Superconductivity in alkali-ammonia complex fullerides

Here we show that low temperature synthesis in liquid ammonia successfully stabilized the combination of one Na and two large alkali ions(K and Rb) by incorporating NH3 molecules. Both (NH3)xNaK2C60 and (NH3)xNaRb2C60 (x approx. 0.8) retained the fcc cell with a = 14.37 angstrom and a=14.52 angstrom, and superconduct at 11K and 13K, respectively. 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. The present system thus provides an example of fcc superconductors involving off-centered cations that exhibit different trend in the well known relationship between transition temperature (Tc) and lattice parameter of face-centered cubic A3C60 (A=alkali metals) superconductors.