Charge densities in CoS2 and NiS2 (pyrite structure)

Abstract

The electron-density distributions in the pyrite-type structures of CoS2 and NiS2 have been determined from high-resolution single-crystal diffraction data [Ag Ka radiation; resolution and temperature (sin0/A)max = 149 /~-I at 295 K for COS2, 1.63 ~-1 at 135 K for NiS2]. The charge densities were refined using a multipolar deformation model [R(IFI) = 0.0119 and 0.0136, respectively]. The X-ray diffraction data of FeS2 [Stevens, DeLucia & Coppens (1980). lnorg. Chem. 19, 813-820; Ag Ka radiation, (sin0/a)m.x = 1.46 A-i, room temperature] were refined using the same deformation model and program in order to facilitate comparison of the results [R(IFI) = 0.0176]. The main features of the resulting deformation maps agree well for all three structures. They consist of important maxima in the immediate vicinity of the metal atoms pointing towards the faces of the coordination octahedron. The heavier the metal atom, the smaller is the distance of the maxima from the atomic centre. These features are interpreted by a preferential occupation of the metal d orbitals which correspond to the cubic t2e orbitals. An analysis of the d-orbital populations indicates that the symmetry of the electron distribution around the metal atom is in all cases very close to cubic, the site symmetry being 3; the t2g orbitals appear to be fully occupied by six electrons while the occupation of the eg orbitals increases in the series Fe, Co, Ni and indicates covalent overlap with the S ligands.