Olga Alcaraz

Static structure and ionic transport in molten AgBr and AgCl

The static structure of molten AgBr and AgCl have been calculated using the hypernetted chain theory of liquids (HNC) and molecular dynamics simulations (MD) with effective potentials based on the functional form originally proposed by Vashishta and Rahman [Phys. Rev. Lett. 40, 1337 (1978)] to study α-AgI. The HNC and MD are in good agreement among themselves as well as in good qualitative agreement with experiment. MD simulations have been also used to calculate the time correlation functions and ionic transport properties of these melts. The results for the velocity autocorrelation functions suggest, in both cases, a mechanism for diffusion akin to that we suggested for molten AgI and CuX (X=Cl, Br, I) [J. Phys. Condens. Matter 2, 6643 (1990)] even though the cations velocity autocorrelation function is no longer purely diffusive. The results for the diffusion coefficients resemble the type of behavior found in superionic melts, as if the transition to a superionic state is finally realized in AgCl and ...

Molecular dynamics study of polarizable point dipole models for molten sodium iodide

The structure, the ionic transport properties, and the dynamics of long-wavelength charge-density fluctuations, for two polarizable point dipole models of molten NaI, have been studied by molecular dynamics simulations. These models are based on a rigid ion potential to which the induced dipole polarization of the anions is added. The polarization is added in such a way that point dipoles are induced on the anions by both local electric field and short-range damping interactions that oppose the electrically induced dipole moments. The two polarizable ion models differ only in the range of the damping polarization interactions. The influence of the induced anion polarization on the different properties of simulated molten NaI is discussed.

Mass and size dependence of single ion dynamics in molten monohalides

This work is concerned with four molten monohalides with different ionic radii ratios (RbCl, NaI, AgCl, and CuCl) 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 difference) 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 backscatteringmotion. 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.

The structure of molten AgCl, AgI and their eutectic mixture as studied by molecular dynamics simulations of polarizable ion model potentials

The structure of molten AgCl, AgI, and their eutectic mixture Ag(Cl(0.43)I(0.57)) is studied by means of molecular dynamics simulations of polarizable ion model potentials. The corresponding static coherent structure factors reproduce quite well the available neutron scattering data. The qualitative behavior of the simulated partial structure factors and radial distribution functions for molten AgCl and AgI is that predicted by the reverse Monte Carlo modeling of the experimental data. The AgI results are also in qualitative agreement with those calculated from ab initio molecular dynamics.

Static dielectric properties of polarizable ion models: Molecular dynamics study of molten AgI and NaI

The fluctuation-dissipation theorem for the static dielectric response function of systems of ions with inducible point dipoles is derived. It is shown that the static longitudinal dielectric function is determined by spatial correlations of both charge and dipole-moment density fluctuations. Moreover, it is deduced that the long-wavelength behavior of the charge structure factor for polarizable ion systems is different from that for systems of rigid ions. Molecular dynamics simulation results of rigid and polarizable ion models for molten AgI and NaI are reported.

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 structure of molten AgCl revisited

Abstract We have studied the total structure of molten AgCl at 1073 K by means of two polarizable ion models via molecular dynamics simulations. The model potentials consisted of either the Vashishta–Rahman (VR) or the Born–Mayer rigid-ion potentials to which the anion-induced polarization contribution is added. Both model potentials reproduce well the main features of the structure of molten AgCl with that using the VR model potential giving marginally better results.

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

This work is concerned with single ion dynamics in molten copper halides (CuI and CuCl) which exhibit fast ionic conduction before melting. The self-dynamic structure factor of the two ionic species in each melt have been calculated by molecular dynamics simulations and the corresponding effective wavelength-dependent self-diffusion coefficients have been studied. The results have been compared with those obtained for molten alkali halides (KCl and RbCl).

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.

The bridge functions of molten salts

The bridge functions of molten NaCl and AgI near melting were obtained by using the model potentials of Born–Huggins–Mayer for NaCl and Vashishta–Rahman for AgI. The calculations of the bridge functions involved molecular dynamics simulations, the extension of the procedure originally proposed by Poll et al. [Phys. Rev. A 37, 1672 (1988)] and the numerical solution of the Ornstein–Zernike equations. The calculated bridge functions do not conform with the universality ansatz. They also differ from the bridge functions obtained for model electrolytes. Following the results obtained for electrolytes and those of this work for molten salts, it is conjectured that the universality ansatz for the bridge functions does not apply for systems whose attractive interactions play a decisive role in their structural ordering.