We report results of electrical-conductivity, thermoelectric-power, and optical experiments on single crystals of ${\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x{\mathrm{S}}_2$ ($0\le x\le 0.12$). The results cannot be explained by means of a conventional one-electron model. However, if it is assumed that both strong electronic correlations and strong electron-phonon coupling exists for electrons in the narrow $d$ bands associated with the transition-metal cations, the experimental results can be understood quantitatively. In the model presented, Ni${\mathrm{S}}_2$ is a Mott insulator, in which the gap is due to a correlation splitting of the $3d-e_g$ band. The introduction of Co substitutionally for Ni results in one hole per Co atom in the lower of these bands. However, strong electron-phonon coupling leads to small-polaron formation, and thus a drastic band narrowing with increasing temperature. For this range of $x$, the $d$-band holes conduct only by means of thermally activated hopping for temperatures above 100 K. Screening effects due to thermally excited carriers lead to a collapse of the energy gap at a critical temperature, but small-polaron hopping remains the predominant conduction mechanism.

• Received 26 June 1978

DOI:https://doi.org/10.1103/PhysRevB.21.1676