Tricritical Lifshitz point in the temperature-pressure-composition diagram for (Pb_ySn_(1-y))_2P_2(Se_xS_(1-x))_6 ferroelectrics

摘要

The heat capacity of Sn_2P_2S_6 ferroelectric crystals has been measured under quasihydrostatic pressures up to 0.7 GPa. The analysis of the heat-capacity and literature data for the birefringence shows that the tricritical point of Sn_2P_2S_6 is in the 0.20-0.25 GPa range. Moreover, in the approximation of a linear change in the free-energy expansion coefficients, with respect to concentration and pressure, thermodynamic trajectories have been constructed for (Pb_ySn_(1-y))_2P_2(Se_xS_(1-x))_6 solid solutions. We have thereby identified the region of the T-p-y-x diagram for (Pb_ySn_(1-y))_2P_2(Se_xS_(1-x))_6 showing the tricritical Lifshitz point.