A combined first-principles calculation and thermodynamic modeling of the F-K-Na system

The phase equilibria and thermodynamic properties of the F-K-Na system were analyzed by combining a first-principles approach and calculation of phase diagram (CALPHAD) technique in the present work. The enthalpies of formation for intermediate compounds (KF, NaF and KNa₂) in the F-K-Na system and the enthalpies of mixing in bcc and halite phases were obtained via first-principles density functional calculations. Using these values together with thermochemical and phase stability data in the literature, computational thermodynamic modeling of the F-K, F-Na, K-Na binary systems and the KF-NaF pseudo-binary system was performed. The isothermal and isopleth sections of the F-K-Na ternary system as well as the liquidus projection were predicted. All calculated results are well within the uncertainties of the experimental data and the results from first-principles calculations. This ternary system forms a basis for better understanding and controlling the extraction process of tantalum.