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

We report the solution growth of single crystals of Ag₈SnS₆ (Canfieldite) from a Ag-Sn-S melt. On cooling from high temperature, Ag₈SnS₆ undergoes a known cubic (F-43m) to orthorhombic (Pna2₁) transition at 460 K. Here, we discover a second structural transition at 120 K (on warming). Single crystal X-ray diffraction shows the low temperature phase adopts a different orthorhombic structure with space group Pmn2₁ that is isostructural to the room temperature forms of the related compounds Ag₈SnSe₆ and Ag₈GeSe₆. The 120 K transition is first-order with large thermal hysteresis. We find the room temperature polymorph can be kinetically arrested into a metastable state by fast cooling to temperatures below 40 K. We lastly compare the room and low temperature forms of Ag₈SnS₆ with its analogues, Ag₈TQ₆ (T = Si, Ge, Sn; Q = S, Se), and identify a trend relating the preferred structures to the unit cell volume, suggesting smaller volume favors the Pna2₁ arrangement.