After an outline of work on rare-gas systems, which serves as a target for parallel work on alkali halides, and an initial brief survey of those parts of this parallel work for which results have been obtained, interionic potential models for alkali halides are considered in some detail. The rigid ion potentials of Fumi and Tosi are discussed and then a major part of the section is devoted to deriving a new set of polarizable ion potentials, which incorporate the ideas behind the lattice dynamical shell model. Extensions which include many-body terms in the potentials are considered briefly and finally the information which can be obtained from alkali halide diatomic molecules is discussed. In the third section methods of computer simulation for ionic liquids are outlined, concentrating on the molecular dynamics method, and some of the properties which can be obtained by analysing the ion trajectories are listed. Results from simulations, including some new work on LiF, NaCl and RbI, are reviewed.

Interionic potentials in alkali halides and their use in simulations of the molten salts

The structure of molten sodium chloride has been investigated by applying the technique of neutron diffraction to isotopically enriched samples. The three partial structure factors relating to Na-Na, Cl-Cl, and Na-Cl correlations have been successfully extracted from the data. Some general conclusions about the structure of molten sodium chloride are made and the relationship between the present work and earlier theoretical studies is discussed.

https://iopscience.iop.org/article/10.1088/0022-3719/8/21/018/pdf

The structure of molten sodium chloride

The method of molecular dynamics is applied to a solvent‐averaged model of electrolyte solutions, described by a generalized Langevin equation. For Brownian motion without solute–solute interactions, we recover the characteristic features of an infinitely dilute solution. For interacting brownons, the results exhibit a noticeable dependence of the calculated self‐diffusion coefficients on the influence of the Coulomb forces. The model system exhibits ion association when the Coulomb forces are made strong enough.

Brownian dynamics: Its application to ionic solutions

Modern techniques of liquid state physics have been successfully used to probe the microscopic structure and dynamics of a variety of multicomponent liquids in which relative ordering of the species is present near freezing. The alkali halides are prototypes for this specific type of short-range order in relation to the nature of bonding, but the systems in question also include other monovalent and polyvalent metal-ion halides, alkali-based intermetallic compounds and chalcogen-based alloys. A viewpoint is taken in this review which gives attention to the relations between liquid and solid phase properties across melting for compound systems at stoichiometric composition. In addition, large deviations for stoichiometry can be realised in the liquid phase, to display trends of evolution of structure, bonding and electronic states with composition.

https://iopscience.iop.org/article/10.1088/0034-4885/49/9/002/pdf

Structure and dynamics of molten salts

The simulation technique of molecular dynamics has been used to investigate a central‐force model for liquid water at 22 °C and 1 g/cm3. In this model, the same atomic pair potentials are used both for intramolecular and intermolecular interactions. Atomic pair correlation functions and velocity autocorrelation functions have been computed. The results demonstrate that stable, nonlinear, vibrating molecules are present, and that they form a random hydrogen‐bond network of the type usually attributed to water. These exploratory calculations suggest that small modifications in the central potentials VHH, VOH, and VOO would produce a more faithful representation of real water.

Study of a central force model for liquid water by molecular dynamics

A significant and characteristic difficulty arising in the use of statistical mechanics to describe the properties of fused salts is due to the extreme range of the electrostatic field; an accurate representation of the Coulomb energy and Coulomb forces may be obtained using Ewald's expansion. Such calculations are tested here on the model case of liquid NaCl at different temperatures. In addition, this method is applied to the description of ionic media by molecular dynamics computations. (A Born-Huggins-Mayer pair potential is used with constants derived from the solid at 298 K.) This treatment introduces cut-off distances and periodic boundary conditions. The influence of these constraints on the conservation of energy and momenta is investigated. Thermodynamic properties of molten NaCl at 1164·5, 1224·5 and 1340·5 K are calculated. The motions of individual ions, anions and cations are described in terms of a velocity self-correlation function and their diffusion coefficients are determined.

Bernard Quentrec

Papers

Application of the molecular dynamics method to a liquid system with long range forces (Molten NaCl)