Orientational Disorder of C60 in Li2CsC60

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Science  27 May 1994:
Vol. 264, Issue 5163, pp. 1294-1297
DOI: 10.1126/science.264.5163.1294


The x-ray diffraction of the nonsuperconducting ternary fulleride Li2CsC60 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] C603- ions, in sharp contrast to their orientational state in superconducting face-centered-cubic K3C60 (merohedral disorder) and primitive cubic Na2CsC60 (orientational order). The orientational disorder of the carbon atoms on the C603- 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.