The MEM charge density study of Rb2CsC60, K2RbC60, and Li2CsC60 by synchrotron radiation powder method

A charge density study of alkali metal doped fullerenes, Rb2CsC60, K2RbC60 and Li2CsC60 was carried out at room temperature by the Maximum Entropy Met hod (MEM) combined with the Rietveld analysis using synchrotron powder data . The MEM charge densities of the C-60 molecules show distinct differences related to the superconducting properties. For both K2RbC60 and Rb2CsC60 wh ich are superconductors at low temperature, the C-60 molecules were in a me rohedral disorder. On the other hand, the C-60 molecules in Li2CsC60; non-s uperconductor even at very low temperature, were in an orientational disord er. In the interatomic region between the C-60 molecules and doped metal at oms, any significant overlapped charge densities related to covalent bond w ere 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 ato ms which should be associated with charge transfer from the metal atoms to the C-60 molecule seems to have a strong correlation to the superconducting transition temperature, Tc, i.e. the higher the Tc, the mon the deficienci es.