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 ${T}ₜvaries from ~500 K ({Ni}0.5Fe0.5S) to below 4 K (Ni0.94{Co}0.06$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}. The changes in {Δ} and T derived from the cluster-model analysis explain the increase in the charge-transfer-type band gap in Ni_1-x{Fe}ₓ$S and hence the increase in ${T}ₜ$ within the cluster model, whereas they do not explain the decrease in ${T}ₜ$ in ${Ni}_{1{{-}}x}CoₓS. In order to explain the dramatic decrease in Tₜ with Co substitution, we propose that the Ni-3d--Co-3d hybridization increases the bandwidths leading to the closure of the band gap.