ORIENTATIONAL FLUCTUATIONS, DIFFUSE SCATTERING AND ORIENTATIONAL ORDER IN SOLID C₆₀. K. H. Michel^*, J. R. D. Copley⁺, ^*Department of Physics, Universiteit Antwerpen UIA, 2610, Antwerpen, Belgium, ⁺National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

Starting from a microscopic model of interactions between C₆₀ molecules on a fcc lattice, we formulate the orientational interaction potential in terms of symmetry adapted rotator functions. All irreducible representations of the cubic group up to the manifold 1 = 12 are taken into account. We introduce new symmetry adapted normal coordinates for orientational motion. Physical quantities such as the free energy are formulated in terms of these normal coordinates. We calculate the diffuse scattering law throughout the Brillouin zone. The theory describes in a very satisfactory way recent X-ray and neutron scattering experiments. We study the orientational order parameter taking into account all T_2g and T_1g representations up to the manifold 1 = 10. The temperature evolution of the order parameter is obtained within molecular field theory from the selfconsistent solution of a transcendental equation. The coupling of representations largely enhances the transition temperature and the first order character of the phase transition.