Preparation and structure of the alkali-metal fulleride A₄C₆₀

Abstract

SUPERCONDUCTING K₃C₆₀ (ref. 1) has been shown² 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 reported⁴¹³C NMR studies of K_xC₆₀ 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_xC₆₀ 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_xC₆₀ can be predicted solely on the basis of electrostatic considerations. We have found no evidence for superconductivity in A₄C₆₀.