Br diffusion in molten NaBr explored by coherent quasielastic neutron scattering.

Molten sodium bromide has been investigated by quasielastic neutron scattering focusing on the wave vector range around the first structure factor peak. The linewidth of the scattering function shows a narrowing around the wave number of the structure factor peak, known as deGennes narrowing. In a monatomic system, this narrowing or in the time domain slowing down, has been related to a self-diffusion process of the caged particle. Here we show that this methodology can be applied to the molten alkali halide NaBr. The incoherent scattering from the sodium ions at small wave vectors provides the self-diffusion coefficient of sodium and the dynamics of bromine ions can be studied at wave numbers around the structure factor peak. With input from molecular dynamics simulations on the partial structure factors, diffusion coefficients of the bromine ions can be obtained. These experimentally derived diffusion coefficients are in good agreement with molecular dynamics simulation results. This methodology to extract self-diffusion coefficients from coherent quasielastic neutron scattering is applicable to binary fluids in general when one particle dominates the scattering response at the structure factor maximum.

Single ion dynamics in molten sodium bromide.

We present a study on the single ion dynamics in the molten alkali halide NaBr. Quasielastic neutron scattering was employed to extract the self-diffusion coefficient of the sodium ions at three temperatures. Molecular dynamics simulations using rigid and polarizable ion models have been performed in parallel to extract the sodium and bromide single dynamics and ionic conductivities. Two methods have been employed to derive the ion diffusion, calculating the mean squared displacements and the velocity autocorrelation functions, as well as analysing the increase of the line widths of the self-dynamic structure factors. The sodium diffusion coefficients show a remarkable good agreement between experiment and simulation utilising the polarisable potential.

The longitudinal optic-like mode in molten alkali halides: a molecular dynamics approximation to inelastic x-ray scattering experiments.

The contribution of the long-wavelength fluctuations in the particle number, mass, and charge densities to the inelastic x-ray scattering dynamical structure factor for molten NaI and other molten alkali halides is accurately analyzed in the high frequencies region of the longitudinal optic-like mode. Molecular dynamics simulation results at low wave numbers are reported for the time correlations between the density components in the reciprocal space, namely, the corresponding intermediate scattering functions, and their spectra, namely, the dynamical structure factors. The time correlations between the longitudinal currents and their spectra are also reported. The importance of cross correlations is discussed. Moreover, the role played by the collective behavior of the two ionic species is also investigated. It is concluded that the longitudinal optic-like mode in molten alkali halides is unlikely to be detected by inelastic x-ray scattering experiments.