Reinvestigation of the System Na 3 AlF 6 ‐Li 3 AlF 6

A partial phase diagram for the system Na 3 AlF 6 ‐Li 3 AlF 6 was constructed from DTA and X‐ray diffraction measurements. A region of solid solution extends from Na 3 AlF 6 to the limiting composition Na 2 LiAlF 6 . At compositions between Na 1.5 Li 1.5 AlF 6 and NaLi 2 AlF 6 a cubic phase resembling the mineral cryolithionite is stable over a narrow range of temperatures, which shifts progressively to higher temperatures with increasing lithium content. A single phase can be quenched from samples annealed within this range except near the ideal cryolithionite composition (Na 1.5 Li 1.5 AlF 6 ) where the upper temperature limit is too low to permit recombination of two solid phases to cryolithionite even with prolonged annealing. Cryolithionite precipitated from aqueous solution, a sample of the mineral, and the solid solutions containing from 53 to 65 mole % Li 3 AlF 6 have identical powder patterns except for differences of lattice constants. Solid solubility of Na 3 AlF 6 in Li 3 AlF 6 reaches 30 mole % at the eutectic temperature.