SUPERCONDUCTING AND STRUCTURAL-PROPERTIES OF RARE-EARTH-BASED CHEVREL-PHASE SELENIDES REMO6SE8 - FIRST SINGLE-CRYSTAL STUDIES

Crystals of some ternary Chevrel-type compounds based on rare-earth mo lybdenum selenides (REMo6Se8) have been grown for RE = La, Ce, Pr, Nd and Sm. Refinement of the crystallographic data has confirmed the stru cture type expected for large ions, that is, the occupancy of the orig in site of the rhombohedral-hexagonal lattice (R (3) over bar symmetry ). However, a systematic deficiency of the rare-earth content is obser ved (similar to 84-94% of the total occupancy), leading to small chang es of the physical properties (e.g. superconducting temperature T-c) a s a function of concentration. Detailed analysis of the structural dat a is presented, both as a function of the cation content and its ionic radius. Lattice constant and interatomic distances are discussed in t erms of electronic and steric effects. Superconducting properties are presented for several compounds since this is the first report on sing le crystal data obtained for rare-earth Chevrel-type selenides. Finall y, a full comparison with the corresponding rare-earth molybdenum sulf ides REMo6S8 is made. (C) 1997 Elsevier Science S.A.