Structure and Thermodynamics of NaF−AlF3 Melts with Addition of CaF2 and MgF2

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Abstract
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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