Experimental Thermodynamic Characterization of the Chalcopyrite-Based Compounds in the Ag–In–Te System for a Potential Thermoelectric Application

The equilibrium concentration space of the Ag–In–Te system in the part AgInTe2–Te–In2Te3 was studied through the modified solid-state electromotive force (EMF) method by dividing into separate phase regions In2Te3–In2Te5–Ag3In97Te147 (I), In2Te5–Te–Ag3In97Te147 (II), Ag3In97Te147–Te–AgIn5Te8 (III), AgIn5Te8–Te–AgIn3Te5 (IV), and AgIn3Te5–Te–AgInTe2 (V), at T≤500 K. The formation of a thermodynamically stable combination of the binary and ternary phases in the (І)–(V) phase regions from a metastable phase mixture of substances was carried out at T≤500 K in the R(Ag+) part of the positive electrode (PE) of the galvanic cells (GCs) of the structure: (−) С |∙| Ag |∙| SЕ |∙| R(Ag+) |∙| PЕ |∙| С (+), where C is the graphite (inert electrode), SE is the solid-state electrolyte (Ag3GeS3Br glass), and Ag is the left (negative) electrode,. The Ag+ ions in the R(Ag+) region functioned as small nucleation centers for the formation of the stable phases. The spatial position of the (I)–(V) phase regions in the concentration space of the Ag–In–Te system relative to the position of silver was used to express the overall potential-forming reactions with the participation of the substances Ag, Te, In2Te5, Ag3In97Te147, AgIn5Te8, AgIn3Te5, and AgInTe2. The subsequent EMF measurements were carried out by applying the same GCs. The temperature dependences of the EMF of GCs with PE of the (I)–(V) phase regions were here used to determine, for the first time, the values of standard thermodynamic functions of the binary and ternary compounds. The determined values of the Gibbs energies of formation of compounds are equal: G_(In_2 Te_5)^°=(182.7±1.9) kJ·mol^(-1), G_(AgInTe_2)^○=(115.0±3.1) kJ·mol^(-1), G_(AgIn_3 Te_5)^○=(301.5±6.5) kJ·mol^(-1), G_(AgIn_5 Te_8)^○=(487.6±11.3) kJ·mol^(-1), and G_(Ag_3 In_97 Te_147)^○=(8954±189) kJmol^(-1). The correctness of the division of the equilibrium phase space of the Ag–In–Te system in the part AgInTe2–Te–In2Te3 involving the AgInTe2, AgIn3Te5, AgIn5Te8, and Ag3In97Te147 compounds was confirmed by the agreement of the calculated and literature thermodynamic data for In2Te5 compound. Compositions of pairs of the ternary compounds for their subsequent practical application were proposed.