THERMODYNAMIC PROPERTIES OF THE AgGaGe3Se8 COMPOUND AT Т≤500 K DETERMINED BY THE EMF METHOD

Abstract

The division of the concentration space of the Ag–Ga–Ge–Se system in the GeSe–Se–Ga2Se3–AgGaGe3Se8–GeSe section was carried out by the EMF method. The concentration space of the Ag–Ga–Ge–Se system contains the four-phase regions GeSe2–Se–Ga2Se3–AgGaGe3Se8 (I) and GeSe–GeSe2–Ga2Se3–AgGaGe3Se8 (II). The spatial positions of these phase regions vs the point of silver were used to express equations of the overall cell reactions of synthesis of the AgGaGe3Se8 compound: 2Ag + 6GeSe2 + Ga2Se3 + Se = 2AgGaGe3Se8 and 2Ag + 7GeSe2 + Ga2Se3 = GeSe + 2AgGaGe3Se8. Reactions were performed in the positive electrodes of the following electrochemical cells (ECCs): (−) С | Ag | SЕ | R(Ag+) | PЕ | С (+), where C is graphite, Ag is the left (negative) electrode, SE is the purely Ag+ ion conducting solid electrolyte (Ag3GeS3Br-glass), PE is the right (positive) electrode, and R(Ag+) is the region of PE that contact with SE. Shifted from the left electrode to the R(Ag+) region for thermodynamic reasons Ag+ ions acted as the small nucleation centers for reconstruction of the metastable mixtures of compounds in the thermodynamically stable mixture of phases. Linear dependences of EMF vs temperature of ECCs were used to calculate the standard thermodynamic properties of the AgGaGe3Se8 equilibrium in the regions (I) and (II): ∆_f G_((I))^○= –(575,0 ± 8,9) kJ·mol −1, ∆_f H_((I))^○= –(574,7 ± 12,0) kJ·mol−1, S_((I))^○ = (515,9 ± 14,1) J·(mol·K)−1 and ∆_f G_((II))^○ = –(588,4 ± 9,4) kJ·mol−1, ∆_f H_((II))^○ = –(590,6 ± 12,9) kJ·mol−1, S_((II))^○ = (507,8 ± 13,2) J·(mol·K)−1, respectively. The relative difference between the calculated values ∆_f G_((I))^○ and ∆_f G_((II))^○ within ~2.3% and the overlap of the uncertainty intervals characterize the four-phase sections (I) and (II) by the combination of compounds of the formula composition. Moreover, values of the partial thermodynamic functions of the potential-determining component (Ag) of the AgGaGe3Se8 compound were calculated.