STRUCTURAL STUDIES OF CYCLOHEXANE C6D12 BY NEUTRON-DIFFRACTION .3. TEMPERATURE-VARIATION MEASUREMENTS FOR THE LIQUID AND PLASTIC CRYSTAL PHASES

Neutron diffraction measurements on C6D12 over a wide temperature rang e, covering liquid crystal, plastic crystal and brittle crystal phases , are presented. The data extend work conducted previously over a rest ricted temperature range and show that the structural features are dep endent on the thermal history of the sample. The results demonstrate t he relative contributions from lattice symmetry (ordered) and diffuse scattering (disordered) at varying temperatures. The spatial distribut ion function undergoes a complex change in which orientational correla tions between adjacent molecules appear to be coupled to translational displacements of the molecular centres. Previous assumptions that cyc lohexane is an 'almost-unhindered' rotator are found to be erroneous a nd the spatial correlations exhibit a complex relationship which is se nsitive to temperature variation. Further methods for the analysis of the order-disorder features are considered.