FULLERIDE SUPERCONDUCTORS - DEPENDENCE OF T-C ON AVERAGE MOLECULAR VALENCE IN NA2CSXC60, 0-LESS-THAN-X-LESS-THAN-1

The effect of molecular valence on the superconducting transition temp erature is studied in a new family of fulleride superconductors Na2Csx C60, 0 < x < 1. By varying the octahedral Cs occupancy, we can for the first time chemically tune the carrier concentration in an isostructu ral sequence of single-phase solid solutions. These all exhibit orient ational phase transitions similar to the end member compounds Na2C60 a nd Na2CsC60; thus, they all belong to the Pa (3) over bar space group at low temperature. Using high-resolution X-ray diffraction, different ial scanning calorimetry (DSC), Raman and SQUID magnetometry, we show that T-c, is optimized at x = 1 (corresponding to three transferred el ectrons per molecule) and is strongly depressed as x decreases from un ity. We observe no superconductivity down to 2 K for less than or equa l to 0.5 (corresponding to less than or equal to 2.5 electrons per C-6 0). Temperature-dependent spin susceptibility from ESR is used to prob e metallic versus non-metallic behavior as a function of carrier conce ntration.