The system Cu3AsS4– Cu3SbS4 and investigations on normal tetrahedral structures

In order to develop a novel model to predict if a so called normal tetrahedral compound crystallizes in a wurtziteor sphalerite superstructure type, the system Cu3AsS4– Cu3SbS4 was reinvestigated. A solid solution series was prepared and the mixed crystals were characterized by powder X-ray techniques. The crystal structures of Cu3As0.330Sb0.670S4, Cu3As0.736Sb0.264S4, and Cu3AsS4 were refined from single crystal X-ray data. The refinements converged to R 1⁄4 0.0209, wR2 1⁄4 0.0484 (Cu3As0.330Sb0.670S4, 201 unique reflections, 15 parameters), R 1⁄4 0.0235, wR2 1⁄4 0.0596 (Cu3As0.736Sb0.264S4, 218 reflections, 15 parameters), and R 1⁄4 0.0241, wR2 1⁄4 0.0669 (Cu3AsS4, 721 reflections, 45 parameters). Z is 2 for all compounds. The volumes of the tetrahedra [MS4] were calculated for the investigated compounds. In addition, the corresponding data were calculated for further solids from literature data. The volumes of the tetrahedra are used to separate compounds with a sphalerite type anion arrangement from compounds with hexagonal packed anions. A closer inspection of the tetrahedra volumes reveals a greater variation for one given material in the case of wurtzite type superstructures than for sphalerite type superstructures. A critical difference in the tetrahedra volumes is derived from these data.