Abstract
The quasi-ternary Ag₂Se–Bi₂Se₃–SnSe₂ system was investigated along the AgBiSe₂–Ag₈SnSe₆ and AgBiSe₂–SnSe₂ sections, as well as by studying a large number of alloys covering the quasi-ternary composition range. The samples were synthesized by the sealed ampoule technique from high-purity elements (99.99 wt.%), which were melted in evacuated (10⁻² Pa) sealed quartz ampoules. The heating rate was 40 K/h, and the maximum synthesis temperature was 1,220 K. Homogenizing annealing was held at 500 K for 300 h. Subsequently, the alloys were cooled to room temperature at a rate of 20 K/h. The known compounds were identified, and the synthesized alloys were characterized by X-ray powder diffraction (DRON-4-13 diffractometer, CuKα radiation), microstructure analysis (Altami MET-1M metallographic microscope), and differential thermal analysis (Thermodent MP-40 furnace with a Pt/Pt–Rh thermocouple).
The phase equilibria in the quasi-ternary Ag₂Se–Bi₂Se₃–SnSe₂ system at 300 K are represented by the isothermal section and two vertical sections, AgBiSe₂–Ag₈SnSe₆ and AgBiSe₂–SnSe₂. Both sections are of the eutectic type, with the eutectic coordinates of ~905 K at 37 mol.% Ag₈SnSe₆, and ~790 K at 70 mol.% SnSe₂, respectively. The composition space of the investigated system is divided into three three-phase regions, Ag₂Se–Ag₈SnSe₆–AgBiSe₂, Ag₈SnSe₆–AgBiSe₂–SnSe₂, and AgBiSe₂–SnSe₂–Bi₂Se₃. All compounds exhibit narrow homogeneity ranges within the investigated composition intervals, with solubility limits up to 5 mol.% for most phases, whereas the solid solution range of AgBiSe₂ extends to about 15 mol.%.
The liquidus surface of the investigated system consists of five primary crystallization fields of solid solution ranges of the binary compounds Ag₂Se, Bi₂Se₃, and SnSe₂ (α, β, and γ), as well as the ternary phases AgBiSe₂ (δ′) and Ag₈SnSe₆ (ε′). Three invariant eutectic reactions occur in the system: LЕ1 ↔α+δ′+ε′ at 875 K and the composition 83 mol.% Ag₂Se / 9 mol.% Bi₂Se₃ / 8 mol.% SnSe₂; LЕ2 ↔δ′+ε′+γ at 740 K and the composition 38 mol.% Ag₂Se / 7 mol.% Bi₂Se₃ / 55 mol.% SnSe2 ; and LЕ3 ↔β+δ′+γ at 765 K and the composition 9 mol.% Ag₂Se / 27 mol.% Bi₂Se₃ / 64 mol.% SnSe₂.