Thermodynamic optimization of Co–Ge binary system

Abstract

Phase diagram of Co–Ge binary system was thermodynamically assessed by using CALPHAD approach in this study. The excess Gibbs energy of the solution phases, liquid, α(Co) and ɛ(Co), were modeled with Redlich–Kister polynomial. Magnetic contribution to the Gibbs energy was also taken into account for α(Co) and ɛ(Co). Considering its crystal structure and solubility range, the intermetallic compound βCo5Ge3, with B82-structure, was particularly described with a three-sublattice model, (Co,Va)1:(Co)4:(Co,Ge)3. And the compound CoGe was described with two-sublattice model according to its crystal structure. Other intermetallic compounds were described as stoichiometric phases because of their narrow homogeneity ranges or unknown crystal structure. In order to obtain a reasonable description of several Co–Ge compounds, first-principle calculations were performed before optimization to determine their formation enthalpies. Finally, a set of thermodynamic parameters was finally obtained so that most data of phase boundaries and thermodynamic properties of various phases were reproduced in present optimization.