Clustering and polymerization of Li15C60

Abstract

The structural and electronic properties of lithium intercalated fullerides (of which Li15C60 is the most representative) are still puzzling and unclear. Above 520 K, x-ray/neutron diffraction shows an fcc phase in which the 15 Li atoms clusterize in the octahedral interstices. However, at lower temperatures, a change in the crystalline symmetry and also in the electronic properties takes place as observed from 13C, 7Li/6Li NMR and x-ray diffraction measurements. X-ray diffraction data suggest the presence of two different stable structures: a tetragonal monomeric and an orthorhombic polymerised phase. Detailed 13C magic angle spinning NMR experiments in the latter phase indicate sp3 bondings among the carbon atoms, whereas the relative (sp2/sp3) intensities, together with x-ray data, suggest the C60 polymerization to be a [2+2] cycloaddition. Multiple quantum NMR experiments on 7Li confirm the presence of lithium clusters, as observed by x-ray diffraction in the high temperature phase, also at lower temperatures. However, the inferred cluster size is significantly smaller than that suggested by the stoichiometry. The distortion in the low-T structure of L15C60 is supposed to induce the migration of Li atoms from octahedral to tetrahedral voids, thus accounting for the lower number of Li atoms in the clusters. Further evidence of this scenario is obtained also from preliminary measurements of line shapes and T1 relaxation times, which exhibit a multiexponential recovery with very different constants that are hardly compatible with a single family of Li atom sites.