Syntheses and crystal structures of the quaternary thio-germanates CuFeGeS and CuCoGeS.

The quaternary thio-germanates CuFeGeS (tetra-copper iron digermanium hepta-sulfide) and CuCoGeS (tetra-copper cobalt digermanium hepta-sulfide) were prepared in evacuated fused-silica ampoules high-temperature, solid-state synthesis using stoichiometric amounts of the elements at 1273 K. These isostructural compounds crystallize in the CuNiSiS structure type, which can be considered as a superstructure of cubic diamond or sphalerite. The monovalent (Cu), divalent (Fe or Co) and tetra-valent (Ge) cations adopt tetra-hedral geometries, each being surrounded by four S anions. The divalent cation and one of the sulfide ions lie on crystallographic twofold axes. These tetra-hedra share corners to create a three-dimensional framework structure. All of the tetra-hedra align along the same crystallographic direction, rendering the structure non-centrosymmetric and polar (space group 2). Analysis of X-ray powder diffraction data revealed that the structures are the major phase of the reaction products. Thermal analysis indicated relatively high melting temperatures, near 1273 K.