Superconductivity of fcc fullerides containing off-centered octahedral cations

Low-temperature synthesis in liquid ammonia yielded the superconductors (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 and a=14.52 Angstrom for K and Rb compounds, respectively, by controlling the ammonia concentration x. T-c of both compounds decreases with increasing 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 displaced by 0.4-0.6 Angstrom from the center of the octahedral site.