The NaF−AlF₃ system with additions of CaF₂ and MgF₂ has been studied with Raman and vapor pressure measurements for 3 ≥ CR (NaF/AlF₃ molar ratio) ≥ 1 and up to 50 mol % additive. The results show that the binary melt can be described using the two equilibria AlF₆³^- = AlF₆²^- + F^- and AlF₅²^- = AlF₄^- + F^- with equilibrium constants 0.25 and 0.05, respectively, at 1293 K. Both reactions have positive reaction enthalpies. The first equilibrium is strongly shifted to the right resulting in a melt mixture with very low AlF₆³^- concentrations even at the Na₃AlF₆ composition. Evidence for nonideal mixing of anions was found. For the ternaries, models based on Raman data are presented and compared with vapor pressure measurements. Good agreement is observed when association between the additives, CaF₂ or MgF₂, with the AlF₅²^- ions in the melt was considered. This association could be experimentally observed through a band broadening and a slight shift in the AlF₅²^- band frequency. Our vapor pressures and Raman data both indicate that MgF₂ clearly acts as an acid when added to NaF−AlF₃ melts of any composition. When CaF₂ is added, a slight decrease of vapor pressure occurs. Raman data indicate a decrease of AlF₄^- concentration, corresponding to a dissociation of CaF₂ with liberation of F^- ions. All these results are, however, very much dependent on the initial melt composition. These data are explained in terms of acid−base, dilution, and association reactions of the solute with the solvent.

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