Optical reflectivity study has been made on the hexagonal (NiAs-type) Ni 1−δ S in order to probe its electronic properties, in particular those associated with the metal-nonmetal transition in this compound. Samples with δ=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 δ 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. Keywords: D. optical properties, D. phase transitionsThe problem of the metal-nonmetal phase transition in the hexagonal NiS has been studied for three decades, but the transition mechanism is not completely understood yet. The high temperature (HT) phase above the transition temperature, T t ∼ 260 K, is a paramagnetic metal. Upon cooling through T t , the resistivity increases suddenly by a factor of ∼ 40, associated with slight increase in the lattice constants (0.3 % in a and 1 % in c) and the appearance of an antiferromagnetic order.1-3 T t is lowered sharply with increasing Ni vacancies, and the transition disappears when the vacancy content exceeds ∼ 4 %.4 Similar behavior is observed also with an applied pressure, and the transition is not observed at pressures above ∼ 2 GPa.5 These behaviors are summarized in the phase diagram of Fig. 1.The nature of the low-temperature (LT) phase below T t has been studied by many experiments. The resistivity (ρ) increases only slightly with cooling, with an activation energy of several meV.4 In contrast, an optical study by Barker and Remeika 6 clearly showed the presence of an energy gap of about 0.15 eV. Hall effect experiment by Ohtani 4 has shown that the majority carrier in the LT phase is the hole, and the density of holes is proportional to that of Ni vacancies, with ∼ 2 holes per Ni vacancy. Namely, the LT phase can be described as a p-type degenerate semiconductor, where the Ni vacancies act as acceptors. Effects of substituting other elements for Ni or S have been also studied in detail. 7,8 Recently, two high-resolution photoemission studies 9,10 have revealed a finite density of states (DOS) around the Fermi energy (E F ) in the LT phase, but they have given contrasting interpretations: Nakamura et al.9 have concluded that there is a small correlation-induced band gap with an unusually sharp band edge, and that the observed finite DOS at E F is due to thermal and instrumental broadenings of the edge. On the other hand, Sarma et al. 10 have concluded that the LT phase is an "anomalous metal".Various models have been proposed to account for the phase transition and the gap opening in NiS. At early stage, it wa...