RAPID AND EFFICIENT SYNTHESIS OF ALKALI-METAL C-60 COMPOUNDS IN LIQUID-AMMONIA

The reaction of stoichiometric amounts of alkali metals with C60 in liquid ammonia provides a rapid and quantitative route to M(x)C60 superconductors (M(x) = K3, Rb3, CsRb2, RbCs2, KRbCs). Annealing of the samples for 24-48 h at 375-degrees-C is required to obtain large superconducting fractions. C-13 and Rb-87 NMR line shapes are reported for Rb3C60. The C-13 line shapes show the degree of rotational motion of the C60 ions varies considerably from sample to sample and is dependent on the method of preparation and subsequent heat treatment. A correlation between the degree of rotational motion and the superconducting fraction is noted and attributed to the amount of disorder in the sample. Rb-87 NMR shows three peaks indicating that Rb3C60 actually contains three different cation sites, rather than the two suggested by X-ray powder diffraction data. The third site is occupied by approximately 15% of the Rb ions in Rb3C60 and is shown to be a subset of the tetrahedral sites in the cubic close-packed C60 lattice. Although the exact nature of the distortion required to produce the third site is unknown, it appears to be unrelated to the orientations of the C60 ions creating the site.