COLLOIDAL DISPERSION OF A RARE-EARTH BORATE, ITS METHOD OF PREPARATION AND ITS USE AS A PHOSPHOR

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The colloidal dispersion of a rare-earth borate of the invention comprises a liquid phase and colloids of said borate dispersed in this phase, these colloids having a mean hydrodynamic diameter measured by DQEL of at most 200 nm and substantially consisting of an elementary particle with a mean size of less than 100 nm. This dispersion is prepared using a method comprising the following steps : (a) a rare-earth oxide is reacted with a controlled amount of a water-soluble monovalent acid, having a pka of between 2.5 and 5.0; (b) the medium obtained after the reaction is heated; (c) boric acid is added to the medium obtained after the previous step and the mixture obtained is heated at a temperature of at least 170°C; and (d) the solid product is separated from the liquid medium thus obtained, and redispersed in a liquid phase, thereby obtaining the colloidal dispersion. The borate of the invention may be used as a phosphor, especially in the manufacture of a luminescent transparent material.