Orientational Disorder in the Plastic Phases of SF6 and CBr4

The coherent, elastic neutron scattering from polycrystalline samples of SF6 and CBr4 in their plastic phases has been measured. The powder line component of the neutron distributions has been analysed in terms of Debye-Waller factors to give information about the orientational disorder in the solids. An extensive diffuse scattering component is also observed in CBr4. The scattering calculated for a simple model of the molecular disorder in CBr4 agrees reasonably well with this observed diffuse scattering, but improvements in the model are clearly necessary.