Syntheses and crystal structures of the quaternary thio-germanates Cu₄FeGe₂S₇ and Cu₄CoGe₂S₇

The quaternary thio-germanates Cu₄FeGe₂S₇ (tetra-copper iron digermanium hepta-sulfide) and Cu₄CoGe₂S₇ (tetra-copper cobalt digermanium hepta-sulfide) were prepared in evacuated fused-silica ampoules via high-temperature, solid-state synthesis using stoichiometric amounts of the elements at 1273 K. These isostructural compounds crystallize in the Cu₄NiSi₂S₇ structure type, which can be considered as a superstructure of cubic diamond or sphalerite. The monovalent (Cu⁺), divalent (Fe²⁺ or Co²⁺) and tetra-valent (Ge⁴⁺) cations adopt tetra-hedral geometries, each being surrounded by four S^2- 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 C2). 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.