Persistent luminescence phenomena in materials doped with rare earth ions

The luminescent efficiency of rare earth ions is usually drastically lowered when defects are present in the host lattice. Persistent luminescence is the most recent rare earth application based on lattice defects. Typical materials are the Eu2+-doped alkaline earth aluminates, MAl2O4:Eu2+ (M=Ca and Sr). The trivalent R3+ ions as co-dopants enhance greatly the duration and intensity of persistent luminescence. As a result of very slow thermal bleaching of the excitation energy from the lattice defects acting as traps, the new persistent luminescent materials yield luminescence still visible to naked eye for more than 10h. Despite the seemingly simple stoichiometry and structure of these materials, the determination of persistent luminescence mechanism(s) presents a very complicated problem. This report presents in detail some of the factors affecting the luminescence properties of the Eu2+-, R3+- doped MAl2O4. The possible mechanisms involved with different defect centers and interactions between them and the emitting Eu2+ ion are discussed based on the results of systematic investigations carried out on the preparation, composition, structure and different luminescence properties. (C) 2003 Elsevier Science (USA). All rights reserved.