Mass and size dependence of single ion dynamics in molten monohalides

Abstract

This work is concerned with four molten monohalides with different ionic radii ratios ͑RbCl, NaI, AgCl, and CuCl͒ and ideal isotopic systems of these salts with different ionic mass ratios. The velocity autocorrelation functions of the two ionic species in each melt have been studied by both a theoretical approximation and molecular dynamics simulations. It is found that their main features may be qualitatively predicted by considering suitable combinations of the second and fourth frequency moments of their spectra. The analysis of these two parameters allows us to determine how the structure ͑strongly dependent on the ionic size difference͒ and the ionic masses contribute to the shape of the velocity autocorrelation functions. The results show that the averaged microscopic motion of the small ions is mainly determined by the first neighboring shell of unlike ions, whereas the nearest shell of like ions also affects the dynamics of the large ions. This effect is more pronounced as the size difference is greater. Furthermore, it is concluded that the size differences encourage the rattling motion of the large ions, whereas the mass difference encourages the backscattering and oscillations of the velocity autocorrelation function of the light ions. A simple rule is derived to determine the interplay between these two effects. Comparison between the mass and nearest distance ratios enables the prediction as to which species will experience a more pronounced backscattering motion. The size difference effects prevail in the hydrodynamics regime and the self-diffusion coefficient of the small ions is higher than that of the large ones. The difference between the self-diffusion coefficient increases as the size differences increases.

Studies of the local structures of molten metal halides

“…The anion motion is, on the contrary, more influenced by the collisions with the other anions than by the coulombic interaction with cation. 99,115 Mixtures AgX 1 -AgX 2 /CuX 1 -CuX 2 . The difference between the silver halides and the copper halides may be attributed to the evolution of the bonding between the cation and the anion.…”

On the Chemla effect in molten alkali nitrates

“…11,12,16 Okada 16 has investigated the isotope effect by running MD simulations of ͑Li,K͒Cl in which the mass of each of the cations were made arbitrarily different. More recently, 33 an extensive study of mass and size dependence of diffusion coefficients and C v (t) in molten monohalides has been reported. Trullàs and Padró 34 have performed a systematic investigation on the difference between single particle and collective velocity correlation functions.…”

Space-dependent self-diffusion processes in molten copper halides: A molecular dynamics study

“…12 We showed that the averaged microscopic motion of the small ions is mainly determined by the firstneighboring shell of unlike ions, whereas the nearest shell of like ions ͑the second-neighboring shell͒ also affects the dynamics of the large ions. Moreover, we showed that the difference between the cation and anion sizes encourages the backscattering and oscillations of the VACF of the large ions, whereas the mass difference encourages the vibrational motion of the light ions.…”