New Materials, Compounds and Applications

This paper presents the results of a comprehensive study of solid-phase equilibria in the Bi₂S₃-Bi₂Se₃-BiI₃ system and the thermodynamic properties of the intermediate phases Bi₁₉S_27(1-х)Sе_27хI₃. The investigations were carried out using differential thermal analysis (DTA), X-ray diffraction (XRD) and electromotive force (EMF) measurements of concentration cells of the type (−) Bi (solid) / liquid electrolyte, Bi³⁺ / (Bi in alloy) (solid) (+) within the temperature range 300–370 K, with morpholine formate ionic liquid as the electrolyte. Based on the experimental data, a solid-phase equilibria diagram of the Bi₂S₃-Bi₂Se₃-BiI₃ system was constructed, according to which it is characterized by the formation of a continuous series of solid solutions along the BiSI-BiSeI section and a wide homogeneity area based on Bi₁₉S₂₇I₃. From the EMF measurement data, the partial thermodynamic functions of bismuth in solid solutions based on Bi₁₉S₂₇I₃ were calculated. Using a fragment of the phase diagram of the quaternary Bi-S-Se-I system, the potential-forming reactions responsible for the indicated partial molar functions were determined and the standard thermodynamic functions of formation and standard entropies of the ternary compound Bi₁₉S₂₇I₃ and the solid solutions Bi₁₉S_27(1-х)Sе_27хI₃ (x = 0.2, 0.4, 0.6) were calculated. In the calculations, in addition to the authors’ own data, literature data on the corresponding thermodynamic functions of the BiSI compound, the solid solutions BiS_1-хSe_xI and the S_1-xSe_x alloys were used.