Collective excitations in an orientationally frustrated solid; neutron scattering and computer simulation studies of SF₆

Abstract

Collective excitations in the orientationally disordered phase of SF 6 have been studied by inelastic neutron scattering and molecular dynamics simulation techniques. Experimental measurements to observe acoustic modes were made along the high symmetry directions at temperatures of 100K and 200 K. The excitations observed showed little evidence of discrete peaks but were all broad and overdamped. They showed little temperature dependence. The dynamical structure factors S(O, co) calculated from the simulation are in qualitative agreement with the observed spectra but quantitatively show discrepancies. For smaller wave vectors than those studied experimentally the calculations show the existence of well-defined, long wavelength acoustic phonons. The wave vector at which the transition occurs between propagating and overdamped excitations was found to be temperature dependent. The results are interpreted in terms of the concept of orientational frustration. Some difficulties in the application of molecular dynamical simulation to the calculation of dynamical correlation functions are discussed in an Appendix.

Influence of rotational motion of molecules on the thermal conductivity of solid SF6, CHCl3, C6H6, and CCl4

“…The interaction between the nearest neighbors in the bcc phase is favorable for molecular ordering caused by the S-F bonds along the {100} direction, and the interaction with the next nearest neighbors is dominated by repulsion between the F atoms. According to x-ray and neutron diffraction data [1,2,4] a strict order is observed in SF 6 (in phase I) just above the phase transition point. The structural dynamical factor  characterizing the degree of the orientational order is close to unity in the interval 95-130 K. This feature sets off SF 6 from other plastic crystals, such as methane, carbon tetrachloride, adamantane and so on, where the long-range orientational order becomes disturbed immediately after the phase transition.…”

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

“…Further diffraction studies identified the crystal structure of the low-temperature ordered phase [8][9][10], the structure of which is shown in figure S1 (https://stacks.iop.org/JPCM/34/ 295401/mmedia) of the supplementary information. Inelastic neutron scattering from the disordered phase has shown that the acoustic modes become over-damped as the wave vector moves from zero into the Brillouin zone, characteristic of a highly disordered material [11].…”

“…Molecular dynamics (MD) simulations [12,13] have suggested that the origin of disorder arises from the nearest fluorine atoms along the direction of the cubic unit cell edge (see figure 1) being unable to approach to distances as short as those implied by the molecular alignment in the average structure. This effect was called 'orientational frustration' [11,13] because the disorder arises from the competition between the orientational interactions between different types of neighbouring molecules. The nearest-neighbour interactions (along the body diagonal of the cubic unit cell) try to order the orientations in the ideal structure shown in figure 1, but next-nearest neighbour interactions (along the unit cell cube edge) oppose the two molecules simultaneously taking these orientations because of the close approach of the nearest fluorine atoms.…”

Calorimetric Study of Phase Transitions in SeF6and WF6Crystals

“…The intermolecular distance may be an important factor that dominates the orientation frustration. A number of physicochemical studies (e.g., X-ray and neutron diffraction, − ,− ,, NMR, − neutron inelastic scattering, , molecular dynamics − ) have been performed for SF 6 and WF 6 . The heat capacity of WF 6 has also been measured by Weinstock et al , but the numerical data of the heat capacity and the detail of the analysis on the transition (e.g., base-line determination) were not stated in their paper.…”