Orientational order inA3C60: Antiferromagnetic Ising model for the fcc lattice

Abstract
By analysis of the electronic contribution to the binding in A3 C60, we show that the problem of orientational ordering can be mapped onto the fcc nearest-neighbor antiferromagnetic Ising model with J at the order of 100 K. For T<1.76J, the system should be found in a state which is antiferromagnetically ordered in two dimensions and disordered in the third. This is not inconsistent with present x-ray powder data. We find that the low-temperature conduction-band structure closely resembles that of the simplest antiferromagnetically ordered (so-called bidirectional) crystal.