Effects of oxygen on the upper critical fields of lead and tin molybdenum sulfide Chevrel phases

The upper critical fields, $H_{c2}$, of ${\mathrm{SnMo}}_6$ ${\mathrm{S}}_{8\mathrm{-}\mathrm{x}}$ ${\mathrm{O}}_{\mathrm{x}}$ and ${\mathrm{PbMo}}_6$ ${\mathrm{S}}_{8\mathrm{-}\mathrm{x}}$ ${\mathrm{O}}_{\mathrm{x}}$ Chevrel-phase materials with controlled oxygen content were measured with dc and pulsed magnetic fields. The present results and those of many earlier investigations are explained by the presence of oxygen. Observations of increases in $T_c$ and the variation of $H_{c2}$ obtained by additions of Gd or Al are explained by the gettering of oxygen by these additives. The highest $T_c$ materials, which have the lowest oxygen content, do not have the highest $H_{c2}$(0).