SINGLE-CRYSTAL STUDIES OF THE CHEVREL-PHASE SUPERCONDUCTOR LAXMO6SE8 - I - CORRELATION BETWEEN T-C AND THE INTERATOMIC DISTANCES

We report, for the first time in the literature, crystal structure stu dies of rare-earth-based Chevrel-phase selenides of formulae LaxMo6Se8 with variable contents (x) of lanthanum, Single crystals were grown f rom off-stoichiometric mixtures that had been melted and slowly cooled from about 1675 degrees C, The origin site of the rhombohedral lattic e may present a considerable deficiency in the lanthanum occupancy, wh ereas no vacancies were found in the Mo6Se8 sublattice. The supercondu cting temperature of this phase depends strongly on the lanthanum conc entration, as we previously reported for powder samples. From the crys tal structure data obtained for three crystals with different lanthanu m contents, a clear correlation was established between T-c and the la nthanum occupancy at the origin site, The critical temperature varied from 9.4 to 11.0 K for x = 0.839(2) to x = 0.9412(2), respectively, th e lanthanum contents being derived from structural data taken on the s ame single crystals. Lattice constants, positional parameters, and int eratomic distances are presented, and their respective variations are discussed in terms of geometrical effects and charge transfer phenomen a. A full comparison is made with the corresponding rare-earth molybde num sulfides REMo6S8. Basic physical properties, i.e., electrical resi stivity, ac susceptibility, and de magnetization, are presented in an accompanying paper; they confirm that LaMo6Se8 is an extremely high-fi eld superconductor. (C) 1998 Academic Press.