SUPERCONDUCTIVITY AND CATION-VACANCY ORDERING IN THE RARE-EARTH FULLERIDE YB2.75C60

INTERCALATED fullerides exhibit remarkable physical properties, including superconductivity at record high temperatures for organic compounds(1-6). The donor intercalants have so far been limited to alkali and alkaline-earth metals. Here we describe the synthesis, structure and magnetic properties of a rare-earth-doped fulleride, Yb2.75C60, which exhibits superconductivity below 6 K. Ytterbium cations occupy off-centred interstitial sites in the face-centred cubic C-60 lattice, and leave eight tetrahedral sites vacant in an orthorhombic unit cell which is thereby doubled in each direction. These cation-vacancy sites exhibit long-range ordering, generating a superstructure which seems to be stabilized by a short-range interaction between charge-deficient single bonds in C-60 and exclusively divalent cations. Our results demonstrate that superconductivity is not limited to C-60 compounds of high structural symmetry.