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Alkali metal fullerides (A x C 60 ) are a subject of considerable current interest because of the occurrence of superconductivity for A x C 60 at temperatures surpassed only by the high T c copper oxides. The preparation and characterization of A x C 60 (A = alkali metal, x = 2,3,4,6) by powder X-ray diffraction, NMR ( 13 C, 23 Na and 87 Rb), and d.c. magnetization are reported. The structures are described as intercalation compounds of the FCC structure of pristine c 60 or of hypothetical BCC or BCT structures. The structures and phase diagrams can be rationalized on the basis of ion size and electrostatic considerations. Only the A 3 C 60 compounds are metallic (and superconducting). The superconducting T c increases nearly linearly with unit cell size. EHT (Extended Huckel Theory) calculations and 13 C NMR relaxation measurements indicate higher densities of states for the higher T c compositions.

INTERCALATING solid C60 with dopant atoms yields materials with a remarkable range of properties1. Alkali metal atoms, for example, readily form charge-transfer compounds2,3, AxC60 (where A is an alkali metal), which can be metallic, superconducting or insulating depending on the dopant concentration4–9. In all cases, the superconducting phase has a face-centred cubic (f.c.c.) structure and stoichiometry A3C60, which suggests a common mechanism for superconductivity dependent, at least in part, on the external coordination number of the C60 molecules. More recently, it has been shown10 that the alkaline earth metal calcium can also be intercalated with fulleride to form a superconducting phase, again with a f.c.c.-derived structure, near a Ca:C60 ratio of 5:1. Here we report the intercalation of fulleride with barium, in which a pure body-centred cubic phase with a lattice constant of 11.171 A is realized near a stoichiometry of Ba6C60. This phase is also superconducting (with a transition temperature of 7 K), suggesting that the mechanism of superconductivity is related to an intrinsic property of the C60 molecules, rather than the external coordination number.