ORIENTATIONAL PHASES FOR M3C60

The mechanism of the orientational ordering of Ceo in alkali-metal-dop ed fullerenes M3C60 is studied. Since the M-C60 (M = K, Rb) interactio ns cause the C60 molecules to assume one of two standard orientations, this model is equivalent to a generalized Ising model on a fcc lattic e. The Ising interactions depend on two type of energies: (1) the dire ct interaction, i.e., the orientationally dependent part of interactio ns between nearest-neighboring C60 molecules (each carrying charge -3e ), and (2) the band energy of the electrons transferred from M+ ions t o the C60(3-) ions. It is shown that the contribution to the pairwise interaction from the direct orientational interaction is ferromagnetic and dominantly nearest neighbor. However, contributions from the band (kinetic) energy of the conduction electrons are found to be antiferr omagnetic for first- and third-nearest neighbors, ferromagnetic for se cond- and fourth-nearest neighbors, and negligible for further neighbo rs. The total first-neighbor interaction is probably antiferromagnetic . A non-negligible four-spin interaction is also obtained. The implica tion of these results for the orientational structure is discussed.