Thermodynamic assessment of the niobium-fluorine system by coupling density functional theory and CALPHAD approach

The complete thermodynamic description of the niobium-fluorine system is presented for the first time in this work. It results from a critical evaluation of the available experimental data and new thermodynamic calculations. In total, three niobium fluoride solid phases (NbF₃, NbF₄ and NbF₅) and seven gaseous species (NbF, NbF₂, NbF₃, NbF₄, NbF₅, Nb₂F₁₀ and Nb₃F₁₅) have been considered during the assessment. Novel data for all the gaseous species were calculated combining density functional theory (DFT), for the prediction of the molecular parameters, and statistical mechanical calculations, for the determination of the thermal functions (i.e. standard entropy and heat capacity). The developed thermodynamic model was found to correctly reproduce all the available experimental data and was used to calculate the Nb-F phase diagram, which is presented in this work as well.