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 Ag2Te-FeTe2-F1.12Te-Ag2Te and Ag8GeTe6-GeTe-FeTe2-AgFeTe2-Ag8GeTe6 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 AgFeTe2, Ag2FeTe2, and Ag2FeGeTe4 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 AgFeTe2, Ag2FeTe2, and Ag2FeGeTe4 were experimentally determined for the first time. The reliability of the division of the Ag2Te-FeTe2-F1.12Te-Ag2Te and Ag8GeTe6-GeTe-FeTe2-AgFeTe2-Ag8GeTe6 phase regions was confirmed by the calculated thermodynamic quantities of AgFeTe2, Ag2FeTe2, and Ag2FeGeTe4 in equilibrium with phases in the adjacent phase regions. Particularly, the calculated Gibbs energies of Ag2FeGeTe4 in two different adjacent 4-phase regions are consistent, which also indicates that it has stoichiometric composition.