The effect of carrier concentration on fulleride superconductivity is
studied by x ray, Raman, magnetization, and ESR of two new cubic famil
ies: Na2CsxC60 (0 < x < 1) and M(3-y)Ba(y)C(60) (0.2 < y < 2, M = K, R
b, or Cs). These are isostructural with known Pa(3) over bar and <Fm(3
)over bar m> trivalent superconductors, respectively, but with variabl
e valence 2 < n < 5. T-c is peaked at or very near n = 3, decreasing r
apidly in either direction to < 0.5 K for n = 2.5 and 5. These results
underscore the failure of both one-electron and strong correlation mo
dels to explain the dependence of T-c on band filling.