Structural constitution of the LaMo6Se8 superconductor

The ternary phase diagram La-Mo-Se shows a broad homogeneity range around the superconducting Chevrel phase LaMo6Se8. A series of single-phase samples of general formula LaMo(6+x)Se(8-x) (0<x<0.75), located inside domain of homogeneity of the superconducting Chevrel phase LaMo6Se8, has been synthesized at 1200 oC and characterized through X-ray, ac-magnetic susceptibility and density measurements. It has been found that the increasing contribution of molybdenum (x) in the series does not affect much the critical temperature (Tc) of the matrix, which remains practically constant and close 10 K. The same true as far as samples' densities are concerned. Based on crystallographical considerations, several models of different distribution of the excess molybdenum (x) have been analytically determined. The resulting densities (d calc), when compared to the experimental values (d exp), served as a basis to evaluate distribution of molybdenum in real samples. The final result, supported by the X-ray powder patterns simulation, indicates on mixed occupation of the Se sublattice consisting of (8-x) Se and the excess molybdenum (x). (author)