Cu8GeSe6–Ag8GeS6 System: Phase Equilibria and High-Entropy Alloys

Argyrodite family compounds and their based phases are of interest as environmentally friendly functional materials. This work presents new phase equilibria data in the Cu8GeSe6–Ag8GeS6 system obtained by differential thermal analysis (DTA) and X-ray powder diffraction (XRD). The system was found to be not quasi-binary due to the incongruent melting of Cu8GeSe6, yet it maintains thermodynamic stability at below the solidus temperature. A continuous solid solution series (δ phase) between the high-temperature phases with both cationic and anionic substitutions was identified. The low-temperature phases were shown to have limited homogeneity regions. Solid solution formation lowers the polymorphic transitions temperatures and extends the homogeneity range of the ion-conducting cubic δ phase down to room temperature and below. At room temperature, the homogeneity range of the δ phase spans 10–70 mol % Ag8GeS6. The δ phase in the range 36–64 mol % Ag8GeS6 can be classified as a high-entropy alloy (HEA), which offers additional facilities for tailoring its properties through entropy-driven structural stabilization.