Preparation, Electrical Properties, Crystal Structure, and Electronic Structure of Cu4GeS4

Cu4GeS4 was synthesized by the solid state reaction of the constituent binary chalcogenides at 830°C. Its crystal structure was determined on the basis of X-ray data of a nonmerohedral twinned crystal: monoclinic P21/c, a=9.790(2), b=13.205(2), c=9.942(3) ? , =100.90(2)°, Z=8. The refinement of 164 parameters with 1373 data converged to R1=0.058 [for I>2(I)], wR2=0.176 (all data), and a twin fraction of 0.274(4). The crystal structure may be described as a monoclinic variant of the Cu4SnS4 type. It is characterized by a three-dimensional framework based on a hexagonal close-packing of sulfur atoms. The Cu and Ge atoms are distributed in an ordered manner over 5/8 of the tetrahedral voids, with half of the Cu atoms distorted into triangular sites. The structure was compared with that of Cu4SnS4, and the difference was ascribed to the formation of more triangular Cu sites in Cu4GeS4. An extended Hückel tight-binding band calculation was performed to analyze the electronic structure, and electrical resistivity measurements confirm the expected semiconducting behavior of Cu4GeS4.