Modeling the vibrational dynamics of solid benzene: Hexafluorobenzene. The anatomy of a phase transition

Abstract

We have studied the lowest temperature phase of the solid, C 6H 6:C 6F 6. The molecular dynamics of this phase (IV) are seen to consist of thermally activated six-fold rotational reorientations of the molecules about their hexad axis together with vibrations of the molecular chains which constitute the solid. In phase III we have observed, in addition to the solid state rotations, the onset of molecular diffusion. We suggest that the transition from phase IV to phase III occurs as a consequence of increasing amounts of disorder arising from these large amplitude librations or rotations. Relatively simple molecular and electrostatic models are employed to rationalize the different types of experimental data.