We investigate the electronic structure of Nickel sulphide (NiS) in the millerite phase using electron spectroscopic measurements and band structure as well as model Hamiltonian calculations. While band structure calculations are found to be relatively more successful in describing the experimental valence band spectrum of this highly conducting phase compared to the hexagonal phase of NiS, cluster calculations including electron correlation effects are found to be necessary for the description of certain features in the experimental spectra, indicating the importance of correlation effects even in a highly metallic system. The electronic parameter strengths obtained from these calculations confirm that the millerite NiS is a highly covalent $\mathrm{pd}$ metal. The comparative study of hexagonal and millerite forms of NiS, provides the information concerning the evolution of the spectral function in a $\mathrm{pd}$ metal as a function of covalency.

• Received 9 February 2002

DOI:https://doi.org/10.1103/PhysRevB.66.115105