Orientational disorder of C₆₀ in Li₂CsC₆₀

The x-ray diffraction of the nonsuperconducting ternary fulleride Li ₂CsC₆₀ reveals at room temperature a face-centered-cubic (Fm3m) disordered structure that persists to a temperature of 13 Kelvin. The crystal structure is best modeled as containing quasispherical [radius of 3.556(4) angstroms] C₆₀^3- ions, in sharp contrast to their orientational state in superconducting face-centered-cubic K₃C ₆₀ (merohedral disorder) and primitive cubic Na₂CsC ₆₀ (orientational order). The orientational disorder of the carbon atoms on the C₆₀^3- sphere was analyzed with symmetry-adapted spherical-harmonic functions. Excess atomic density is evident in the (111) directions, indicating strong bonding Li⁺-C interactions, not encountered before in any of the superconducting alkali fullerides. The intercalate-carbon interactions and the orientational state of the fullerenes have evidently affected the superconducting pair-binding mechanism in this material.