Persistent luminescence and its mechanism of Ba5(PO4)3Cl:Ce3+,Eu2+

We investigated the persistent luminescence in europium-doped barium chloroapatite upon codoping with cerium. Luminescence properties of the phosphors, including photoluminescence, luminescence decay, and thermoluminescence are systematically studied. This material holds the key to the unsettled questions in the field of persistent luminescence. We formulated a model of persistent luminescence on the basis of the experimental results. In our model, some Eu2+ ions are oxidized to Eu3+ under short UV excitation, and the released electrons are trapped at positive substitutional defect introduced by Ce3+. The persistent luminescence arises from the recombination of these trapped electrons with the photo-ionized Eu3+ ions. The effects of Coulomb attraction between the substitutional defect and trapped electron, along with electron–electron repulsion, construct a metastable exciton-like trap. To a first approximation, the third ionization energy and ionic radius of lanthanide codopant can be used to rationalize their influence on persistent luminescence of Eu2+. Our model can also explain other systems, such as SrAl2O4:Eu2+,R3+ (R = rare earth elements).