The synthesis of thermodynamically stable phases of the Ag9GaSe6–Ag8GeSe6 (І) system at T<600 K were performed in the 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 (Ag2GeS3-glass), PE is the right (positive) electrode, and R(Ag+) is the region of PE that contact with SE. PEs of ECCs were prepared from finely ground non-equilibrium mixtures of the compounds (Ag9GaSe6)1–х and (Ag8GeSe6)х, х=0.05, 0.1, 0.2, … , 0.9. Shifted from the left electrode to the R(Ag+) region for thermodynamic reasons Ag+ ions acted as the nucleation centers for the equilibrium phases of the x compositions, that is as the catalysts for reconstruction of the metastable mixtures of ternary compounds.
The reproducibility of the EMF vs T dependences in the heating-cooling cycles is a result of the completion of reconstruction in the R(Ag+) region. Experimental dependences EMF vs T of ECCs with PE of mixtures of compounds indicated x, ECCs with PE of the Ag9GaSe6 and Ag8GeSe6 compounds are characterized by several discrete linear regions with different temperature intervals and functional dependences on temperature. The equations of the temperature dependences of the partial Gibbs energies of Ag-component in alloys for each discrete section of the specific x=0, 0.3, 0.5, 0.7, 1.0 were established for the first time and values of the standard partial functions were calculated. The bypass lines drawn through the points of EMF values of the cells at T=const for arbitrarily selected temperatures 298 K, 341 K, 395 K, 445 K, and 495 K in the range 280-500 K determine the phase composition of the equilibrium T-x space of (I) as: a solid solution based on compound Ag17GaGeSe12 in the range of 0.25≤x≤0.75, solid solutions based on the compounds (Ag9GaSe6)1–х for 0≤х≤0.15 and (Ag8GeSe6)х for 0.83≤х≤1, and two two-phase sections in the ranges 0.15≤x≤0.25 and 0.73≤x≤0.77. Some parameters of the crystal structure, the values of the total and ionic components of conductivity, the transfer numbers in the range of 290–380 K, and the integral values of the standard thermodynamic functions of the Ag17GaGeSe12 compound were established for the first time.
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