Dynamics of molten alkali halides: LiCl and DCl

The authors have measured the inelastic neutron scattering spectra of three samples of molten 7 LiCl 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 - <1) the former variables are appropriate when m + 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.