Chevrel phases, actinide based compounds An(1+x)Mo6Se8 (An=Np, Pu, Am)

Chevrel's phases are ternary chalcogenides of molybdenum of the general formula M Mo6Se8 where M represents a monovalent cation (Na+, Li+), divalent (Pb2+, Mg2), trivalent (TR3+) or even tetravalent cation (Th4+). The rare earth based compounds are of particular interest because the selenides are superconductors with high critical temperature and particularly with very important critical field. The ternary selenides An(1+x)Mo6Se8 with x=0 and 0.2 and An=237Np, 239Pu, 241Am were prepared and studied. These compounds are isomorphous of their rare earth counterparts and crystallize in a rhombohedral system. Their idealized structure is formed of Mo6Se8 clusters around the actinide ions. The An cations are surrounded with 8 selenium ions in a quasi cubic symmetry. The magnetic susceptibilities of the compounds, measured between 2.5 and 300 K show that the actinides are trivalent, thereby agreeing with the value of their lattice parameters. The self-irradiaton effects with the Pu 239 and particularly the Am 241 are sufficient to destroy the superconductivity