Structural and electronic properties of sodium-intercalated C₆₀

Abstract

ALKALI metal fuller ides 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 Na_xC₆₀ has been reported, although measurements of electrical conductivity¹⁰ and Raman¹¹, ESR¹¹ and photoemission¹² spectra of thin films have given clear indications of Na_xC₆₀ formation. Here we report the synthesis and initial characterization of bulk Na_xC₆₀ (2≤x≤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.