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The heat capacity of Sn 2 P 2 S 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 2 P 2 S 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 y Sn 1−y ͒ 2 P 2 ͑Se x S 1−x ͒ 6 solid solutions. We have thereby identified the region of the T-p-y-x diagram for ͑Pb y Sn 1−y ͒ 2 P 2 ͑Se x S 1−x ͒ 6 showing the tricritical Lifshitz point.