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 Ni0.95S 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 K² 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 Rln(2S+1). The entropy of transition is proportional to the transition temperature Tₜ for a series of samples with 105<Tₜ<315 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 {μ}m with slightly different stoichiometries.