The MEM Charge Density Study of Rb2CsC60, K2RbC60, and Li2CsC60 by Synchrotron Radiation Powder Method

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A charge density study of alkali metal doped fullerenes, Rb2CsC60, K2RbC60 and Li2CsC60, was carried out at room temperature by the Maximum Entropy Method (MEM) combined with the Rietveld analysis using synchrotron powder data. The MEM charge densities of the C60 molecules show distinct differences related to the superconducting properties. For both K2RbC60 and Rb2CsC60, which are superconductors at low temperature, the C60 molecules were in a merohedral disorder. On the other hand, the C60 molecules in Li2CsC60; non-superconductor even at very low temperature, were in an orientational disorder. In the interatomic region between the C60 molecules and doped metal atoms, any significant overlapped charge densities related to covalent bond were not found for all three materials. This is the clear evidence for that superconductivity of these materials and covalency of doped metals have no correlation at all. Instead, the charge deficiencies of the doped metal atoms which should be associated with charge transfer from the metal atoms to the C60 molecule seems to have a strong correlation to the superconducting transition temperature, Tc, i.e. the higher the Tc, the more the deficiencies.