Superconductivity in sodium-and lithium-containing alkali-metal fullerides

THE discovery¹ of superconductivity, with a transition temperature T_c of 18 K, in potassium-doped C₆₀ was followed by the synthesis of other superconducting M₃C₆₀ phases: Rb₃C₆₀ (T_c = 28K and 30 K; refs 2, 3), CsRb₂C₆₀ (T_c = 31K; ref. 4) and Cs₂RbC₆₀ (Tc = 33 K; ref. 4). A monotonic relationship is observed⁵ between Tc and lattice constant a₀ for these face-centred cubic M₃C₀ compounds, all of which have a₀ values larger than that of pure C₆₀. Here we study this relationship over a wider range using mixed alkali compounds incorporating sodium (Na₂MC₆₀, where M is K, Rb or Cs) and lithium (Li₂CsC₆₀). The Na₂MC₆₀ compounds have face-centred cubic structures and are superconducting, with T_c = 12 K (M = Cs), 2.5 K (M = Rb) and 2.5 K (M = K); these Tc_s are lower than those predicted on the basis of high-pressure studies^6-8. This departure from the previous relationship between T_c and a₀ for lattice parameters smaller than that of pristine C₆₀ may provide new insights into what controls superconductivity in these materials.