The Equilibrium Phase Formation and Thermodynamic Properties of Functional Tellurides in the Ag–Fe–Ge–Te System

Equilibrium phase formations below 600 K in the parts Ag 2 Te–FeTe 2 –F 1.12 Te–Ag 2 Te and Ag 8 GeTe 6 –GeTe–FeTe 2 –AgFeTe 2 –Ag 8 GeTe 6 of the Fe–Ag–Ge–Te system were established by the electromotive force (EMF) method. The positions of 3- and 4-phase regions relative to the composition of silver were applied to express the potential reactions involving the AgFeTe 2 , Ag 2 FeTe 2 , and Ag 2 FeGeTe 4 compounds. The equilibrium synthesis of the set of phases was performed inside positive electrodes (PE) of the electrochemical cells: (−)Graphite ‖LE‖ Fast Ag + conducting solid-electrolyte ‖R[Ag + ]‖PE‖ Graphite(+), where LE is the left (negative) electrode, and R[Ag + ] is the buffer region for the diffusion of Ag + ions into the PE. From the observed results, thermodynamic quantities of AgFeTe 2 , Ag 2 FeTe 2 , and Ag 2 FeGeTe 4 were experimentally determined for the first time. The reliability of the division of the Ag 2 Te–FeTe 2 –F 1.12 Te–Ag 2 Te and Ag 8 GeTe 6 –GeTe–FeTe 2 –AgFeTe 2 –Ag 8 GeTe 6 phase regions was confirmed by the calculated thermodynamic quantities of AgFeTe 2 , Ag 2 FeTe 2 , and Ag 2 FeGeTe 4 in equilibrium with phases in the adjacent phase regions. Particularly, the calculated Gibbs energies of Ag 2 FeGeTe 4 in two different adjacent 4-phase regions are consistent, which also indicates that it has stoichiometric composition.