The orientational disorder in crystalline sulfur hexafluoride, SF6, has been studied using a combination of neutron total scattering and the reverse Monte Carlo method. Analysis of the atomic configurations has shown the extent of the disorder through the evaluation of the S-F bond orientational distribution function, consistent with, but improving upon, the results of earlier neutron powder diffraction data. The correlations between orientations of neighbouring molecules have been studied through analysis of the distributions of F-F distances, showing that nearest-neighbour F-F close contacts are avoided, consistent with previous molecular dynamics simulation results. The results present a new case study of the application of neutron total scattering and the reverse Monte Carlo methods for the study of orientational disorder, where in this instance the disorder arises from orientational frustration rather than from a mismatch of molecular and site symmetries.
We report a study of the orientational order and phase transitions in crystalline deuterated methane, carried out using neutron total scattering and the reverse Monte Carlo method. The resultant atomic configurations are consistent with the average structures obtained from Rietveld refinement of the powder diffraction data, but additionally enable us to determine the C-D bond orientational distribution functions (ODF) for the disordered molecules in the high-temperature phase, and for both ordered and disordered molecules in the intermediate-temperature phase. We show that this approach gives more accurate information than can been obtained from fitting a bond ODF to diffraction data. Given the resurgence of interest in orientationally-disordered crystals, we argue that the approach of total scattering with the RMC method provides a unique quantification of orientational order and disorder.
