Orientational disorder and the phase transition in hexafluoroethane

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Abstract
The crystal structure of hexafluoroethane in its high temperature phase is investigated by neutron diffraction from polycrystalline samples. The structure is body centred cubic with pronounced molecular orientational disorder and is qualitatively similar at both 110 K and 95 K. The disorder is analysed in terms of symmetry-adapted functions. An expression is derived for the diffuse scattering in terms of the coefficients which specify the orientational disorder. The calculated diffuse scattering is only in qualitative agreement with that observed. A structural transition to a lower-symmetry phase occurs in the temperature range 70 K to 40 K and in this range the two phases coexist. The supposedly structural transition previously reported at 104·1 K has not been observed.