Phase Equilibria in the Ag2Se-GeSe2-SnSe2 System and Thermodynamic Properties of Ag8Ge1-xSnxSe6 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.