Rb2n(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 family of reduced molybdenum sulfides with the general formula Rb2n(Mo9S11)(Mo6nS6n+2) (n=1 to 4) in addition to the previously known compound Rb2Mo6S6. The new rubidium-reduced molybdenum sulfides have been prepared by solid-state reaction of Rb2MoS4, MoS2, and Mo heated at temperatures ranging between 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 structures consist of an equal mixture of Mo9S11 and Mo6nS6n+2 (n=1 to 4) cluster units interconnected through Mo–S bonds. The Rb+ cations occupy large voids between the different cluster units. When n goes from 1 to 4, the a parameter of the hexagonal unit cell decreases from 9.433(1) ? to 9.237(1) ?, whereas the c parameter increases linearly from 56.27(1) ? to 137.28(2) ?. Extended Hückel (tight-binding) calculations revealed that the clusters are hypoelectronic and the Fermi level crosses a rather flat band for all members of the family. Resistivity measurements carried out on single crystals from 290 K to 2 K confirmed the metallic behavior and each member was found to be a superconductor with transition temperatures ranging from 4.2 K to 10.9 K.