Optical reflectivity spectra. of the hexagonal Ni1-deltaS have been measure
d to study its electronic structures, in particular those associated with t
he metal-nonmetal transition in this compound. Samples with delta similar t
o 0.002 and 0.02 are studied, which have transition temperatures T-1 simila
r to 260 K and 150 K, respectively. Upon the transition, a pronounced dip a
ppears in the infrared region of the reflectivity spectra. The optical cond
uctivity spectra suggest that the nonmetallic phase is a carrier-doped semi
conductor with an energy gap of similar to 0.2-0.3 eV. The spectra also sho
w that the gap becomes larger with decreasing temperature, and smaller with
increasing delta. It is found that the overall spectrum in the nonmetallic
phase can be explained in terms of a charge-transfer semiconductor, consis
tent with recent theoretical and photoemission studies of NiS. (C) 1999 Els
evier Science Ltd. All rights reserved.