Investigation of the ordering of Ba<sub>1–x</sub>La<sub>x</sub>F<sub>2+x</sub> solid solutions during phase formation from a solution in a sodium nitrate melt

Matrices based on inorganic fluorides have garnered significant interest from researchers for the development of effective phosphors. In this study, fluorite-like phases of the composition Ba1–xLnxF2+x, with an LnF3 content of approximately 40 mol. % for Ln = La–Lu, were synthesized by crystallization of fluorides from a NaNO3 melt. It was observed that the by-product of the synthesis, BaF2, dissolves and is removed from the system during the washing of samples with water. A cubic solid solution with a fluorite structure was formed for rare earth elements within the cerium subgroup. Notably, sodium was incorporated into the samples with Ln = Gd–Lu. The formation of trigonal fluorite-like phases with the Ba4Ln3F17 structure occurred during synthesis only for lanthanoides with smaller ionic radii (Tm–Lu). For intermediate-sized rare earth ions (Gd–Ho), fluorite-like tetragonal phases were formed, exhibiting very weak superstructural reflections on the X-ray diffraction patterns. The resulting matrices have potential applications in the development of up-conversion luminophores and optical thermometers.