PHASE FORMATION IN THE Ag₂S−Sb₂S₃−GeS₂ SYSTEM

The quasi-ternary system Ag₂S−Sb₂S₃−GeS₂ is of interest since the initial binary compounds and ternary compounds Ag₃SbS₃, AgSbS₂, Ag₈GeS₆, Ag₁₀Ge₃S₁₁ and Ag₂GeS₃ of the limiting quasi-binary systems exhibit promising properties for applications in solar energy, thermoelectricity, optoelectronics and biomedicine.

The samples for the study of the Ag₂S–Sb₂S₃−GeS₂ system were obtained by direct single-temperature synthesis from high-purity (at least 99.99 wt.%) elements silver, antimony, germanium and sulfur in evacuated quartz containers. The ampoules were heated to the maximum synthesis temperature of 1220 K, cooled to 500 K, annealed at 500 K for 500 h, and then quenched in air.

The identification of the synthesized samples employed X-ray phase analysis, microstructure analysis, and differential thermal analysis.

Two compounds are formed in the Ag₂S−Sb₂S₃−GeS₂ system at 500 K, Ag₁₁Sb₃GeS₁₂ (intersection of AgSbS₂–Ag₈GeS₆ and Ag₃SbS₃–Ag₂GeS₃ sections) and Ag₂₃Sb₃Ge₇S₃₀ (intersection of Sb₂S₃–Ag₁₀Ge₃S₁₁ and Ag₃SbS₃–Ag₂GeS₃ sections). The isothermal section thus consists of eight two-phase equilibria on the boundary sides, and eleven in the middle of the quasi-ternary system, separating ten three-phase fields.

Vertical section Ag₃SbS₃–Ag₂GeS_3,which is a quasi-binary system, was investigated. Both quaternary compounds Ag₁₁Sb₃GeS₁₂ and Ag₂₃Sb₃Ge₇S₃₀ form at this section at 25 mol.% Ag₂GeS₃ and 70 mol.% Ag₂GeS₃, respectively, and melt congruently at 1047 K and 1077 K, respectively. Ag₁₁Sb₃GeS₁₂ has two polymorphous transitions at 527 K and 605 K; Ag₂₃Sb₃Ge₇S₃₀ has a polymorphous transition at 880 K.

The crystal structure of both compounds was determined by powder XRD. Ag₁₁Sb₃GeS₁₂ (Ag_0.917Ge_0.083Sb_0.250S) crystallizes in cubic space group F-43m, and Ag₂₃Sb₃Ge₇S₃₀ (Ag_1.600Ge_0.400Sb_0.268S₂) crystallizes in tetragonal space group I-42d.