Thermodynamic descriptions of the Ag-X (X = S, As, Lu) systems

Based on the critical evaluation of experimental data available in the literature, for the first time the Ag-X (X = S, As, Lu) binary systems were assessed thermodynamically using the CALPHAD (CALculation of PHAse Diagram) method. The solution phases including liquid, (Ag), (alpha S), (beta S), (alpha As), (epsilon As) and (Lu) were described by the substitutional solution model, and their excess Gibbs energies were described with the Redlich-Kister polynomial equation. In view of the possible existence of associate species Ag2S and the difficulty to describe the liquid phase by the substitutional solution model, an associate model was used to describe the liquid phase in the Ag-S system. The intermetallic compounds, i.e. alpha Ag2S, AgLu, Ag2Lu and Ag4Lu, were all modeled as stoichiometric compounds. And the intermetallic compounds beta Ag2S and gamma Ag2S were described by two-sublattice model (Ag, S)(2)S-7 in view of the solubilities. Self-consistent thermodynamic parameters to describe the Gibbs energies of phases in the Ag-S, Ag-As and Ag-Lu systems were obtained. The calculated results were in good agreement with the reported phase equilibria and thermodynamic properties.