Fluorite Solid Solutions of Congruent Melting in the PbF2–CdF2–RF3 Systems

The congruent melting compositions in the MF₂–M'F₂–RF₃ systems (M ≠ M' = Ca, Sr, Cd, Ba, Pb; R are rare-earth elements (REEs)) have been determined based on thermodynamic topological analysis. Crystals of fluorite solid solution in the PbF₂–CdF₂–RF₃ (R = Tb, Ho, Er, Tm, Yb, Lu) systems were grown by the vertical directional crystallization technique. Their phase composition and distribution of components over the crystalline boule length were investigated. Crystals of congruently melting solid solutions (Pb_0.67Cd_0.33)\(_{{1-x}}\)R_xF\(_{{2 + x}}\) (R = Tb, Ho, Er, Tm, Yb, Lu) were grown for the first time. Traces of low-temperature transformation—a phase isostructural to the Pb₂YF₇ compound (sp. gr. I4/m), in which Y sites are occupied by the corresponding R cations and Pb sites can be partially occupied by Cd cations—were found in the crystals with R = Ho, Er, Tm, and Yb. The crystals with R = Tb and Lu have a high degree of homogeneity and are suitable for optical studies.