Method for preparing rare-earth doped fluoride nanoparticles

A process comprising combining at least 0.1 normal aqueous solution of a fluoride selected from the group consisting of alkali metal fluorides, ammonium fluoride, hydrogen fluoride, and mixtures thereof, with at least 0.1 normal aqueous solution of a host multi-valent metal salt, comprising a host multi-valent metal cation and an anion, and an aqueous solution of a salt comprising a rare earth metal cation dopant, the amount of the rare-earth dopant being in the range of 0.1 to 25 mol-% of the molar concentration of the host multi-valent metal cation, forming a reactant mixture resulting in a precipitate of aqueously insoluble rare-earth doped multi-valent metal fluoride nanoparticles, the rare-earth doped multi-valent metal fluoride being characterized by an aqueous solubility of less than 0.1 g/100 g of water.