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 (F (4) over bar 3m) to orthorhombic (Pna2(1)) phase transition at approximate to 460 K. By studying the magnetization and thermal expansion between 5-300 K, we discover a second structural transition at approximate to 120 K. Single crystal X-ray diffraction reveals the low-temperature phase adopts a different orthorhombic structure with space group Pmn2(1) (a = 7.662 9(5) angstrom, b = 7.539 6(5) angstrom, c = 10.630 0(5) angstrom, 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, Ag(8)TQ(6) (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 Pna2(1) arrangement. We support this picture by showing that the transition to the Pmn2(1) phase is avoided in Ge alloyed Ag8Sn1-xGexS6 samples as well as in pure Ag8Ge6.