Phase Equilibria and Thermodynamic Properties of Compounds in the Ag2FeS2-Ga2S3 Cross-Section of the Ag-Fe-Ga-S System Determined by the EMF Method

The phase equilibria and division of the Ag2S-GaS-Ga2S3-FeS2-FeS-Ag2S region (A) of the Ag-Fe-Ga-S system below 600 K were established by the modified EMF method. The electrochemical cells (ECs) of the following structure were assembled: (-)C||Ag||SE||R(Ag+)||PE||C(+), where C is the graphite; Ag is the left electrode; SE is the solid-state electrolyte; PE is the right electrode; R(Ag+) is the region of Ag+ diffusion in the PE. Initially assembled PEs are a nonequilibrium phase mixture of binary sulfides with the ratios of simple substances covering all composition space of (A). The catalyst for the reactions in R(Ag+) were Ag+ ions acting as small nucleation centers of equilibrium mixtures of compounds. The division of (A) was realized with the participation of the binary as well as more complex compounds AgGaS2, Ag9GaS6, AgFeS2, Ag2FeS2, Ag2FeGa20S32, Ag2FeGa2S5, and Ag18Fe9Ga2S21. The spatial position of the three- and four-phase regions relative to the Ag point was employed to establish the overall potential-forming reactions for synthesizing quaternary phases in the PEs of ECs. The temperature dependencies of the electromotive force of ECs were used to calculate the values of the standard thermodynamic functions of the quaternary compounds.