Lattice instability and superconductivity in the Pb, Sn, and Ba Chevrel phases

Abstract

High-resolution neutron powder diffraction measurements show that sintered samples of ${\mathrm{PbMo}}_6$ ${\mathrm{S}}_8$, ${\mathrm{SnMo}}_6$ ${\mathrm{S}}_8$, and ${\mathrm{BaMo}}_6$ ${\mathrm{S}}_8$ all transform from a high-temperature rhombohedral R3¯ structure to a low-temperature triclinic P1¯ structure, but with a broad temperature range over which both phases are present. The temperature of the structural transition, which may be varied with pressure or composition, is empirically correlated with the superconducting transition temperature $T_c$.