THERMODYNAMIC INVESTIGATION OF THE Ag7GeSe5I COMPOUND AND Ag8-xGeSe6-xIx SOLID SOLUTIONS BY THE EMF METHOD WITH SOLID ELECTROLYTE

Abstract

Compounds of the argyrodite family and phases based on them are of considerable interest as environmentally friendly multifunctional materials exhibiting thermoelectric, photovoltaic, and optical properties, as well as high ionic conductivity. In this work, the thermodynamic properties of the Ag7GeSe5I compound and the solid solutions Ag8-xGeSe6-xIx were studied for the first time using the electromotive force (EMF) method with a solid Ag⁺-conducting electrolyte Ag4RbI5. Based on EMF measurements of the concentration cells of the (-)Ag(s) | Ag4RbI5(s) | (Ag–in alloy) (+) type in the 300–390 K temperature range, linear equations describing the temperature dependences of the EMF were obtained. Using these equations, the relative partial thermodynamic functions of silver in the alloys were calculated. Based on the solid-phase equilibria diagram of the Ag₂Se–AgI–GeSe₂ system and several boundary systems, a fragment of the phase diagram of the quaternary Ag–Ge–Se–I system was constructed. Using this diagram, the equations of the potential-forming reactions responsible for the above-mentioned partial molar functions were derived, and the standard thermodynamic functions of formation and the standard entropies of the compound Ag7GeSe5I and the solid solutions Ag8-xGeSe6-xIx (х=0.2, 0.4, 0.6, and 0.8) were calculated.