SOLID SOLUTION REGIONS AND PHASE EQUILIBRIA IN THE Ag8GeS6‒Ag7SiS5I SYSTEM

In this work, a physicochemical study of phase equilibria in the Ag8GeS6–Ag7SiS5I system was carried out for the first time, which is of key importance for the development of new functional materials and solid electrolytes with enhanced ionic conductivity for all-solid-state power sources and energy storage systems. The relevance of the study is driven by the necessity to search for superionic conductors based on Ag+ silver cations that crystallize in the argyrodite structural type and combine high ionic mobility, chemical stability, and operational safety. Synthesized polycrystalline samples of the Ag8GeS6–Ag7SiS5I system were investigated by differential thermal analysis (DTA) and X-ray powder diffraction (XRD) with indexing of diffraction patterns using the EXPO 2014 software. Based on experimental data, the complete phase diagram of the Ag8GeS6–Ag7SiS5I system was constructed for the first time. It was established that a continuous series of solid solutions (Roozeboom Type I) is formed in the high-temperature region above the phase transition temperatures of the initial components. The smooth course of the liquidus and solidus curves confirms the complete mutual solubility of the components in the liquid and high-temperature solid states. Upon cooling, a solubility gap is observed, accompanied by the formation of a two-phase region coexisting terminal Ag8-xGe1-xSixS6-xIx solid solutions with the orthorhombic structure of LT-Ag8GeS6 (SG Pna21) and Ag8-xGe1-xSixS6-xIx solid solutions with a cubic structure based on Ag7SiS5I (space group F-43m). The homogeneity ranges of the terminal solid solutions exceed 25 and 60 mol.%, respectively. The compositional dependence of the unit cell volumes of the alloys is in good agreement with the identified boundaries of the phase regions. The formation mechanism of substitution solid solutions with the composition Ag8-xGe1-xSixS6-xIx was described via defect reactions in Kröger–Vink notation. It was demonstrated that simultaneous isovalent (Ge+4↔Si+4) and heterovalent (S-2↔I-) two-ion substitution is accompanied by the formation of additional silver cation vacancies. The emergence of new cation vacancies creates favorable conditions for reducing the activation energy of Ag+ ion migration and serves as a prerequisite for increasing ionic conductivity. The asymmetry in the extent of the boundary solid solutions was explained within the polarity of isomorphism, taking into account the size factor and unit cell volume. It was shown that the cubic structure of Ag7SiS5I, featuring a larger cell volume and longer interatomic distances, significantly more effectively relaxes local structural stresses upon the incorporation of the Ge+4 ion, which accounts for its wider homogeneity range. The obtained results establish a fundamental framework for the targeted design of new solid electrolytes with controlled defect structure and high ionic conductivity.