Orientational order and phase transitions in deuterated methane: a neutron total scattering and reverse Monte Carlo study

Abstract

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.

Unique features of the structural phase transition in acetylene showing simultaneous characteristics of reconstructive, displacive and order–disorder

“…The crystal structure of acetylene is particularly interesting because the molecule is small and linear (an iterative step from HCN), and because the crystal structure defined by the centres of mass of the molecules is particularly simple, namely the cubic closed packed structure. In this sense acetylene might have reminded us of other molecular crystals, such as methane 45 and adamantane, 46 which both have orientational order-disorder phase transitions with a high-temperature phase that have the cubic close-packed structure. Indeed, it had been suggested that acetylene might also have an orientationally disordered state in the high-temperature cubic phase.…”

Orientational order/disorder and network flexibility in deuterated methylammonium lead iodide perovskite by neutron total scattering

“…Whereas in the analysis of crystallographic data the coefficients might be fitted to data for the structure factors of Bragg reflections, 46 the orthonormality of the functions means that we can obtain the values of directly from the distribution of atoms in the RMC configurations rather than by fitting, making use of the relationshipwhere the averages are over all molecules in a set of configurations. This approach has been used recently to analyse the RMC configurations of the cubic phases of KCN, 18 BaCO 3 , 19 methane 20 and SF 6 . 21…”

“…The RMC method is used to generate supercell congurations of atomseffectively instantaneous imagesthat are simultaneously consistent with the long-range order revealed by the information contained in the Bragg scattering and with the uctuations of the local structure revealed by the PDF. The congurations can therefore give information about the distributions of orientations of molecules and molecular ions in ordered, partially ordered and disordered phases, [15][16][17][18][19][20][21] and about uctuations within inorganic crystal networks. 6,22,23 We believe that the total scattering method coupled with RMC analysis gives a unique view of disordered crystalline materials.…”

“…This is exactly the sort of question that total scattering methods analysed by the RMC method can tackle and provide useful, and unique, new insights. [15][16][17][18][19][20][21] 3 Methods…”

Orientational disorder in sulfur hexafluoride: a neutron total scattering and reverse Monte Carlo study

“…In this sense, the paper also serves as a demonstration of the power of the RMC method to determine structural details of orientationally disordered crystals beyond what is possible from Bragg diffraction alone, given that Bragg diffraction provides information only about single-atom distribution functions whereas total scattering provides information about the relative positions of pairs of atoms. Within this general context, the study of SF 6 provides a useful contrast to recent studies demonstrating the power of the RMC method for studying orientational disorder-previous examples are KCN [17], BaCO 3 [18] and methane [19]-because SF 6 is an example of orientational frustration, rather than of site disorder due to the symmetry of the molecule being lower than that of the site. These demonstrations, together with the unique information they provide, are important given the current revival of interest in orientationally disordered materials due to the recognition that they possess a number of important properties for modern applications [20].…”