Collective dynamics of alkali chloride glasses: Molecular dynamics analyses of the dynamic structure factors

Abstract

The collective dynamical properties of two glassy alkali chlorides [pure LiCl and the mixture 50(mol %) LiCl–40 KCl–10 CsCl] have been investigated by using molecular dynamics simulations. On the basis of van Hove’s description of the microscopic density fluctuations, the dynamic structure factors S(k,ω) have been calculated for 0.3≤k≤5.9 Å−1 to reveal phonon excitation and motional correlations between each ionic species. Examining the phonon dispersion curves, it is found that there are one longitudinal acoustic and two longitudinal optic branches in both glasses. In the LiCl glass, collective vibrations of the Li+ and Cl− ions are spatially delocalized for k≤1.6 Å−1 and are strongly correlated with each other. On the other hand, in the mixture glass, collective motions of the Li+ ions are almost independent of those of the Cl− ions. In this glass, there are the intense longitudinal optic (LO) vibrations localized on single Li+ ions, similar to an Einstein oscillation. The LO mode frequency for k≥1.5 Å−1 is constant at ∼280 cm−1 which corresponds to the Einstein frequency. In addition to the standard method to calculate the dynamic structure factors by means of van Hove’s description, we have successfully calculated the dynamic structure factors of the glasses in terms of the normal modes for Stillinger and Weber’s inherent structures. In this calculation, we have utilized Zemach and Glauber’s normal mode description which was originally formulated to give the dynamic structure factors for polyatomic molecules in gas phase. In this scheme, the effects of various physical factors on the k and ω-dependence of the dynamic structure factors are analyzed. Moreover, it is revealed that the mass-effect rather than the interionic forces dominates the differences in the phonon density of states between the two systems.