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.