Inelastic neutron scattering studies of the dynamics of molten alkali halides

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Abstract
The authors have measured the inelastic neutron scattering spectra of samples of molten RbCl, CsCl and NaI using the time-of-flight technique. These show broadening of the quasi-elastic peak associated with diffusion together with an extended inelastic spectrum associated with collective motions. They present a set of definitions of correlation functions for binary systems in terms of which the spectra are analysed. Single-ion motion is shown to be consistent with simple diffusion up to the first peak in S+-(Q) after which it becomes more gas-like. de Gennes narrowing of the quasi-elastic peak due to collective motions is observed. The inelastic scattering is discussed in terms of these collective motions which are represented by mass ('acoustic' type mode) and charge ('optic' type mode) current correlations. Quantitative comparison is made with the results of computer simulation and of the inelastic neutron scattering spectra at various temperatures in both the solid and molten states.

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