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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