Phase equilibria in the Ag8SiSe6-Ag8SiTe6 system and characterization of the solid solutions Ag8SiSe6-xTex

Abstract

Due to polymorphism and complex crystal structure, compounds of the argyrodite family and phases based on them exhibit several interesting functional properties, such as thermoelectric, photoelectric, optical, as well as ionic conductivity for Cu+ and Ag+ cations. The paper presents the results of the study of phase equilibria in the Ag8SiSe6-Ag8SiTe6 system by DTA, XRD, and SEM methods. Refined data on the melting temperature (1278 K) and polymorphic transitions (315 K and 354 K) of the Ag8SiSe6 compound are presented. The crystallographic parameters of LT-Ag8SiSe6 (Cubic, F-43m, a = 1.0965 nm) and IT-Ag8SiSe6 (Cubic, P4232, a = 1.0891 nm) are also determined. It has been established that the investigated system is quasi-binary and its phase diagram is characterized by the formation of a continuous series of substitutional solid solutions between HT-Ag8SiSe6 and Ag8SiTe6. This process is accompanied by a strong decrease in the temperatures of polymorphic transformations of Ag8SiSe6, which leads to the stabilization of the ion-conducting cubic phase at room temperature in the >10 mol% Ag8SiTe6 compositions area. The crystal lattice parameters of the synthesized solid solutions are calculated by indexing the powder diffraction patterns. The stabilization of the high-temperature cubic phase at room temperature achieved by us presents new opportunities for the development of environmentally friendly thermoelectrics and ion-electronic conductors based on silicon argyrodites with desired composition and properties.