PHASE EQUILIBRIA IN THE Ag7PS6 – Ag8SiS6 SYSTEM

Abstract

Ternary silver (I) chalcogenides with an argyrodite structure are of considerable scientific and practical interest as promising thermoelectric, optical, and superionic materials. The functional parameters of these phases are determined by the motif of the crystal structure: tetrahedral close packing and a large number of voids that can be occupied by mobile cations. The result is a significant electrical conductivity and low phonon thermal conductivity. However, the constant growth of requirements for the parameters of functional materials leads to their continuous improvement by increasing their component composition. The alloys of the Ag7PS6 - Ag8SiS6 system were obtained by melting ternary sulfides in vacuumed quartz ampoules in appropriate stoichiometric ratios. The maximum synthesis temperature was 1015°C. All the obtained polycrystalline alloys of the Ag7PS6 - Ag8SiS6 system were studied by DTA and XRD methods. Based on the results of the study of the alloys of the Ag7PS6 - Ag8SiS6 system, a corresponding phase diagram was built and it was found that the Ag7PS6 - Ag8SiS6 system is quasi-binary. It was found that high-temperature modifications of Ag7PS6 and Ag8SiS6 compounds crystallizing in the face-centered cubic SG F-43m form a continuous series of solid solutions. At a lower temperature, the region of the solid solutions region narrows. The areas of two-phase equilibrium between the high and low temperature modifications are realized separately and independently for each of the initial components. At the annealing temperature, the region of boundary solid solutions based on HT-Ag7PS6 does not exceed 25 mol%, and HT-Ag8SiS6 45 mol%.