Mean-field theory for C70.

Orientational ordering transitions in C70 are studied by constructing a Landau free energy in terms of order parameters describing long-range orientational order. This theory predicts that the transition from the orientationally disordered state into a partially ordered state, where the long axes of the molecules are parallel to one another, is discontinuous with an elastic distortion. Order parameters describing the lower temperature transition, where spinning about the long axis becomes hindered, are also discussed.