NMR and high-resolution x-ray diffraction evidence for an alkali-metal fulleride with large interstitial clusters:Li12C60

NMR and high-resolution x-ray diffraction of several lithium-doped fullerides ${\mathrm{Li}}_x{\mathrm{C}}_{60}$ show that the stoichiometry $x=12\mathrm{}$ represents a stable Li doping for a high symmetry ${\mathrm{C}}_{60}$-like phase, which might extend to much larger x. ${\mathrm{Li}}_{12}{\mathrm{C}}_{60}$ is fcc at high temperature, and it distorts to tetragonal upon cooling. We have evidence from ${}^{13}\mathrm{C}$ NMR that the ${\mathrm{C}}_{60}$ units are static in the tetragonal phase at least up to $T=373\mathrm{}\hspace{0.5em}\mathrm{K}.$ Rietvelt refinement of the fcc phase favors a hybrid cluster intercalated structure with shortening of selected Li-C distances.