STRUCTURAL AND ELECTRONIC-PROPERTIES OF SODIUM-INTERCALATED C-60

ALKALI metal fullerides exhibit superconductivity 1-4 at temperatures surpassed only by the copper-oxide-based ceramics. Three types of stoichiometry and structure have been identified: A3C60 (ref. 5) (where A is K or Rb) with A+ cations intercalated in the face-centred cubic (f.c.c.) C60 structure, and A6C60 (ref. 6) or A4C60 (ref. 7) (where A is K, Rb or Cs) with body-centred arrays of C60 molecules. All A3C60 compounds reported so far are superconductors with transition temperatures that increase with unit cell size 8,9. No successful preparation of bulk NaxC60 has been reported, although measurements of electrical conductivity 10 and Raman 11, ESR 11 and photoemission 12 spectra of thin films have given clear indications of NaxC60 formation. Here we report the synthesis and initial characterization of bulk NaxC60 (2 less-than-or-equal-to x less-than-or-equal-to 6) and mixed alkali phases Na2AC60 (where A is K, Rb, or Cs). All of these phases have intercalated f.c.c. structures. The Na6C60 structure has a Na4 Cluster centred on the octahedral site. The Na2AC60 Compounds superconduct for the larger A cations, but a crossover to nonsuperconducting behaviour occurs with decreasing cation size and correlates with a minimum in the unit cell volume.