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 Im3m) for SF6 and face-centred cubic (space group Fm3m) for CBr4. The bond-orientation distribution function, f(Ω), has maxima in the directions for SF6 and in the 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 tr...

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