Ag2Se-Tl2Se-Bi2Se3 quasi-ternary system

Abstract

The Ag2Se-Tl2Se-Bi2Se3 quasi-ternary system (system A) was studied using DTA, X-ray powder diffraction, microstructure examination, and microhardness measurements. TlBiSe2-AgBiSe2, AgTlSe-AgBiSe2, AgTlSe-Bi2Se3, and Tl2Se-AgBiSe2 polytherms, isothermal sections at 500 and 800 K, and liquidus surface projection of system A were constructed. System A is congruently triangulated into the following subordinate triangles: Tl2Se-AgTlSe-Tl9BiSe6 (I), AgTlSe-Tl9BiSe6-TlBiSe2 (II), Ag2Se-AgTlSe-TlBiSe2 (III), Ag2Se-AgBiSe2-TlBiSe2 (IV), and AgBiSe2-TlBiSe2-Bi2Se3 (V). Subsystems I, III, and V are ternary systems with three-phase eutectic equilibrium; system II has a three-phase eutectic, and system IV is characterized by several invariant and monovariant peritectic and eutectic equilibria. Primary crystallization and homogeneity fields were outlined, and the types and coordinates of invariant and monovariant equilibria in system A were determined. A characteristic feature of the title system is an extensive field of solid solutions between high-temperature cubic AgBiSe2 and TlBiSe2 phases; this field lies as a continuous belt along the AgBiSe2-TlBiSe2 quasibinary section and covers about one-fourth of the surface area of the triangular diagram of system A.