Photochromism of rare earth doped barium haloapatite

A series white-amethyst photochromic materials, Ba5(PO4)3Cl:Eu2+,R3+ (R = Y, La–Nd and Sm–Lu) are synthesized. These materials are sensitive to electromagnetic waves from short UV to X-ray. The materials change their color from white to amethyst by short UV or X-ray irradiation. The photo-colored samples can be bleached by green light irradiation or heat treatment. A model that involves ionization of the 5d electron of Eu2+ to conduction band states and subsequent trapping by a positive lattice defect is proposed for the photochromism in these compounds.