In order to reveal the microscopic origin of the metal-nonmetal transition in NiS, we have studied the Ni 2p x-ray-absorption spectra of substituted systems Ni1-xMxS (M=Fe,Co), whose transition temperature Tt varies from 500 K (Ni0.5Fe0.5S) to below 4 K (Ni0.94Co0.06S). Spectra taken at room temperature have been analyzed using a configuration-interaction cluster model to deduce changes in the electronic structure parameters, namely, the S-3p-to-Ni-3d charge-transfer energy and the Ni-3dS-3p transfer integral T. We find that Fe substitution does not increase the local Ni-S distance as would be expected from the lattice parameters but affects energy levels as is reflected in . The changes in and T derived from the cluster-model analysis explain the increase in the charge-transfer-type band gap in Ni1-xFexS and hence the increase in Tt within the cluster model, whereas they do not explain the decrease in Tt in Ni1-xCoxS. In order to explain the dramatic decrease in Tt with Co substitution, we propose that the Ni-3dCo-3d hybridization increases the bandwidths leading to the closure of the band gap.
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