X-ray absorption study of the metal-nonmetal transition in nis and ni_1-xm_xs (M=fe and co)

We have studied the electronic structure of NiS using Ni 2p X-ray absorption spectroscopy and subsequent cluster-model analysis. The results indicate that with Fe substitution or with decreasing temperature, the S 3p-to-Ni 3d charge-transfer energy increases and the Ni 3d-S 3p transfer integral decreases leading to the increase in the band gap and with Co substitution the band widths increase through the Ni 3d-to-Co 3d hybridization leading to the collapse of the bandgap.