Effect of pressure and oxygen defects in divalent Chevrel-phase superconductors

The pressure dependence of the superconducting transition temperature $T_c$ is measured for several samples each of the divalent Chevrel-phase systems Sn${\mathrm{Mo}}_6$ ${\mathrm{S}}_8$, Pb${\mathrm{Mo}}_6$ ${\mathrm{S}}_8$, and Eu${\mathrm{Mo}}_6$ ${\mathrm{S}}_8$. The pressure dependence of $T_c$ scales with $T_c$ and each system has the same scale factor. The data are discussed in terms of oxygen defects systematically introduced into each system and in terms of the lattice constants. Oxygen defects inadvertently introduced into Chevrel-phase superconducting systems may account for the wide variation of $T_c$ quoted in the literature.