Thermodynamic properties of the Yb-Sb system predicted from first-principles calculations

In the present work, thermodynamic properties of the Yb-Sb system have been studied by first-principles based quasiharmonic phonon approach, reporting the temperature-dependent isobaric heat capacity, entropy, formation enthalpy, isothermal bulk modulus, and thermal expansion. The effects by using different exchange-correlation functionals and the pseudopotentials on thermodynamic properties are examined. The results reveal that the combined Perdew-Burker-Ernzerhof functional revised for solids (PBEsol) together with the pseudopotential with all f-electrons frozen in the core is the best choice. This combination leads to an overall good description of the relative phase stabilities and thermodynamic properties, except for the rocksalt YbSb compound which is calculated to have an energy of ~25 kJ/mol-atom above the convex hull. The discrepancy is due probably to the change of the Yb valence states between the rocksalt YbSb structure and the other Yb-Sb compounds. Furthermore, the present calculations show that the Yb16Sb11 compound is stable. The analysis of stretching force constants demonstrates that the extremely strong Yb-Sb bonding is responsible for the stabilities of Yb11Sb10, Yb16Sb11, h-Yb5Sb3, and Yb4Sb3 at high temperatures.