Infrared band gap at the metal-nonmetal transition in NiS

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Abstract
Optical-reflection spectra have been measured for hexagonal NiS in the energy range 0.01 - 5.0 eV at temperatures both above and below the metal-nonmetal transition near 260 K. Using Kramers-Kronig analysis it is found that the conductivity below 0.1 eV drops on cooling through the transition. Above this energy a peak appears which reaches a maximum near 0.4 eV. The peak is identified with an energy gap which appears in the low-temperature antiferromagnetic phase. The low energy of this peak is contrasted with the predictions of localized-electron models. Comparisons made with recent augmented-plane-wave band calculations which include exchange strongly suggest an itinerant electron model for the transition.