Heat capacity of nickel sulfide and its semimetal-metal transition

Heat-capacity measurements have been made from 5 to 330 K on NiS which shows a nonmetal-metal transition at 265 K, and on ${\mathrm{Ni}}_{0.95}$S which is metallic at all temperatures. The low-temperature data yield electronic specific-heat coefficients of 0.9 ± 0.2 and 6.2 ± 0.4 mJ/mole ${\mathrm{K}}^2$ and characteristic temperatures of 373 and 336 K, respectively. We deduce that NiS is a semimetal, not a semiconductor, at low temperatures. Of the entropy of transition, 5.32 J/mole K, 25% is electronic and 75% is due to the lattice. Any magnetic contribution is small compared to $R\ln \mathrm{}(2S+1)\mathrm{}$. The entropy of transition is proportional to the transition temperature $T_t$ for a series of samples with $105<\mathrm{}{T}_t<315\mathrm{}$ K. A strong electron-phonon interaction is indicated. Structure in the specific-heat anomaly of tiny crystals (∼ 1 mg) is ascribed to the presence of domains of dimensions ∼ 100 μm with slightly different stoichiometries.