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

Collective excitations in the orientationally disordered phase of SF6 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 100 K 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(Q, ω) 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.

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