Thermodynamic assessment of the La–Sb and the Ho–Sb systems using the associate model

The La–Sb and the Ho–Sb systems were thermodynamically assessed by CALPHAD (CALculation of PHAse Diagram) approach based on the available experimental data including thermodynamic properties and phase equilibria. The formation enthalpies (at 0 K) of the compounds of the La–Sb and the Ho–Sb systems were calculated by the first-principles method and used in the present thermodynamic optimization. The liquid phases of both systems were modeled as the associate model, with the constituent species La, Sb and La3Sb2 for the La–Sb liquid and Ho, Sb and HoSb for the Ho–Sb liquid. The excess Gibbs energy was formulated with the Redlich–Kister polynomial. The intermetallics, La2Sb, La3Sb2, LaSb, LaSb2, Ho5Sb3, α-Ho4Sb3, β-Ho4Sb3, α-HoSb, β-HoSb and HoSb2, were treated as stoichiometric compounds. The calculated results showed the good agreement with the experimental data. The inconsistency of the calculated formation enthalpy of the compound HoSb2 with the reported experimental value was analyzed.