METAL-NONMETAL TRANSITION IN NIS INDUCED BY FE AND CO SUBSTITUTION - X-RAY-ABSORPTION SPECTROSCOPIC STUDY

In order to reveal the microscopic origin of the metal-nonmetal transi tion in NiS, we have studied the Ni 2p x-ray-absorption spectra of sub stituted systems Ni1-xMxS (M = Fe,Co), whose transition temperature T( t) varies from approximately 500 K (Ni0.5Fe0.5S) to below 4 K (Ni0.94C o0.06S). Spectra taken at room temperature have been analyzed using a configuration-interaction cluster model to deduce changes in the elect ronic structure parameters, namely, the S-3p-to-Ni-3d charge-transfer energy DELTA and the Ni-3d-S-3p transfer integral T. We find that Fe s ubstitution does not increase the local Ni-S distance as would be expe cted from the lattice parameters but affects energy levels as is refle cted in DELTA. The changes in DELTA and T derived from the cluster-mod el analysis explain the increase in the charge-transfer-type band gap in Ni1-xFexS and hence the increase in T(t) within the cluster model, whereas they do not explain the decrease in T(t) in Ni1-xCoxS. In orde r to explain the dramatic decrease in T(t) with Co substitution, we pr opose that the Ni-3d-Co-3d hybridization increases the bandwidths lead ing to the closure of the band gap.