SUPERCONDUCTIVITY OF FCC FULLERIDES CONTAINING OFF-CENTERED OCTAHEDRAL CATIONS

Low-temperature synthesis in liquid ammonia yielded the superconductor s (NH3)(x)NaA(2)C(60) (0.5<x<1, A=K and Rb). These compounds retained the fee cell with a tunable lattice constant around a=14.37 Angstrom a nd a=14.52 Angstrom for K and Rb compounds, respectively, by controlli ng the ammonia concentration x. T-c of both compounds decreases with i ncreasing lattice parameter, in a striking contrast with the empirical correlation. A Na-NH3 cluster occupies the octahedral interstice and remaining K or Rb occupies the tetrahedral site. The Na ion is displac ed by 0.4-0.6 Angstrom from the center of the octahedral site.