ALKALI-METAL STOICHIOMETRY AND STRUCTURE OF K4C60 AND RB4C60

Alkali-metal fulleride samples KxC60 and RbxC60 with stoichiometries c lose to x=4 were examined with x-ray powder-diffraction measurements a t room temperature. We have shown through Rietveld refinements that A( 4)C(60) is a line phase with charge per lattice site that is closer to integer than in any other fulleride. The C-60 molecules are disordere d between two possible orientations within the body-centered-tetragona l lattice with the three orthogonal twofold rotation axes aligned with the unit-cell translation vectors in space group I4/mmm. The x-ray di ffraction results exclude any quadrupole distortion of the fullerene m olecule beyond about 0.04 Angstrom. These results suggest that K4C60 a nd Rb4C60 may be Mott insulators.