Optical study of the metal-nonmetal transition in Ni_1-δS

Optical reflectivity spectra of the hexagonal Ni_1-δS have been measured to study its electronic structures, in particular those associated with the metal-nonmetal transition in this compound. Samples with δ approx. 0.002 and 0.02 are studied, which have transition temperatures T_t approx. 260 K and 150 K, respectively. Upon the transition, a pronounced dip appears in the infrared region of the reflectivity spectra. The optical conductivity spectra suggest that the nonmetallic phase is a carrier-doped semiconductor with an energy gap of approx. 0.2-0.3 eV. The spectra also show that the gap becomes larger with decreasing temperature, and smaller with increasing δ. It is found that the overall spectrum in the nonmetallic phase can be explained in terms of a charge-transfer semiconductor, consistent with recent theoretical and photoemission studies of NiS.