Processes for producing particles of rare earth-containing phosphor materials, in which the particles have a core-shell structure and the shell has a lower rare earth content than the core. Such a process may include contacting a core particle with a precursor comprising Na(La,Ce,Tb)P2O7 to form a mixture, and then heating the mixture to a temperature sufficient to decompose the Na(La,Ce,Tb)P2O7 to evolve and melt an NaPO3 flux and initiate deposition of a (La,Ce,Tb)PO4 shell on each core particle in the presence of the molten NaPO3 flux.