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.