What does Landau theory tell us about the orientational state of fullerenes?

Abstract

Various orientation transitions in the fullerenes are described within the Landau theory of phase transitions. Among these we review the description of the orientational ordering transition from an fcc to an sc unit cell which occurs in undoped C 60 at a temperature T≈260 K. We also discuss the symmetry of the orientational glass transition which is found via elastic neutron scattering at T≈90 K. We comment on the seemingly spurious transition (so called “2-a transition”) which was claimed to double the size of the unit cell from simple cubic to fcc with eight molecules per unit cell. We also discuss the orientational state of the doped systems M 3C 60, where M denotes an alkali metal. In this system the C 60 molecules are restricted to two “standard” orientations. The resulting Ising model has antiferromagnetic nearest neighbor interactions and nonnegligible four-spin interactions whose properties have recently been discussed. The Landau description of the orientational ordering in C 70 is also presented. Here we include a description of the elastic distortion which accompanies orientational ordering.