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.

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The temperature dependence of density, viscosity, and surface tension was determined in the system 55 mol % NaF + 45 mol % AlF3. The interfacial tension between aluminum and the above melt plus the melt 55 mol % KF + 45 mol % AlF3 was measured at 750 °C. These molten salts are potential low-temperature electrolytes for aluminum electrolysis. It was found that the temperature dependence of density and surface tension could be described by linear equations within the measured temperature ranges. The temperature dependence of viscosity can be described by a quadratic equation.

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