Superconductivity of fcc fullerides containing off-centered octahedral cations

Low-temperature synthesis in liquid ammonia yielded the superconductors ${\left(\mathrm{N}{\mathrm{H}}_3\right)}_x\mathrm{Na}{A}_2{\mathrm{C}}_{60}$ ($0.5<x<\mathrm{}1\mathrm{}$, $A=\mathrm{K}\mathrm{and}\mathrm{Rb}$). These compounds retained the fcc cell with a tunable lattice constant around $a=14.37\mathrm{}$ Å and $a=14.52\mathrm{}$ Å 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-N${\mathrm{H}}_3$ cluster occupies the octahedral interstice and remaining K or Rb occupies the tetrahedral site. The Na ion is displaced by 0.4-0.6 Å from the center of the octahedral site.