Alkali-metal stoichiometry and structure ofK4C60andRb4C60

Abstract

Alkali-metal fulleride samples ${\mathrm{K}}_{\mathrm{x}}$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{\mathrm{x}}$${\mathrm{C}}_{60}$ with stoichiometries close to x=4 were examined with x-ray powder-diffraction measurements at room temperature. We have shown through Rietveld refinements that ${\mathrm{A}}_{4}$${\mathrm{C}}_{60}$ is a line phase with charge per lattice site that is closer to integer than in any other fulleride. The ${\mathrm{C}}_{60}$ molecules are disordered between two possible orientations within the body-centered-tetragonal lattice with the three orthogonal twofold rotation axes aligned with the unit-cell translation vectors in space group I4/mmm. The x-ray diffraction results exclude any quadrupole distortion of the fullerene molecule beyond about 0.04 \AA{}. These results suggest that ${\mathrm{K}}_{4}$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{4}$${\mathrm{C}}_{60}$ may be Mott insulators.