Superconducting and normal-state properties of nonavalent fullerides

We found that A(3)Ba(3)C(60) (A = K, Rb, and Cs) forms an isostructural series of compounds with a body centered-cubic structure and tunable lattice parameter, while retaining the nominal molecular valence as C-60(9-). In sharp contrast with the trivalent face-centered-cubic A(3)C(60)-type superconductors, the density of states at the Fermi energy and T-c decrease with increasing lattice parameter. Analysis by a phonon-mediated pairing model gives relevant parameters which considerably differ from those proposed for the A(3)C(60) superconductors.