Synthesis and properties of mixed alkali-metal-alkaline-earth fullerides

A new class of fullerides, MBa(2)C(60) (M=K, Rb, or Cs) and MBaCsC(60), allows us to study the effect of molecular valence on electronic structure while maintaining the crystal structure of M(3)C(60) superconductors. X-ray diffraction and Raman scattering show that these are face-centered cubic with pentavalent and quadrivalent C-60's, respectively. We find no evidence for superconductivity down to 0.5 K, while electron spin resonance indicates weakly metallic behavior. C-13 NMR indicates strong dynamical disorder above room temperature, which may be related to the anisotropic crystal field induced by cations of different valence.