Intercalation of sodium heteroclusters into the C₆₀ lattice

INTERCALATION of sodium into Chas been shownto yield a range of compounds NaC(2<x<6). Unlike the other alkali-metal-doped compounds, there is no evidence of superconductivity within this range of doping. Furthermore, Naretains the face-centred-cubic (f.c.c.) structure of undoped Cwhereas the analogous x = 6 phases with K, Rb or Cs are body-centred cubic (b.c.c.) Here we report the preparation of sodium-doped Cwith x up to about 10; the structure remains f.c.c. throughout. Rietveld refinement of X-ray diffraction data yields a structure with ideal composition Na in which the octahedral interstitial site contains a nine-atom body-centred cluster of sodium atoms while the tetrahedral sites are singly occupied. Clusters of four to nine sodium atoms in the octahedral site provide sufficient 'chemical pressure' to prevent the low-temperature lattice distortion that occurs when this site is singly occupied, as in Na(ref. 1). This distortion is thought to suppress superconductivity in the latter compound. Thus we suggest that new sodium-doped superconduct-ing phases may be found in the range 4< x< 11.