Enthalpies of Mixing in Binary Liquid Alkali Fluoride Mixtures

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Abstract
The enthalpies of mixing in the binary liquid systems LiF–NaF, LiF–KF, LiF–RbF, and LiF–CsF have been determined over wide ranges of composition in a high-temperature calorimeter suitable for temperatures up to 1100°C. A few measurements near the 50–50 composition are reported for the systems NaF–KF, NaF–RbF, and KF–RbF. The results are compared with comparable data for the alkali nitrates, chlorides, bromides, and iodides previously studied by Kleppa and Hersh. On the whole the enthalpies of mixing of the fluorides are somewhat less exothermic (or more endothermic) than predicted from the earlier results. It is suggested that this may be related to the lower polarizability of the fluoride anion with the attendant weakening of the nearest-neighbor cation–anion ion–induced-dipole–ion interaction, and to the radius ratio effect, i.e., to the core repulsion between next-nearest-neighbor cations in the presence of the small fluoride anion.