Superconductivity of fcc fullerides containing off-centered octahedral cations

Low-temperature synthesis in liquid ammonia yielded the superconductors (NH₃)ₓNaA₂C₆₀ ($0.5<x<1$, A=KandRb). These compounds retained the fcc cell with a tunable lattice constant around $a=14.37$ {} and $a=14.52$ {} for K and Rb compounds, respectively, by controlling the ammonia concentration x. Tc 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 {} from the center of the octahedral site.