ORIENTATIONAL ORDER IN A3C60 - ANTIFERROMAGNETIC ISING-MODEL FOR THE FCC LATTICE

By analysis of the electronic contribution to the binding in A3C60, we show that the problem of orientational ordering can be mapped onto th e fcc nearest-neighbor antiferromagnetic Ising model with J at the ord er of 100 K. For T < 1.76J, the system should be found in a state whic h is antiferromagnetically ordered in two dimensions and disordered in the third. This is not inconsistent with present x-ray powder data. W e find that the low-temperature conduction-band structure closely rese mbles that of the simplest antiferromagnetically ordered (so-called bi directional) crystal.