Heat capacity of nickel sulfide and its semimetal-metal transition

Abstract

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 \ifmmode\pm\else\textpm\fi{} 0.2 and 6.2 \ifmmode\pm\else\textpm\fi{} 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 $R\mathrm{ln}(2S+1)$. The entropy of transition is proportional to the transition temperature ${T}_{t}$ for a series of samples with $105<{T}_{t}<315$ K. A strong electron-phonon interaction is indicated. Structure in the specific-heat anomaly of tiny crystals (\ensuremath{\sim} 1 mg) is ascribed to the presence of domains of dimensions \ensuremath{\sim} 100 \ensuremath{\mu}m with slightly different stoichiometries.