Process for the preparation of phosphor

A process for the preparation of a cerium activated rare earth oxyhalide phosphor having the following formula:

• LnOX:xCe
• in which Ln is at least one rare earth element selected from the group consisting of Y, La, Gd and Lu;
• X is at least one halogen selected from the group consisting of Cl, Br and I;
• and x is a number satisfying the condition of 0<x.ltoreq.0.1, which comprises:
• adding to a mixture of starting materials for the phosphor or a heat-treated product of said mixture the heat treatment being effected at 300.degree. to 500.degree. C. at least one compound selected from the group consisting of tetrafluoroboric acid compounds, hexafluorosilicic acid compounds and metal fluorides in an amount of not more than 100% by weight per the amount of said mixture or said heat-treated product thereof to mix them;
• and
• firing the so obtained mixture at a temperature of from 900.degree. to 1500.degree. C., wherein said at least one compound is present in an amount sufficient to effect a cerium activated rare earth oxyhalide phosphor having the above formula, exhibiting a higher luminance of stimulated emission upon excitation with an electromagnetic wave having a wavelength within the region of 450 to 900 nm after exposure to X-ray radiation, than said phosphor prepared as above, absent said compound.