Mass and size dependence of single ion dynamics in molten monohalides

This work is concerned with four molten monohalides with different ionic ra dii ratios (RbCl, NaI, AgCl, and CuCl) and ideal isotopic systems of these salts with different ionic mass ratios. The velocity autocorrelation functi ons of the two ionic species in each melt have been studied by both a theor etical approximation and molecular dynamics simulations. It is found that t heir main features may be qualitatively predicted by considering suitable c ombinations of the second and fourth frequency moments of their spectra. Th e analysis of these two parameters allows us to determine how the structure (strongly dependent on the ionic size difference) and the ionic masses con tribute to the shape of the velocity autocorrelation functions. The results show that the averaged microscopic motion of the small ions is mainly dete rmined by the first neighboring shell of unlike ions, whereas the nearest s hell 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 co ncluded that the size differences encourage the rattling motion of the larg e ions, whereas the mass difference encourages the backscattering and oscil lations of the velocity autocorrelation function of the light ions. A simpl e rule is derived to determine the interplay between these two effects. Com parison between the mass and nearest distance ratios enables the prediction as to which species will experience a more pronounced backscattering motio n. The size difference effects prevail in the hydrodynamics regime and the self-diffusion coefficient of the small ions is higher than that of the lar ge ones. The difference between the self-diffusion coefficient increases as the size differences increases. (C) 2000 American Institute of Physics. [S 0021-9606(00)50347-X].