Orientational order in A3C60: effects on valence bands and infrared optical spectra

By analysis of the electronic contribution to the binding in A3C60, we show that the problem of orientational ordering can be mapped onto the fcc nearest-neighbor antiferromagnetic Ising model with J of the order of 100K. The ground state crystal structure should be therefore 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 bi-directional) crystal. Calculated optical conductivity can be fitted by a Drude-like contribution and an ''interband'' Lorenzian peak at 400-500 cm-1. (orig.)