Structure and Thermodynamics of Alkali Fluoride−Aluminum Fluoride−Alumina Melts. Vapor Pressure, Solubility, and Raman Spectroscopic Studies

The MF−AlF 3 −Al 2 O 3 (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 Al 2 O 3 solubility has been measured as a function of the melt composition and temperature. These data show that the ions AlF 4 -, AlF 5 2-, AlF 6 3-, and F - exist together with the oxyanions Al 2 OF 6 2-, Al 2 O 2 F 4 2-, and possibly Al 3 O 4 F 4 3- . For the NaF- and KF-containing systems, the solubility of Al 2 O 3 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−AlF 3 mixtures, 16.8 mol % at 1027 °C for the KF−AlF 3 mixtures). For the LiF−AlF 3 system the Al 2 O 3 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 AlF 6 3- = AlF 5 2- + F -, AlF 5 2- = AlF 4 - + F -, 2Al 2 OF 6 2- + 2F - = Al 2 O 2 F 4 2- + 2AlF 5 2-, 2Al 2 O 2 F 4 2- + F - = Al 3 O 4 F 4 3- + AlF 5 2-, can give a reasonable explanation of the observed Raman, vapor pressures, and solubility data.