Energy Bands of Hexagonal NiS

Energy bands are calculated for hexagonal NiS using lattice constants and crystal potential appropriate to the metallic phase. Group theoretic results for s, p, and d bands in the NiAs structure are obtained, and the calculation is performed using a first-principles tight-binding method. The energy band structure is characterized by a $3d$ band about 3 eV wide, hybridized with a broad s-p band, indicating s-d-type conductivity. The Fermi energy lies just below the top of the d bands and the density of states exhibits a strong sharp peak about 1 eV below the Fermi energy. Since the energy bands suggest considerable structure in the low-energy region of the reflectivity, selection rules, polarizations, and energies for some of the spectra are presented. The band structure is discussed in terms of the metal-to-semiconductor transition which has been observed for hexagonal NiS.