Band model for the metal-semiconductor transition in NiS

A simplified form of Slater's model for treating exchange effects in an itinerant antiferromagnet is applied to hexagonal NiS. In this calculation, which involves the augmented-plane-wave (APW) method, the antiferromagnetic superlattice is introduced by adding a constant $+V_s$ and $-V_s$ to the potentials within the two nickel APW spheres in the nonmagnetic unit cell. It is found that this superlattice opens a narrow band gap at the NiS Fermi level when $V_s\approx 0.04$ Ry. As a result, we conclude that the observed low-temperature nonmetallic behavior in NiS is due to the antiferromagnetic superlattice and does not involve a Mott-Hubbard transition.