Phase Equilibria in Ag8SnS6 – Cu2SnS3 and Ag2SnS3 – Cu2Sn4S9 Systems

Abstract

Purpose. Chalcostannates of copper and silver are among the important functional materials
of modern technology. Among them, there are materials with valuable semiconductor,
photosensitive, and thermoelectric properties. By complex methods of physical-chemical analysis
(differential thermal, X-ray phase, microstructural analysis, microhardness measurement, and
density determination) phase equilibria have been studied in systems Ag₈SnS₆–Cu₂SnS₃ and
Ag₂SnS₃–Cu₂Sn₄S₉ and their T–x phase diagrams have been constructed.
Results. It was shown that the Ag₈SnS₆–Cu₂SnS₃ system is a quasi-binary cross-section of the
Ag₂S–SnS₂–Cu₂S quasi-triple system and belongs to the simple eutectic type with limited
solubility regions based on the initial sulphides. The coordinates of the eutectic point are:
50 mol % Ag₂SnS₃ and T = 900 K.
The solubility based on Ag₈SnS₆ and Cu₂SnS₃ at the eutectic temperature stretches to 20 and
28 mol %, respectively. With decreasing temperature, the solid solutions decompose, and at
300 K they have values of 5 and 10 mol %. The phase transition a-Ag₈SnS₆ ↔ b-Ag₈SnS₆ occurs
at 410 K and has a eutectoid character. It was established that with an increase in the concentration
of Ag₈SnS₆ in solid solutions (Cu₂SnS₃)1–х (Ag₈SnS₆)х, the cubic lattice parameter increases
from a = 0.5445 nm (for pure Cu₂SnS₃) to a = 0.725 nm (for composition x = 0.1). The concentration
dependence of the lattice parameter on the composition is linear.
Conclusion. The Ag₂SnS₃–Cu₂Sn₄S₉ system, due to the peritectic melting of Cu₂Sn₄S₉, has a
complex character and is a partial quasi-binary section. The quasi-binary state is violated in the
range of 65–100 mol % Cu₂Sn₄S₉ and above temperature 900 K. The part of the system in the
concentration range of 0–65 mol % Cu₂Sn₄S₉, is of a eutectic type. Solid solutions based on
Ag₂SnS₃ – Cu₂Sn₄S₉ are narrow and at 300 K they are 10; 2.5 mol %, respectively.

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