Calculation of the stability of higher C₆₀ anions in alkali-doped crystals

The energy of icosahedral C₆₀^x- anions are calculated for 1 ≤ x ≤ 10 by means of the Roothaan open shell theory modified to take account of the icosahedral point group symmetry. The volume π electron approximation is used. It is shown that the energies of C₆₀^x- can be fitted by a quadratic dependence on x similar to that used for atomic ions. The influence of the crystalline field on C₆₀^x- in the alkali-doped crystals is estimated analytically by evaluation of the Madelung constants. As a whole the obtained data on the relative stability of the various crystalline phases correspond well to the results obtained by Fleming et al. (Nature, 352 (1991) 701).