Abstract
The formation of solid solutions in the ternary system NaF-KF-AlF3 has been studied by X-ray diffraction and thermal analysis. Chiolite has been shown to form solid solutions with the composition (Na(5−x)Kx)Al3F14, in the limited range of 0<x<0.4. The lattice parameters change in the intervals for (a) 7.010 (3) −7.050 (3) Å, for (c) 10.365 (10) −10.400 (10)Å. According to the investigation of the crystal structure, potassium cations substitute sodium only in the 2-fold position in the amount of ~40%. The solid solutions are stable in the range from room to melting point temperature. A wide range of solid solutions based on β-cryolite (Na3AlF6) and elpasolite (K2NaAlF6) above 540°C has been studied in detail. It is only the 8-fold cationic position in the β-cryolite structure which appears to have contributed into the substitution in the full range of solid solutions. The solid solution decays into a mixture of α-Na3AlF6 and K2NaAlF6 upon calcination below 540°C., followed by further cooling without changing the α-Na3AlF6 composition. Elpasolitе initially containing an excess of sodium ions, has yielded cryolite and stoichiometric K2NaAlF6 below 340°C. The phase K2NaAl3F12 present in two polymorphic forms, has not formed a wide range of solid solutions; however, a slight excess of potassium ions has improved the stability of the high-temperature form.
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