Phase Equilibria in the System CaF2‐AlF3‐Na3AlF6 and Part of the System CaF2‐AlF3‐Na3AlF6‐Al2O3

The complete phase‐equilibrium diagram of the system CaF2‐AlF3‐Na3AlF6 and the subsolidus portion of the system CaF2‐AlF3‐Na3AlF6‐Al2O3 were established from microscopic, powder X‐ray diffraction, quench, and DTA data obtained from samples encapsulated in sealed tubes and either reacted in the solid state or melted and recrystallized. The system Na3AlF6‐CaF2 contains a simple eutectic with no compound formation or solid solution. The system CaF2‐AlF3‐Na3AlF6 contains two ternary compounds, NaCaAlF6 and NaCaAl2F9, which melt incongruently at 735° and 712°C, respectively; NaCaAlF6 exists in three polymorphic forms with transitions at 610° and 722°C and NaCaAl2F9 is body‐centered cubic with a0= 10.765 Å. The three binary and two ternary compounds divide the system into eight compatibility triangles. Along the NaCaAlF6‐Na3AlF6 join, 7 mol% NaCaAlF6 is soluble in α‐cryolite at 525° and 42% in β‐cryolite at 731°C. The quaternary system Na3AlF6‐AlF3‐CaF2‐Al2O3 contains eight compatibility tetrahedra.