THERMODYNAMICALLY STABLE PHASES OF THE Ag9GaSe6 – Ag8GeSe6 SYSTEM AT T<600 K AND THEIR PHYSICO-CHEMICAL PROPERTIES

Abstract

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.