Molecular dynamics computer simulations of molten zinc chloride

Thermal and Transport Properties of Molten Chloride Salts with Polarization Effect on Microstructure

Molten chloride salt is recognized as a promising heat transfer and storage medium in concentrating solar power in recent years, but there is a serious lack for thermal property data of molten chloride salts. In this work, local structures and thermal properties for molten chloride salt—including NaCl, MgCl2, and ZnCl2—were precisely simulated by Born–Mayer–Huggins (BMH) potential in a rigid ion model (RIM) and a polarizable ion model (PIM). Compared with experimental data, distances between cations, densities, and heat capacities of molten chloride slats calculated from PIM agree remarkably better than those from RIM. The polarization effect brings an extra contribution to screen large repulsive Coulombic interaction of cation–cation, and then it makes shorter distance between cations, larger density and lower heat capacity. For NaCl, MgCl2, and ZnCl2, PIM simulation deviations of distances between cations are respectively 3.8%, 3.7%, and 0.3%. The deviations of density and heat capacity for NaCl between PIM simulation and experiments are only 0.6% and 2.2%, and those for MgCl2 and ZnCl2 are 0.7–10.7%. As the temperature rises, the distance between cations increases and the structure turns into loose state, so the density and thermal conductivity decrease, while the ionic self-diffusion coefficient increases, which also agree well with the experimental results.

The structure of molten salts

The three-dimensional structures of thirteen MX and MX 2 molten salts (M, metal; X, halide) have been modelled from neutron diffraction data by using the Reverse Monte Carlo method. Although the structures are highly disordered the dominant structural symmetries of the short-range order can be determined by using bond angle correlations and spherical harmonic invariants. It is found that cations in the alkali chlorides tend to be octahedrally coordinated by anions (and vice versa), although the large number of ‘vacancies’ leads to coordination numbers lower than six. CuCl has a tetrahedral coordination of anions about cations. In MX 2 melts small cations are octahedrally coordinated except in ZnCl 2 where the coordination is tetrahedral. As the cation size increases the coordination tends to cubic. In all cases the local structural symmetries in the melt are similar to those in the corresponding crystals. It is shown that the ‘pre-peak’ which occurs in the partial structure factor A MM ( Q ) for MX 2 melts with small cations, and which has been associated with intermediate range ordering, arises from a local density fluctuation due to cation ‘clustering’. This is the first time that a visual picture of such intermediate-range order has been obtained.