High temperature crystal structure and DSC of Sn₂P₂S₆

We report the single crystal X-ray diffraction of Sn₂P₂S₆ at 110°C. The crystals are monoclinic, with space group P2₁/n, a=9.362(2) Å, b=7.493(1) Å, c=6.550(3) Å, β=91.17(3)°, and Z=2. The structure consists of P₂S₆^-4 anions linked together via S-Sn contacts. Each Sn atom is coordinated to eight S atoms. The S-Sn distances range from 2.914-3.227 Å. At room temperature, Sn₂P₂S₆ crystallizes in the space group P_c and has a strong ferroelectric response. Using differential scanning calorimetry, the transition to the paraelectric state is determined to be 60±2°C. The DSC data are also consistent with the phase transition being second order. The major change in the structure results from movement of the tin atoms. The P₂S₆^-4 units remain essentially unchanged. Neither the room temperature nor the high temperature phase exhibits a layered structure characteristic of the transition metal phosphorus chalcogenides, M₂P₂X₆.