Neutron diffraction study of the plastic phases of polycrystalline SF6 and CBr4

An investigation of the plastic phases of polycrystalline specimens of sulphur hexafluoride (SF₆) and carbon tetrabromide (CBr₄) by neutron elastic scattering and neutron diffraction experiments is reported. A theory of neutron diffraction in plastic crystals, which treats the Bragg scattering and the diffuse scattering from a unified point of view, is developed and applied in the interpretation of the neutron results. The Debye-Scherrer peaks are analysed, both by a cumulant expansion technique and a cubic harmonic analysis, to determine the crystal structures of the plastic phase which are found to be body-centred cubic (space group Im3_m) for SF₆ and face-centred cubic (space group Fm3m) for CBr₄. The bond-orientation distribution function, f(Ω), has maxima in the <100> directions for SF₆ and in the <110> directions for CBr₄. Since, in both cases, f(Ω) is appreciably different from zero for all orientations, it is apparent that significant thermal reorientation takes place in both these crystals. The translational and librational displacements in CBr₄ are exceptionally large and give rise to extensive diffuse scattering which is analysed on the basis of a simple Einstein model. The model predicts that the centre-of-mass thermal vibration and the orientational disorder give approximately equal contributions to the total diffuse scattering. The calculated scattering is in good agreement with experiment for all wave vector transfers outside the range 2 to 3 Å^-1. Inside this range discrepancies occur which are interpreted as evidence for the existence of orientational short-range order in CBr₄.