Non-activation synthesis and thermodynamic properties of ternary compounds of the Ag–Te–Br system

Abstract

Based on the literature data on the phase formation of the Ag–Te–Br system in the part AgBr–Ag2Te–Te (I) and the results of the electromotive force (EMF) measurements of the electrochemical cells (ECCs) the alleged scheme division of (I) into 10 three-phase regions was analyzed. The overall potential-forming reactions were expressed for the phase regions AgBr–Ag19Te6Br7–Te, Ag19Te6Br7–Ag3TeBr–Te, Ag3TeBr–Ag10Te4Br3–Te, Ag3TeBr–Ag5Te2Br–Ag10Te4Br3, Ag5Te2Br–Ag2Te–Ag23Te12Br, and Ag10Te4Br3–Ag23Te12Br–Te. Reactions were performed by applying ECCs of the type (−) IE | Ag | Solid-state electrolyte | R(Ag+) | PE | IE (+), where IE is the inert electrode (graphite), Ag is the negative (left) electrode, PE is the positive (right) electrode, R(Ag+) is the region of Ag+ diffusion into PE. PEs of ECCs were prepared from finely ground non-equilibrium mixtures of pure components Ag, Te, and AgBr. The component ratios were determined from the equations of the potential-forming reactions in respective phase regions. Non-activation synthesis of the equilibrium set of phases was performed in the R(Ag+) region of ECCs. The Ag+ ions displaced from the left to the right electrode for thermodynamic reasons, act as the nucleation centers for stable compounds and as the catalysts for the low-temperature synthesis of an equilibrium set of phases. The synthesis duration of the initial mixture of phases with a particle size ∼5 μm and T = 500 K is under ≤5 h. Linear dependences ЕMF vs T of ECCs between T=(440 and 500) K were used for calculations of the standard values of the Gibbs energies, enthalpies, and entropies of equilibrium compounds in (I) consisting of Ag19Te6Br7, Ag3TeBr, Ag10Te4Br3, Ag5Te2Br, and Ag23Te12Br. The agreement calculated values of the Gibbs energy of the Ag23Te12Br compound in two different three-phase regions Ag5Te2Br–Ag2Te–Ag23Te12Br and Ag10Te4Br3–Ag23Te12Br–Te confirms the correctness of the alleged scheme division of (I).