Thermodynamic study of the Ge–Mn–Si system

[Display omitted] •Mixing enthalpies in the solid solution D88 are calculated using DFT calculations.•Phase separation at 900K in the D88 solid solution is experimentally checked.•Ternary interaction parameters are assessed.•Isothermal sections are drawn using assessed parameters. Among the potential materials for thermoelectric applications, higher manganese silicides (HMS) MnSix (with x around 1.75) exhibit interesting figures-of-merit at intermediate temperatures (573–873K). Moreover it appears that the figure-of-merit can be improved by germanium doping. The optimization of the elaboration of such alloys needs the knowledge of the ternary Ge–Mn–Si system. The solid–solid equilibrium in the ternary system seems to be highly driven by the D88 phase an isostructural form of Mn5Si3 and Mn5Ge3. However, the existence of a complete solid solution between these two compounds is not clearly established in the literature. First principles calculations are performed to clarify the mixing enthalpies in this potential solid solution. The predicted phase separation at low temperature is then experimentally confirmed by metallographic examinations on 3 annealed samples. On the basis of these new data, a thermodynamic description of the ternary system is performed using the Calphad method.