A small particle size of the phosphor preparation method

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A process for preparing phosphor particles having a particle size of 1 micron or less and that are spherical in shape. Such phosphors are useful, for example, as upconverter phosphors in labels for immunoassays. The process comprises preparing a dilute solution of soluble salts of the rare earth metals in amounts required to make the desired phosphor, adding a chemical agent that releases hydroxyl ions, precipitating small spherical particles of a rare earth hydroxycarbonate compound, heating the particles in oxygen at a limited temperature to form the corresponding trioxide particles, and heating the trioxide particles in a sulfur-containing flux in the absence of oxygen at a temperature no higher than about 900 DEG C to convert the trioxide particles to the corresponding oxysulfide phosphor particles.