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

A new class of fullerides, MBa2C60 (M=K, Rb, or Cs) and MBaCsC60, allows us to study the effect of molecular valence on electronic structure while maintaining the crystal structure of M3C60 superconductors. X-ray diffraction and Raman scattering show that these are face-centered cubic with pentavalent and quadrivalent C60 close-quote s, respectively. We find no evidence for superconductivity down to 0.5 K, while electron spin resonance indicates weakly metallic behavior. 13C NMR indicates strong dynamical disorder above room temperature, which may be related to the anisotropic crystal field induced by cations of different valence. copyright 1996 The American Physical Society