Orientational Disorder of C 60 in Li 2 CsC 60

Abstract

The x-ray diffraction of the nonsuperconducting ternary fulleride Li 2 CsC 60 reveals at room temperature a face-centered-cubic (Fm3̅m) 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] C 60 3- ions, in sharp contrast to their orientational state in superconducting face-centered-cubic K 3 C 60 (merohedral disorder) and primitive cubic Na 2 CsC 60 (orientational order). The orientational disorder of the carbon atoms on the C 60 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.