Cluster model for the electronic structure of the Chevrel-phase compound PbMo6S8

A simplified model for the electronic structure of the Chevrel-phase compound Pb${\mathrm{Mo}}_6$ ${\mathrm{S}}_8$ is obtained by means of a tight-binding calculation for a cubic ${\mathrm{Mo}}_6$ ${\mathrm{S}}_8$ cluster. The interaction parameters for this cluster calculation are derived from a tight-binding fit to the results of an ab initio augmented-plane-wave energy-band calculation for a hypothetical ${\mathrm{Mo}}_3$S crystal with the ${\mathrm{Cu}}_3$Au structure. It is found that the molecular-orbital states near the ${\mathrm{Mo}}_6$ ${\mathrm{S}}_8$ Fermi level $E_F$ consist of partially-filled nonbonding Mo $d_{x^2-y^2}$-type orbitals. These results suggest the presence of narrow $d$-type conduction bands in compounds such as Pb${\mathrm{Mo}}_6$ ${\mathrm{S}}_8$ since bandwidths resulting from intercluster interactions are estimated to be small.