Nonstoichiometry and superconductivity in La/sub x/Mo/sub 6/S/sub y/

Lanthanum Chevrel type compounds, La/sub x/Mo/sub 6/S/sub y/ (0.9 ≤x≤1.1, and 7.0≤y≤8.0 in nominal content), are prepared by sintering at 1623 and 1773K in a vacuum sealed Mo container. All of the samples heated at 1623K accompany with impurity phases. The hexagonal c/a ratio varies from 1.2488 to 1.2620, and the superconducting critical temperature T/sub c/ is a monotonic function of c/a. On the other hand, the samples heated at 1773K have a single phase in the region of 1.0≤x≤1.1, and 7.4≤y≤8.0. They show the c/a ratios from 1.220 to 1.254, but T/sub c/ is almost constant and independent of c/a. These relations between T/sub c/ and c/a are explained by the dependence of the Fermi level on the c/a ratio