Joaquim Trullàs

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 Polarization Effects on AgI

Three different models of AgI are studied by molecular dynamics simulations. The first one is the rigid ion model (RIM) with the effective pair potential of the Vashishta and Rahman form and the parametrization proposed by Shimojo and Kobayashi. The other two are polarizable ion models in which the induced polarization effects have been added to the RIM effective pair potential. In one of them (PIM1), only the anions are assumed to be polarizable by the local electric field. In the other one (PIM2s), the silver polarization is also included, and a short-range overlap-induced polarization opposes the electrically induced dipole moments. This short-range polarization is proved to be necessary to avoid overpolarization when both species are assumed to be polarizable. The three models reproduce the superionic character of alpha-AgI at 573 K and the liquid behavior of molten AgI at 923 K. The averaged spatial distribution of the cations in the alpha-phase obtained for PIM1 appears to be in better agreement with experimental data analysis. The PIM1 also reproduces the structure factor prepeak at about 1 A(-1) observed from neutron diffraction data of molten AgI. The three models retain in the liquid phase the superionic character of alpha-AgI, as the mobility of the cations is significantly larger than that for the anions. The ionic conductivity for the polarizable ion models is in better agreement with experimental data for alpha-AgI and molten AgI.

Langevin dynamics simulation of ions in solution: influence of the solvent structure

The Langevin dynamics simulation method has been used to study the structural and dynamical properties of ions in electrolyte solutions. Time evolutions of Na+ and Cl− ions in aqueous solutions have been simulated using different solvent averaged ion–ion interaction models. The changes observed in the properties of ions when nonoscillatory solvent averaged potentials are exchanged for more realistic ones including the solvent structure effects are discussed. We have also investigated the formation of ionic associations, paying special attention to the lifetime of the detected dimers.

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.

A polarizable ion model for the structure of molten CuI

The results are reported of the molecular dynamics simulations of the coherent static structure factor of molten CuI at 938 K using a polarizable ion model. This model is based on a rigid ion potential to which the many body interactions due to the anions induced polarization are added. The calculated structure factor reproduces the clear sharp prepeak observed in neutron diffraction data. The corresponding partial structure factors and the related radial distribution functions calculated by molecular dynamics are compared with those found in the literature derived from a combination of neutron and x-ray diffraction data with the aid of the reverse Monte Carlo simulation technique, as well as those calculated by ab initio MD simulations.The results are reported of the molecular dynamics simulations of the coherent static structure factor of molten CuI at 938 K using a polarizable ion model. This model is based on a rigid ion potential to which the many body interactions due to the anions induced polarization are added. The calculated structure factor reproduces the clear sharp prepeak observed in neutron diffraction data. The corresponding partial structure factors and the related radial distribution functions calculated by molecular dynamics are compared with those found in the literature derived from a combination of neutron and x-ray diffraction data with the aid of the reverse Monte Carlo simulation technique, as well as those calculated by ab initio MD simulations.

Neutron diffraction data and molecular dynamics simulations of the molten mixture Ag(Br0.7I0.3).

The structure factors of the ionic liquid mixture Ag(Br(0.7)I(0.3)) at three temperatures, 723, 923, and 1023 K, as well as of the pure molten AgI at 923 K and the pure molten AgBr at 773 and 923 K, were studied experimentally and by means of molecular dynamics simulations. The experiments were carried out using the high intensity total scattering time-of-flight spectrometer, HIT-II, at the KENS spallation neutron source in Japan. The experimental data are very reliable, with the possible exception of the small momentum transfer region, whose accessibility is limited by neutron energy and detector positions. The simulations made use of the semiempirical rigid ion potentials of the Vashishta-Rahman [Phys. Rev. Lett. 40, 1337 (1978)] type using a new set of parameters appropriate for the mixture. Within the known constraints of the pairwise rigid ion potentials, the simulated structure factors are in fair agreement with experiment. The results for the pair distribution functions suggest that the molten mixture retains the superionic character found in previous calculations of both the AgI and AgBr melts. This suggestion is confirmed by the results for the self-diffusion coefficients. Values obtained for the ionic conductivities are also presented.

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.

Langevin dynamics study of NaCl electrolyte solutions at different concentrations

Langevin dynamics simulations of NaCl electrolyte solutions at 25° C and several concentrations (0.1–2 M) were performed. The solvent‐averaged potentials of Pettitt and Rossky were assumed. These potential models show remarkable oscillatory shapes that are associated with the discrete structure of the solvent. The changes in the ionic structure and self‐diffusion coefficients with the increase of the concentration are discussed. Special attention is paid to both the mean number of ionic pairs and the time that these ionic associations survive. The results are compared with the ones obtained from other potential models and with the available experimental data.

MOLECULAR DYNAMICS STUDY OF THE VELOCITY CROSS-CORRELATIONS IN LIQUIDS

Velocity cross-correlations for both soft-sphere fluids at different densities and temperatures and a simple molten salt are investigated by molecular dynamics simulation. Time correlation functions between the velocity of a tagged particle and velocities of particles within specified ranges of initial separations are calculated. In the case of the soft-sphere fluids, separation ranges corresponding to the first, second and third shells of neighbors are considered whereas up until six different shells are analyzed for the molten salt. The calculated functions allow us to build a picture of the spread of the initial momentum of a tagged particle over the successive shells of neighbors. It is observed that collisions with intermediate particles are the main mechanism for the transfer of momentum. A balance between the momentum exchanged by particles in a given shell with those in the two adjacent shells should be carried out in order to analyze the resulting velocity cross-correlation functions. The rate of...