Fluoride solid solutions exhibit exceptional optical and thermodynamic properties that make them valuable for advanced technological applications, and the PbF₂-EuF₃ system represents a particularly promising quasi-binary system for developing high-performance materials. However, the comprehensive understanding of the thermodynamic conditions governing phase equilibria and the precise boundaries of homogeneity regions in this system remains incomplete, limiting the rational design of single-phase materials with desired properties. Therefore, we conducted a comprehensive investigation of the thermodynamic conditions (temperature and composition) controlling the existence of cubic and rhombohedral phases within the homogeneity regions of the PbF₂-EuF₃ system. Solid solution samples were synthesized using both solid-phase synthesis and co-precipitation techniques from aqueous nitrate solutions. Phase equilibria were systematically investigated in two critical regions: the solvus line spanning 0–10 mol% EuF₃ and the ordered rhombohedral R-phase region spanning 35–45 mol% EuF₃. Structural characterization was performed at temperatures below the phase transition temperature in lead fluoride (365 °C) using X-ray phase analysis, optical probing, and Raman scattering. Our investigation successfully demonstrated the possibility of obtaining cubic preparations of high purity across the 0–37 mol% EuF₃ composition range. Additionally, we precisely defined the region of existence of the ordered rhombohedral R-phase within the concentration range of 37–39 to 43–44 mol% EuF₃. These findings provide essential thermodynamic data for the rational design of PbF₂-EuF₃ solid solutions and establish clear compositional boundaries for obtaining desired phase structures in this technologically important fluoride system.

A comprehensive study of the thermodynamic conditions (temperature, composition) of the existence of the cubic phase within the limits of homogeneity region in the quasi-binary PbF2-EuF3 system was carried out. Solid solution samples were obtained by solid-phase synthesis and co-precipitation technique from aqueous nitrate solutions. Phase equilibria were investigated in two regions: the solvus line in the range of 0-10 mol% EuF3 and the region of existence of the ordered rhombohedral R-phase in the range of 35-45 mol% EuF3. The structure of phases in the PbF2-EuF3 system was examined at temperatures below the phase transition temperature in lead fluoride (365°C). The possibility of obtaining a single-phase preparation of a cubic phase of high purity in 0-37 mol% EuF3 composition range has been demonstrated. The region of existence of the ordered rhombohedral R-phase in the concentration range from 37-39 to 43-44 mol% EuF3 was defined using X-ray phase analysis, optical probing, and Raman scattering.