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 ${\mathrm{Ni}}_{1\mathrm{-}\mathit{x}}$ ${\mathit{M}}_{\mathit{x}}$S (M=Fe,Co), whose transition temperature ${\mathit{T}}_{\mathit{t}}$ varies from ∼500 K (${\mathrm{Ni}}_{0.5}$ ${\mathrm{Fe}}_{0.5}$S) to below 4 K (${\mathrm{Ni}}_{0.94}$ ${\mathrm{Co}}_{0.06}$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 ${\mathrm{Ni}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Fe}}_{\mathit{x}}$S and hence the increase in ${\mathit{T}}_{\mathit{t}}$ within the cluster model, whereas they do not explain the decrease in ${\mathit{T}}_{\mathit{t}}$ in ${\mathrm{Ni}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Co}}_{\mathit{x}}$S. In order to explain the dramatic decrease in ${\mathit{T}}_{\mathit{t}}$ with Co substitution, we propose that the Ni-3d–Co-3d hybridization increases the bandwidths leading to the closure of the band gap.