Phase Diagrams in the Development of the Argyrodite Family Compounds and Solid Solutions Based on Them

The paper analyzes the works of the last decade on the study of ionic conductivity, thermoelectric, photovoltaic, photocatalytic, optical, etc. properties of the argyrodite family compounds (prototype Ag₈GeS₆), as well as phases and composite materials based on them. Considered works allow us to characterize them as valuable environmentally friendly functional materials with great potential for practical application. The main approaches which have been used in these works to enhance functional properties, as well as to improve the application capabilities of this class of substances have been analyzed. The importance of further systematic investigations on their design based on the “composition-structure-property” relationship has been noted. In the present review, special attention is paid to the analysis of works on phase equilibria in the corresponding systems since the information accumulated in phase diagrams is extremely important for optimizing the properties of compounds through targeted variation of composition and structure. Available data on the phase equilibria and thermodynamic properties of ternary and quaternary systems forming Cu/Ag argyrodite compounds and solid solutions based on them are presented and analyzed. It has been shown that anionic and Cu ↔ Ag substitutions in compounds lead to a strong decrease in their polymorphic phase transition temperatures and an expansion of the temperature-composition ranges of the existence of high-temperature ion-conducting phases up to room temperature and below. The possibility of replacing part of the chalcogen atoms in the compounds with halogens, and the Cu(Ag) atoms with elements of the zinc subgroup, which expands the range of argyrodite phases is also shown. The significance of expanding studies of phase equilibria and thermodynamic properties of these systems, especially five-component and more complex systems possessing a big possibility of the formation of high-entropy argyrodite phases, which have thermodynamic stability in a wide temperature-composition range and better applied characteristics has also been specified.