Phase relationships in the 1.5GeS₂ + Cu₂GeSe₃ ↔ 1.5GeSe₂ + Cu₂GeS₃ reciprocal system

Highlights: • The phase diagram of the 1.5GeS₂ + Cu₂GeSe₃ ↔ 1.5GeSe₂ + Cu₂GeS₃ reciprocal system is constructed. • The Cu₂GeS₃–Cu₂GeSe₃ system investigated. • The existence of the continuous Cu₂GeS_3−xSe_x solid solutions was revealed. • The thermodynamic functions of the Cu₂GeS_3−xSe_x solid solutions were studied. - Abstract: The phase relations in the 1.5GeS₂ + Cu₂GeSe₃ ↔ 1.5GeSe₂ + Cu₂GeS₃ system have been investigated over the entire concentration range mainly by thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques, with the aid of microhardness and electro-motive force measurements (EMF) applied to equilibrated alloys. The liquidus surface projection, isothermal section at 800 K and the phase diagrams of the two vertical sections have been constructed. The homogeneity ranges and primary crystallization fields of phases as well as the character and temperature of non- and monovariant equilibria were determined in the system. It was found that the system is reciprocally reversible and is featured by continuous solid solubility fields along the boundary GeS₂–GeSe₂ and Cu₂GeS₃–Cu₂GeSe₃ systems. The partial molar thermodynamic functions (ΔG^‾,ΔH^‾,ΔS^‾) of copper were calculated for the Cu₂GeS_3−xSe_x (for x = 0.6; 1.2; 1.8; 2.4) solid solutions based on the results of the EMF measurements