Patent ID: 8858833

Claim:
A process comprising reacting aqueous solutions consisting essentially of an aqueous solution of one or more fluorides with an aqueous solution of a host multi-valent metal salt and one or more rare-earth salts to form aqueously insoluble rare-earth doped multi-valent metal fluoride nanoparticles characterized by d50<100 nanometers, the rare-earth doped multi-valent metal fluoride being characterized by an aqueous solubility of less than 0.1 g/100 g of water; wherein the concentrations of the solution of the one or more fluorides and the solution of the host multi-valent metal salt and the one or more rare earth salts are in stoichiometric amounts forming a rare-earth doped multi-valent metal fluoride, and the aqueous solution of one or more fluorides and the aqueous solution of a host multi-valent metal salt and the one or more rare-earth salts are independently at least 0.1 normal; wherein the relative concentration of the rare earth cation with respect to that of the multi-valent metal cation is in the range of 2 to 20% on a molar basis; and, wherein the fluoride is selected from alkali metal fluorides, ammonium fluoride, hydrogen fluoride, or mixtures thereof.