Phase relations in the Cu3SbS4-Sb2S3-S system were determined experimentally over the entire concentration range by means of differential thermal analysis (DTA) and powder X-ray diffraction (PXRD) techniques. One boundary, two internal polythermal sections, and the liquidus surface projection of the system were constructed. Primary crystallization fields of existing phases, as well as, types and coordinates of non- and monovariant equilibria were determined. It was defined that, the concentration triangle under study is an independent subsystem of the Cu-Sb-S ternary system and belongs to the monotectic type with a wide stratification field of two liquids.

The nature of phase equilibria in the Cu3SbS3-CuI binary system over the entire concentration range were studied by means of the powder X-ray diffraction analysis (PXRD) for the first time at room temperature. It was found that the sample containing 66.7 mol.% CuI composed of a single phase and has a powder diffraction pattern completely different from the constituent phases of the system under study. The crystal lattice type and parameters, that were determined on the basis of the X-ray diffraction pattern of this sample using the TOPAS 4.2 and EVA computer programs are fully consistent with the literature data of the Cu5SbS3I2 four-component compound. The copper (I) iodide rich samples of the system consist of a two-phase mixture of Cu5SbS3I2 and CuI phases. However, the system is unstable in the Cu5SbS3I2-Cu3SbS3 composition range. In this concentration interval, the system is characterized by complex physico-chemical interaction of the initial components.