Influence of cation coordination numbers on transport properties of ionic liquid mixtures

We test a conjecture, based on spectacular differences between FeCl₃-based and CrCl₃-based solutions, that transport properties in molten salt solutions are conditioned by the coordination number that the highest z/r cation adopts in the solution. To do this we study a case in which change of anion type dictates a change of cation coordination number from 4 to 6. We use additions of fluoride ion to a molten chloride solution containing Co²⁺ in one case and Ni²⁺ in a second, and compare the ionic conductivities with those of isostoichiometric solutions containing only chloride anions. In the presence of fluoride both solutions show lower conductivities than in the all-chloride equivalent, consistent with formation of some MCl₆^4- sites and the difference is greater for Ni(II) solutions, consistent with the greater preference of the d⁸ ion for octahedral sites.