Dynamics of molten alkali halides: LiCl and DCl

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Abstract
The authors have measured the inelastic neutron scattering spectra of three samples of molten 7LiCl with different isotopic chlorine compositions, and of one sample of molten KCl, using the time-of-flight spectrometer IN6 at the Institut Laue-Langevin. It is observed that the main peak in the total dynamical structure factor for LiCl is sharply peaked in both Q and omega directions, while that for KCl is much weaker and broader. The spectra for LiCl show evidence for two types of collective mode with characteristic energies of 10-20 and 40-50 meV. Using the differences between the samples, it is possible to show that appropriate pairs of dynamical variables for these modes are m+, -m- (low energies) and q+, -q- (high energies), whereas in molten CsCl the variables have been shown to be m+, m- and q+, q-. They propose that for the alkali chlorides (where the ionic radius ratio r+/r-+m-. There is a crossover between the variables, resulting in strong-mode coupling, when m+=m-. The data for KCl are consistent with this proposition.