Thermodynamic re-modeling of the Yb-Sb system aided by first-principles calculations

The thermodynamic description of the Yb-Sb binary system is developed by means of the CALculations of PHAse Diagrams (CALPHAD) method by combining experimental data in the literature and predictions from first-principles calculations based on density functional theory (DFT) in the literature and the present work. Two pseudopotentials of Yb are compared in the present DFT-based calculations with 14 and 13 f-electrons frozen in the core, i.e., 5p⁶6s² and 5p⁶6s²5 d¹ electrons as valence electrons, termed Yb_2 and Yb_3, respectively. It is shown that the phonon spectrum of the YbSb phase calculated using the Yb_3 pseudopotential does not have imaginary phonon modes and is subsequently used to predict its temperature dependent thermodynamic properties by the DFT-based quasiharmonic phonon calculations. The present thermodynamic database includes the Yb₁₆Sb₁₁ phase in addition to five intermetallic phases that were considered in previous modeling studies, i.e., YbSb₂, YbSb, Yb₁₁Sb₁₀, Yb₄Sb₃, and Yb₅Sb₃. The high temperature orthorhombic structure of the Yb₅Sb₃ phase is not considered in the present work as it was stabilized by hydrogen. The associate solution model is used to describe the short-range ordering behavior in the liquid phase. The calculations from the present thermodynamic model show good agreement with thermochemical and phase equilibrium data from both the present work and the literature.