Structural aspects of the formation of solid solutions in the NaF-KF-AlF3 system

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) angstrom, for (c) 10.365 (10) -10.400 (10) angstrom. According to the investigation of the crystal structure, potassium cations substitute sodium only in the 2-fold position in the amount of similar to 40%. The solid solutions are stable in the range from room to melting point temperature. A wide range of solid solutions based on beta-cryolite (Na3AlF6) and elpasolite (K2NaAlF6) above 540 degrees C has been studied in detail. It is only the 8-fold cationic position in the beta-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 alpha-Na3AlF6 and K2NaAlF6 upon calcination below 540 degrees C., followed by further cooling without changing the alpha-Na3AlF6 composition. Elpasolite initially containing an excess of sodium ions, has yielded cryolite and stoichiometric K2NaAlF6 below 340 degrees 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.