PHASE EQUILIBRIA IN THE Ag8SіSе6–Ag7PSе6 SYSTEM

Abstract

Interactions in the quasi-binary system Ag8SiSe6–Ag7PSe6 were investigated. Alloys of the system were obtained from high purity elements (at least 99.99 mol.% of the principal) by a direct single-temperature method in an MP-60 automatic muffle furnace. The maximum synthesis temperature was 1220 K. The starting compounds and obtained alloys were identified by X-ray phase analysis, X-ray structure analysis and DTA methods. The Ag8SiSe6–Ag7PSe6 system is a quasi-binary section of the quasi-ternary system Ag2Se–SiSe2–P2Se5 and is characterized by continuous mutual solubility of the high-temperature modifications of these compounds in both liquid and solid state. The phase diagram belongs to Type I of Rooseboom classification. The sub-solidus region at 300 K features two two-phase regions between three single-phase regions, a small one in the range of 2–4 mol. % Ag7PSe6 (SG P213 + SG F–43m), and a quite significant one from 37 to 78 mol. % Ag7PSe6 (SG P213 + SG F–43m). The formation of continuous solid solutions of substitution expressed as Ag8–xSi1–xPxSe6 (x=0–1) with increasing temperature expands the region of the high-temperature cubic phase (SG F–43m). The formation of the solid solutions of low-temperature modifications significantly lowers the temperature of the polymorphous transition of both starting selenides. The crystal structure of two separate compositions of the solid solutions, Ag7.2P0.8Si0.2Se6 and Ag7.7Si0.7P0.3Se6, was investigated by X-ray powder method. The transtion from Ag7PSe6 to Ag7.2P0.8Si0.2Se6 features the substitution P(V) → Si(IV) + Ag(I), i.e. one atom (P) is replaced by two atoms (Si and Ag). Si atoms replace P atoms to form a statistical mixture M (P + Si). An additional site of Ag atoms (Ag4) appears at the same time. In the transtion from Ag8SiSe6 to Ag7.7Si0.7P0.3Se6, Si(IV) + Ag(I) → P(V) substitution takes place, i.e. two atoms (Si and Ag) are replaced by one atom (P). As P atoms replace Si atoms, a statistical mixture M (Si + P) is formed. At the same time, the occupation of the sites of Ag atoms decreases.