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 transition in NiS, we have studied the Ni 2p x-ray-absorption spectra of substituted systems Ni 1-x M x S (M=Fe,Co), whose transition temperature T t varies from ∼500 K (Ni 0.5 Fe 0.5 S) to below 4 K (Ni 0.94 Co 0.06 S). 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 Δ and the Ni-3d-S-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 Δ