Thermal expansion of the lowest-temperature phase of benzene: hexafluorobenzene

Abstract

Neutron diffraction measurements of the anisotropic thermal expansion of the molecular complex C₆D₆∶ C₆F₆ have been used to investigate the intermolecular forces and vibrational properties in the solid low-temperature phase (IV). Assuming a simple first-order approximation to the thermal expansion an intermolecular bond strength and average frequency have been derived which are in agreement with other estimates, and consistent with a Lennard-Jones type of potential. The thermal expansion is approximately isotropic below 150 K, and becomes slightly anisotropic as a precursor to the phase transition at 205 K. The nature of the anisotropy suggests that the phase transition is to some extent associated with the onset of rotational motion of the aromatic molecules about their six-fold axes. This conclusion is consistent with previous quasielastic neutron scattering observations.