Process for the preparation of rare earth oxyhalide phosphor

Claim 1

A process for the preparation of lanthanum and gadolinium oxyhalide phosphor crystals selected from the group consisting of oxychlorides and oxybromides activated with thulium ion consisting essentially of recrystallized phosphor crystals having an average median particle size in the range extending from approximately 3.0 microns diameter up to approximately 9.0 microns diameter along with a more narrow particle size distribution and polyhedral shape comprising the following steps: (a) heating a mixture of particles of oxides selected from lanthanum and gadolinium oxides, including mixtures thereof, which further contains thulium ion in activator amounts from about 0.0005 to 0.01 mole per mole of the phosphor and an ammonium halide whose halide ion is that of the selected oxyhalide, along with sufficient scandium ion to produce a scandium level in said phosphor from about 0.001 to 0.2 mole per mole of the phosphor for a time and temperature sufficient to form the selected oxyhalide phosphor, and (b) reheating said phosphor with an alkali metal halide flux mixture whose halide ions are that of the selected oxyhalide phosphor to a temperature not exceeding approximately 900° C. for a time sufficient to recrystallize said selected oxyhalide phosphor.