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

Abstract

A new class of fullerides, M${\mathrm{Ba}}_{2}$${\mathrm{C}}_{60}$ (M=K, Rb, or Cs) and M${\mathrm{BaCsC}}_{60}$, allows us to study the effect of molecular valence on electronic structure while maintaining the crystal structure of ${\mathit{M}}_{3}$${\mathrm{C}}_{60}$ superconductors. X-ray diffraction and Raman scattering show that these are face-centered cubic with pentavalent and quadrivalent ${\mathrm{C}}_{60}$'s, respectively. We find no evidence for superconductivity down to 0.5 K, while electron spin resonance indicates weakly metallic behavior. $^{13}\mathrm{C}$ NMR indicates strong dynamical disorder above room temperature, which may be related to the anisotropic crystal field induced by cations of different valence. \textcopyright{} 1996 The American Physical Society.