A Low-Temperature Structural Transition in Canfieldite, Ag₈SnS₆, Single Crystals

Canfieldite, Ag₈SnS₆, is a semiconducting mineral notable for its high ionic conductivity, photosensitivity, and low thermal conductivity. We report the solution growth of large single crystals of Ag₈SnS₆ of mass up to 1 g from a ternary Ag-Sn-S melt. On cooling from high temperature, Ag₈SnS₆ undergoes a known cubic (F4̅3m) to orthorhombic (Pna2₁) phase transition at ≈460 K. By studying the magnetization and thermal expansion between 5-300 K, we discover a second structural transition at ≈120 K. Single crystal X-ray diffraction reveals the low-temperature phase adopts a different orthorhombic structure with space group Pmn2₁ (a = 7.662 9(5) Å, b = 7.539 6(5) Å, c = 10.630 0(5) Å, Z = 2 at 90 K) that is isostructural to the room-temperature forms of the related Se-based compounds Ag₈SnSe₆ and Ag₈GeSe₆. The 120 K transition is first-order and has a large thermal hysteresis. On the basis of the magnetization and thermal expansion data, the room-temperature polymorph can be kinetically arrested into a metastable state by rapidly cooling to temperatures below 40 K. We last compare the room- and low-temperature forms of Ag₈SnS₆ with its argyrodite analogues, Ag₈TQ₆ (T = Si, Ge, Sn; Q = S, Se), and identify a trend relating the preferred structures to the unit cell volume, suggesting smaller phase volume favors the Pna2₁ arrangement. We support this picture by showing that the transition to the Pmn2₁ phase is avoided in Ge alloyed Ag₈Sn_1-GeS₆ samples as well as in pure Ag₈GeS₆.