Fulleride superconductors: dependence of Tc on average molecular valence in Na2Cs C60, 0 < x < 1

The effect of molecular valence on the superconducting transition temperature is studied in a new family of fullende superconductors Na2CsxC60, 0 < x < 1. By varying the octahedral Cs occupancy, we can for the first time chemically tune the carrier concentration in an isostructural sequence of single-phase solid solutions. These all exhibit orientational phase transitions similar to the end member compounds Na2C60 and Na2CsC60; thus, they all belong to the Pa— space group at low temperature. Using high-resolution X-ray diffraction, differential scanning calorimetry (DSC), Raman and SQUID magnetometry, we show that Tc is optimized at x = 1 (corresponding to three transferred electrons per molecule) and is strongly depressed as x decreases from unity. We observe no superconductivity down to 2 K for x ≤ 0.5 (corresponding to less than or equal to 2.5 electrons per C60). Temperature-dependent spin susceptibility from ESR is used to probe metallic versus non-metallic behavior as a function of carrier concentration.