Band Structure of NiS as Calculated Using a Simplified Linear-Combination-of-Muffin-Tin-Orbitals Method

Hexagonal NiS undergoes a first-order metal-to-nonmetal transition as the temperature is lowered below 264 K. NiS appears to be a "normal Pauli-paramagnetic $d$-band metal" above the transition temperature $T_t$, while below $T_t$ it is antiferromagnetic (moment $\sim 1.7{\mu }_B$) and either metallic or semimetallic. By use of a simplified (linear-combination-of-muffin-tin-orbitals) method that no longer requires a secular equation to be calculated as a function of energy, the band structure for NiS is calculated. These results indicate that the S $p$ bands overlap the bottom of the $d$ bands as hypothesized by White and Mott. It appears that this overlap increases with temperature, as a result of lattice vibrations which are included in the calculation through a Debye-Waller factor. We suggest a Mott-Hubbard transition occurs for a critical electron-phonon coupling which may be associated with a critical temperature.