CONDUCTIVITY ANISOTROPY AND STRUCTURAL PHASE-TRANSITION IN COVELLITE CUS

Covellite CuS consists of alternating layers with compositions CuS and Cu2S2. We examine the anisotropy of the electrical conductivity and the origin of the structural phase transition in CuS by performing extended Huckel tight-binding band electronic structure calculations. Our study shows that CuS is a three-dimensional metal, but its conductivity should be much greater along the directions perpendicular than parallel to the c-axis. The partially empty valence bands of CuS have strong contributions from the sulfur 3p orbitals of the Cu2S2 layers, so that the oxidation state of CuS is better described by (Cu+)3(S2-)(S2-) than by (Cu+)3(S-)(S22-). The structural phase transition of CuS is not driven by Fermi surface instability. A probable cause for this phase transition cannot be the bonding interactions between the Cu+ ions but the van der Waals interactions of the S...S contacts between the CuS and Cu2S2 layers.