Phase formation in LaF3–NaGdF4, NaGdF4–NaLuF4, and NaLuF4–NaYF4 systems: Synthesis of powders by co-precipitation from aqueous solutions

Detailed studies of LaF3–NaGdF4, NaGdF4–NaLuF4, and NaLuF4–NaYF4 systems have revealed that LaF3–NaGdF4 system precipitates, formed in aqueous media, contained gagarinite-type NaGd1−xLaxF4 (x ≤ 0.0625) solid solution, tysonite-type La1−xGdxF3 (x ≤ 0.50) phase, and cubic fluorite-type NaGdF4-based phase, whereas NaGdF4–NaLuF4 precipitates contained hexagonal gagarinite-type (x ≤ 0.25) and cubic fluorite-type (x ≥ 0.675) NaGd1−xLuxF4 solid solutions. Furthermore, there were continuous series of single-phase cubic fluorite-type NaLuF4–NaYF4 solid solutions formed in the third investigated system. Crystallization of Na0.5−xLu0.5+xF2+2x solid solutions from aqueous media occurred in an incongruent manner; the use of 5-fold excess of NaF led to precipitation of NaLuF4 (x = 0) with almost stoichiometric composition.