Low-Temperature Structural Phase Transitions in Thermoelectric Tetrahedrite, Cu12Sb4S13, and Tennantite, Cu12As4S13

Tetrahedrite (Cu12Sb4S13) and tennantite (Cu12As4S13) crystallize in isomorphous cubic structures (I (4) over bar 3m) at room temperature and exhibit phase transitions at 85 and 124 K, respectively. We have investigated how the crystal structures change through the phase transitions using single crystal synchrotron X-ray diffraction data. The low-temperature structure of Cu12Sb4S13 belongs to the I (4) over bar 2m space group, which is described by the 2a x 2a x 2c supercell. The distortion of S(2)Cu(2)(6) octahedra is found to be responsible for the structural transformation. In contrast, the structure of Cu12As4S13 preserves its cubic symmetry and periodicity below the transition temperature. The low-temperature structure is characterized by positional disorder of S(1) and As(1) atoms along with a significant change in the occupancy of disordered sites of the Cu(2) atom. Besides distinct low-temperature structures for Cu12Sb4S13 and Cu12As4S13, the structural phase transition is further accompanied by negative thermal expansion of lattices (per formula unit) in both compounds.