The structure of binary molten salt mixtures: A neutron diffraction study.

Structural modification in a series of polyvalent metal chloride - alkali chloride binary molten salt mixtures has been investigated using the pulsed neutron diffraction technique. Structure factors have been measured for NiCl2- KCl, NiCl2-LiCl and ZnCl2-LiCl samples spanning the entire composition range. The key finding was that the degree of structural modification is dependent on the relative size and polarising power of the two cation species. The mixtures of NiCl2, and ZnCl2, with LiCl largely appear to be admixtures of the two pure salt structures, whereas adding KCl to NiCl2 results in a better ordered, more regularly tetrahedral local structure around the metal cation and enhancement of the first sharp diffraction peak (FSDP). A simple model involving charge ordering of discrete tetrahedral units by alkali counter-ions is proposed as an explanation for the enhanced intermediate range order. In order to identify some of the partial structure factor contributions to the enhanced FSDP, the scattering was measured for three isotopically-enriched NiCl2+2KCl samples. A complementary isotopic substitution experiment was performed on three ZnCl2+2KCl samples. The results generally confirm the findings of the composition study, with a strong similarity between the two molten salt systems also being evident. In addition, RMC modelling supports the proposed model for intermediate range order in the mixtures. Structure factors were also measured for AlCl3-LiCl and AlCl3-NaCl samples covering the entire composition range. Several features consistent with strong charge ordering of discrete tetrahedral units by alkali counter-ions were identified. In addition, RMC modelling of the data for pure AICl3 strongly challenges the 'established' view of the structure and an alternative 'sparse network liquid' model is proposed which emphasises the similarity to ZnCl2.