X-RAY-ABSORPTION STUDY OF THE METAL-NONMETAL TRANSITION IN NIS AND NI1-XMXS (M=FE AND CO)

We have studied the electronic structure of NiS using Ni 2p X-ray abso rption spectroscopy and subsequent cluster-model analysis. The results indicate that with Fe substitution or with decreasing temperature, th e S 3p-to-Ni 3d charge-transfer energy increases and the Ni 3d-S 3p tr ansfer 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 band gap.