LITHIUM DOPING OF FULLERENE: Li12C60

We have studied by NMR, magnetometry and x-ray diiraction several lithium doped fullerides Li x C 60. 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 x. We have followed the temperature dependence of the structure of the x = 12 phase. It is fcc at high temperature , in analogy with pristine C 60 , and it distorts to tetragonal upon cooling. We have evidence from 13 C NMR that the C 60 units are static in the tetragonal phase at least up to T=373 K. We have attempted a Rietveld reenement of the high temperature diiraction proole, comparing several structural models, including the superfulleroid proposed by Lett 198, 472 (1992)). We nd 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.