Abstract: Methods for producing metal/metalloid oxide particles comprise rare earth metals herein include reacting a reactant stream in a gas flow. The reactant stream includes a rare earth metal precursor and an oxygen source. A collection of particles comprising metal/metalloid oxide have an average particle size from about 15 nm to about 1 micron. The metal/metalloid oxide comprises a non-rare earth metal oxide wherein less than about 25 percent of a non-rare earth metal is substituted with a rare earth metal. The metal/metalloid oxide particles can be reacted with H2S or C2S to form corresponding metal/metalloid sulfide particles. The metal/metalloid sulfide particles can be doped with rare earth metals. The particles are useful as phosphors, for example for use in displays.