Syntheses and crystal structures of the quaternary thio­germanates Cu(4)FeGe(2)S(7) and Cu(4)CoGe(2)S(7)

The quaternary thio­germanates Cu(4)FeGe(2)S(7) (tetra­copper iron digermanium hepta­sulfide) and Cu(4)CoGe(2)S(7) (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 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 tetra­valent (Ge(4+)) cations adopt tetra­hedral geometries, each being surrounded by four S(2−) 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.