THE discovery1 of superconductivity, with a transition temperature Tc of 18 K, in potassium-doped C60 was followed by the synthesis of other superconducting M3C60 phases: Rb3C60 (Tc = 28K and 30 K; refs 2, 3), CsRb2C60 (Tc = 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 Tc = 12 K (M = Cs), 2.5 K (M = Rb) and 2.5 K (M = K); these Tcs are lower than those predicted on the basis of high-pressure studies6-8. This departure from the previous relationship between Tc and a0 for lattice parameters smaller than that of pristine C60 may provide new insights into what controls superconductivity in these materials.
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