Thermodynamic Examination of Quaternary Compounds in the Ag–Fe–(Ge, Sn)–Se Systems by the Solid-State EMF Method

Funding Information: This work was partially supported by the Ministry of Education and Science of Ukraine (Grant No. 0119U002208). This work was also financially supported by the Academy of Finland project ?Thermodynamic investigation of complex inorganic material systems for improved renewable energy and metals production processes? (Decision number 311537), as part of the activities of the Johan Gadolin Process Chemistry Center at ?bo Akademi University. Publisher Copyright: © 2021, The Minerals, Metals & Materials Society. Copyright: Copyright 2021 Elsevier B.V., All rights reserved.The equilibrium phase space of the Ag–Fe–X–Se (X: Ge, Sn) systems in the parts Ag8XSe6–XSe–FeSe2–AgFeSe2–Ag8XSe6 consists of four quaternary-phase regions formed with the participation of low-temperature modifications of the Ag2FeGeSe4 and Ag2FeSnSe4 compounds. The kinetic barriers of the formation of equilibrium four-phase regions that are observed under conditions of vacuum ampoule synthesis below 600 K were overcome by synthesis of phases at the positive electrodes of electrochemical cells (ECCs): (−)C | Ag | SE | R (Ag+) | PE | C(+), where C is graphite, Ag is left (negative) electrode, SE is the solid-state electrolyte, PE is the right (positive) electrode, and R (Ag+) is the region of Ag+ diffusion into PE. Silver cations Ag+ that shifted from the left to the right electrode of ECCs acted as the seed centers of an equilibrium set of phases. Based on the temperature dependences of the EMF of the cells in the temperature range 430–485 K, the standard thermodynamic functions of the Ag2FeGeSe4 and Ag2FeSnSe4 compounds were calculated for the first time. The observed experimental results and thermodynamic calculations are in good agreement.Peer reviewe