Structure of the Oxygen Point Defect in Sn${\mathrm{Mo}}_6$${\mathrm{S}}_8$ and Pb${\mathrm{Mo}}_6$${\mathrm{S}}_8$

Rietveld structural refinement of neutron powder diffraction data for Chevrel-phase samples in both the Sn-Mo-S-O and Pb-Mo-S-O systems has led to a model for an oxygen-containing point defect. The oxygen substitutes for the sulfur located in the special position along the $\overline{3}$ axis and the metal ion is displaced ≃ 0.8 Å toward the oxygen impurity, presumably forming a covalent bond. This defect is responsible for the wide variation reported for $T_c$ and $\frac{c}{a}$ (hexagonal, lattice-constant ratio).