Phase behavior in Cu₈SiS₆-Ag₈SiS₆ and Cu₈SiSe₆-Ag₈SiSe₆ systems and new cubic argyrodyte phases with variable composition

Argyrodite-family compounds and their derivative phases are environmentally friendly functional materials with a wide range of valuable properties, including thermoelectric, photoelectric, and optical properties, as well as high ionic conductivity arising from the high mobility of Cu⁺ and Ag⁺ ions. In this study, phase equilibria in the Cu₈SiS₆–Ag₈SiS₆ and Cu₈SiSe₆–Ag₈SiSe₆ argyrodite systems were investigated using differential thermal analysis, X-ray diffraction, and microhardness measurements, and the corresponding temperature–composition (T–x) phase diagrams were constructed. It was established that both systems exhibit quasi-binary behavior and are characterized by the formation of continuous solid solutions (δ-phase) between the high-temperature modifications of the parent compounds, as well as extensive solid-solution regions based on their low-temperature modifications. The formation of these solid solutions is accompanied by a systematic decrease in the temperatures of polymorphic transitions of the parent phases, thereby stabilizing the δ-phase at room temperature. Furthermore, it was determined that, at room temperature, the homogeneity range of the δ-phase spans 10–50 mol% Ag₈SiS₆ for the sulfide system and 10–85 mol% Ag₈SiSe₆ for the selenide system. The lattice parameters of the synthesized solid solutions were determined from X-ray diffraction data, and showed a linear dependence on composition consistent with Vegard’s law. The formation of continuous cubic solid solutions is accompanied by an increase in microhardness due to solid-solution strengthening.