  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.
