Structural and electronic properties of the noncubic superconducting fullerides A ' C-4(60) (A '= Ba, Sr)

The absence of superconductivity in noncubic alkali fullerides, A(4)C(60) ( A = K,Rb,Cs) has been associated with the effects of electron correlation a nd electron-phonon interaction on the narrow t(1u)-derived conduction band. We find that the structurally related alkaline earth fullerides, A(4)'C-60 (A' = Sr,Ba), are superconducting with small values of the densities of st ates at the Fermi level, N-epsilon F. Close contacts between Ba2+ ions and neighboring C-60 units in Ba4C60 imply a strong orbital hybridization, whic h is evidently responsible for stabilizing the noncubic superconducting pha se, in contrast to insulating A(4)C(60).