Syntheses and crystal structures of the quaternary thiogermanates Cu4FeGe2S7 and Cu4CoGe2S7

The quaternary thiogermanates Cu4FeGe2S7 (tetracopper iron digermanium heptasulfide) and Cu4CoGe2S7 (tetracopper cobalt digermanium heptasulfide) 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 tetravalent (Ge4+) cations adopt tetrahedral geometries, each being surrounded by four S2− anions. The divalent cation and one of the sulfide ions lie on crystallographic twofold axes. These tetrahedra share corners to create a three-dimensional framework structure. All of the tetrahedra 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.