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The quaternary thio-germanates Cu 4 FeGe 2 S 7 (tetra-copper iron digermanium hepta-sulfide) and Cu 4 CoGe 2 S 7 (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 Cu 4 NiSi 2 S 7 structure type, which can be considered as a superstructure of cubic diamond or sphalerite. The monovalent (Cu + ), divalent (Fe 2+ or Co 2+ ) and tetra-valent (Ge 4+ ) 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 C 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.

acta crystallographica. section e, crystallographic communications

Syntheses and crystal structures of the quaternary thiogermanates Cu4FeGe2S7and Cu4CoGe2S7