Rb-2n(Mo9S11)(Mo6nS6n+2) (n=1 to 4): A novel family of superconducting molybdenum cluster compounds

Reinvestigation of the Rb-Mo-S system has led to the discovery of a novel f amily of reduced molybdenum sulfides with the general formula Rb-2n(Mo9S11) (Mo6nS6n+2) (n = 1 to 4) in addition to the previously known compound Rb2Mo 6S6. The new rubidium-reduced molybdenum sulfides have been prepared by sol id-state reaction of Rb2MoS4, MoS2, and Mo heated at temperatures ranging b etween 1600 K and 2000 K in a sealed Mo crucible. All members of the family crystallize in the R-3c space group with Z = 6 in the hexagonal setting. X -ray diffraction studies on single crystals showed that their crystal struc tures consist of an equal mixture of Mo9S11 and Mo6nS6n+2 (n = 1 to 4) clus ter units interconnected through Mo-S bonds. The Rb+ cations occupy large v oids between the different cluster units. When n goes from 1 to 4, the a pa rameter of the hexagonal unit cell decreases from 9.433(1) Angstrom to 9,23 7(1) Angstrom, whereas the c parameter increases linearly from 56.27(1) Ang strom to 137.28(2) Angstrom. Extended Huckel (tight-binding) calculations r evealed that the clusters are hypoelectronic and the Fermi level crosses a rather flat band for all members of the family. Resistivity measurements ca rried out on single crystals from 290 K to 2 K confirmed the metallic behav ior and each member was found to be a superconductor with transition temper atures ranging from 4.2 K to 10.9 K. (C) 2000 Academic Press.