Abstract:
Tetrahedrite (Cu sub(12) Sb sub(4) S sub(13)) and tennantite (Cu sub(12) As sub(4)S sub(13)) crystallize in isomorphous cubic structures (I43m) at room temperature and exhibit phase transitions at 85 K 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 Cu sub(12) Sb sub(4) S sub(13) belongs to the I42m space group, which is described by 2a x 2a x 2c supercell. The distortion of S(2)Cu(2) sub(6) octahedra is found to be responsible for the structural transformation. In contrast, the structure of Cu sub(12) As sub(4)S sub(13) 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 Cu sub(12) Sb sub(4)S sub(13) and Cu sub(12) As sub(4)S sub(13), the structural phase transition is further accompanied by negative thermal expansion of lattices (per formula unit) in both compounds.
