Charge density study of C 60 superconductors by MEM/Rietveld analysis

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The charge density study of alkali metal doped fullerenes, Rb 2CsC 60, K 2RbC 60, Na 2RbC 60 and Li 2CsC 60, was carried out at room temperature by MEM/Rietveld analysis, which is the combination of the Maximum Entropy Method (MEM) and Rietveld analysis using synchrotron radiation powder data. The distinctly different features of charge densities of the C 60 molecules relating to the superconducting properties were revealed in the obtained MEM charge densities. For Na 2RbC 60, K 2RbC 60 and Rb 2CsC 60, the rotations of the C 60 molecules were virtually quenched while the C 60 molecule in Li 2CsC 60 was in an orientational disorder. In the interatomic region, the overlapped charge density indicating the existence of a covalent bond was not found significantly between the C 60 molecules and doped metal atoms for the four materials. However, in the MEM charge densities, the relative differences among them were found significantly for the charge deficiencies of the doped metal atoms which should be associated with charge transfer from the metal atoms to the C 60 molecule.