Phase equilibria in the Ag2Se–GeSe2–SnSe2 system and thermodynamic properties of Ag8Ge1–x Sn x Se6 solid solutions

The Ag2Se–GeSe2–SnSe2 system has been studied in the temperature range 290–430 K using differential thermal analysis, X-ray diffraction, and emf measurements concerning silver concentration cells with Ag4RbI5 as a solid electrolyte. We have constructed a number of vertical sections; the 300-, 800-, and 900-K isothermal sections of the phase diagram; and a projections of the liquidus surface. It has been shown that the only pseudobinary join, Ag8GeSe6–Ag8SnSe6, contains a continuous series of solid solutions between the two crystalline phases of the constituent selenides and divides the system into two independent subsystems. Ag2Se–Ag8GeSe6–Ag8SnSe6 is a univariant, eutectic subsystem, and GeSe2–Ag8GeSe6–Ag8SnSe6–SnSe2 is an invariant eutectic subsystem. Experimental emf data were used to evaluate partial thermodynamic functions of the silver in the high-temperature and low-temperature Ag8Ge1–xSnxSe6 solid solutions.