the results for temperature dependences of optical birefringence and optical indicatrix rotation derived for Sn₂P₂S₆ crystals, we show that at the atmospheric pressure, those crystals are under the conditions of tricritical point, with the critical exponent of the order parameter ? . At the same time, we have found that the temperature behaviour of the rotation angle of spontaneous electric polarization obeys the law ? . The action of hydrostatic pressure leads to only small changes in the critical exponent β, thus suggesting that the tricritical point on the P, T-diagram of Sn₂P₂S₆ should occur in the pressure range ? .