Lithium doping of fullerene:: Li12C60.

We have studied by NMR, magnetometry and x-ray diffraction several lithium doped fullerides LixC60. We have found that the stoichiometry x=12 represents the minimum Li doping for a phase whose structure is strikingly similar to that of pristine C-60 and it extends to much larger Li doping levels I. We have followed the temperature dependence of the structure of the x = 12 phase. It is fee at high temperature, in analogy with pristine C-60, and it distorts to tetragonal upon cooling. We have evidence from C-13 NMR that the C-60 units are static in the tetragonal phase at least up to T=373 K. We have attempted a Rietveld refinement of the high temperature diffraction profile, comparing several structural models, including the superfulleroid proposed by J. Kohanoff, W. Andreoni and M. Parrinello (Chem Phys Lett 198, 472 (1992)). We find that the most likely structure is that of an hybrid intercalation compound in which the Li atoms do form a cluster centered in the octahedral void but some of them show a partially covalent bond the C-60 units.