Optical study of the metal-nonmetal transition in NiS
  Optical reflectivity study has been made on the hexagonal (NiAs-type) Ni_{1-d}S in order to probe its electronic properties, in particular those associated with the metal-nonmetal transition in this compound. Samples with d=0.005 and 0.02 are studied, which have transition temperatures Tt=246 K and 161 K, respectively. A pronounced dip appears in the reflectivity spectra upon the transition, and the optical conductivity spectra show that the electronic structure below Tt is similar to that of a carrier-doped semiconductor with an energy gap of ~ 0.15 eV. The optical spectra indicate that the gap becomes larger with decreasing temperature, and it becomes smaller as d increases. It is also found that the overall spectrum including the violet region can be described based on a charge-transfer-type semiconductor, consistent with recent photoemission results. 
H. Okamura, J. Naitoh, N. Nanba, M. Matoba, M. Nishioka, S. Anzai