Experimental investigation and thermodynamic assessment of phase equilibria in the GaTe–AgGa5Te8–Te system below 600 K

Abstract

Equilibrium T–x space of the Ag–Ga–Te system in the GaTe–AgGa₅Te₈–Te part was divided below 600 K into three-phase regions Ga₂Te₅–AgGa₅Te₈–Te, Ga₂Te₃–AgGa₅Te₈–Ga₂Te₅, Ga₇Te₁₀–AgGa₅Te₈–Ga₂Te₃, Ga₃Te₄–AgGa₅Te₈–Ga₇Te₁₀, and GaTe–AgGa₅Te₈–Ga₃Te₄ by the electromotive force (EMF) method. To accomplish accurate experimental data, the following electrochemical cells (ECs) were assembled: (−)IE|NE|SSЕ|R{Ag⁺}|PЕ|IE(+), where IE is the inert electrode (graphite powder), NE is the negative electrode (silver powder), SSE is the solid-state electrolyte (glassy Ag₃GeS₃Br), PE is the positive electrode, R{Ag⁺} is the region of PE that contact with SSE. At the stage of cell preparation, PE is a nonequilibrium phase mixture of the well-mixed powdered compounds Ag₂Te, GaTe, Ga₂Te₃, and tellurium, taken in ratios corresponding to two or three different points in each of the mentioned regions. The equilibrium set of phases was formed in the R{Ag⁺} region at 600 K for 48 h with the participation of the Ag⁺ ions. Silver cations, displaced for thermodynamic reasons from the NE to the PE of the ECs, acted as catalyst, i.e., small nucleation centers of equilibrium phases. The spatial position of the established three-phase regions relative to the silver point was used to assign the overall potential-determining reactions of synthesis of the ternary AgGa₅Te₈ and binary Ga₂Te₅, Ga₇Te₁₀, Ga₃Te₄ compounds. For the first time, the values of the standard thermodynamic functions (Gibbs energies, enthalpies, and entropies) of these compounds were determined based on the temperature dependences of the EMF of the ECs.