The MF−AlF3−Al2O3 (M = Li, Na, K) systems have been studied in the liquid state. Raman spectra were recorded to obtain information on the structure and the ionic composition of these melts, vapor pressure measurements were performed to obtain thermodynamic information, and the Al2O3 solubility has been measured as a function of the melt composition and temperature. These data show that the ions AlF4 -, AlF5 2-, AlF6 3-, and F- exist together with the oxyanions Al2OF6 2-, Al2O2F4 2-, and possibly Al3O4F4 3-. For the NaF- and KF-containing systems, the solubility of Al2O3 increases very much with the mole ratio n°MF/n°AlF 3 and is maximum at n°MF/n°AlF 3 = 4 (8.1 mol % at 1040 °C for the NaF−AlF3 mixtures, 16.8 mol % at 1027 °C for the KF−AlF3 mixtures). For the LiF−AlF3 system the Al2O3 solubility is much smaller with a maximum of ≈2.2 mol % probably around n°MF/n°AlF 3 = 1.5 at 1000 °C. The four equilibria AlF6 3- = AlF5 2- + F-, AlF5 2- = AlF4 - + F-, 2Al2OF6 2- + 2F- = Al2O2F4 2- + 2AlF5 2-, 2Al2O2F4 2- + F- = Al3O4F4 3- + AlF5 2-, can give a reasonable explanation of the observed Raman, vapor pressures, and solubility data.

The NaF-AlF(3) system with additions of CaF(2) and MgF(2) has been studied with Raman and vapor pressure measurements for 3 >/= CR (NaF/AlF(3) molar ratio) >/= 1 and up to 50 mol % additive. The results show that the binary melt can be described using the two equilibria AlF(6)(3)(-) = AlF(6)(2)(-) + F(-) and AlF(5)(2)(-) = AlF(4)(-) + 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 AlF(6)(3)(-) concentrations even at the Na(3)AlF(6) 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, CaF(2) or MgF(2), with the AlF(5)(2)(-) ions in the melt was considered. This association could be experimentally observed through a band broadening and a slight shift in the AlF(5)(2)(-) band frequency. Our vapor pressures and Raman data both indicate that MgF(2) clearly acts as an acid when added to NaF-AlF(3) melts of any composition. When CaF(2) is added, a slight decrease of vapor pressure occurs. Raman data indicate a decrease of AlF(4)(-) concentration, corresponding to a dissociation of CaF(2) 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.