PREPARATION AND STRUCTURE OF THE ALKALI-METAL FULLERIDE A4C60

SUPERCONDUCTING K3C60 (ref. 1) has been shown 2 to have an intercalated face-centred cubic (f.c.c.) structure, and other A3C60 compounds (where A is K, Rb or mixtures of K, Rb and/or Cs) form an isostructural series with superconducting transition temperatures up to 31.3 K (ref. 3). Recently we reported 4 C-13 NMR studies of K(x)C60 for x < 3, and concluded that the system contained two phases (x = 0 and x = 3) for 0 < x < 3. Here we report an investigation of A(x)C60 in the range 3 < x < 6 and find evidence for an additional phase at x = 4. In contrast to f.c.c. A3C60 (refs 2, 3) and body-centred cubic A6C60 (ref. 5), A4C60 (where A is K, Rb, Cs) has a body-centred tetragonal structure. We show that all of the experimentally observed phases of A(x)C60 can be predicted solely on the basis of electrostatic considerations. We have found no evidence for superconductivity in A4C60.