Phase relations and thermodynamic properties in the ternary reciprocal system LiFNaFNa 3AlF 6Li 3AlF 6

Abstract

The ternary reciprocal sytem LiFNaFNa 3AlF 6Li 3AlF 6 has been investigated by thermal analysis, differential thermal analysis, quenching, X-ray diffraction, microscopy, and calorimetry. The phase diagrams of the following systems are given: LiFNaF (revised), LiFAlF 3, Na 3AlF 6LiF, and LiFNaFNa 3AlF 6Li 3AlF 6. Some values of heat of mixing and heat content in the system have been measured. It is shown that molten mixtures in this system can be treated as consisting of the following species: Li +, Na +, AlF 3- 6, AlF 3 and F -. At high contents of alkali fluoride the dissociation of the AlF 3- 6 ion to AlF 3 and F - will, however, be negligible. On the basis of the calorimetric data, heats of mixing and dissociation, together with the degree of dissociation of AlF 3- 6, in the systems LiFAlF 3 and LiFNa 3AlF 6 have been calculated. The partial Gibbs free energy, enthalpy and entropy of Na 3AlF 6 in the system LiFNa 3AlF 6 have also been calculated. Finally the activity of Na 3AlF 6 in the latter system has been calculated by treating it as a part of the ternary reciprocal system 3LiF+Na 3AlF 6→Li 3AlF 6+3NaF A satisfactory agreement between the Flood, Førland and Grjotheim theory and the experimental values is obtained at small Na 3AlF 6 concentrations.