Role of defect states in persistent luminescence materials

The Eu2+ doped alkaline earth aluminates, MAl2O4:Eu2+ (M = Ca and Sr) co-doped with selected R3+ ions are efficient new persistent luminescence materials. The detailed mechanisms of persistent luminescence are not yet known. The importance of the defects to induce persistent luminescence was studied here by EPR, Moessbauer, thermoluminescence (TL) and persistent luminescence measurements. The EPR results show that even the non-doped material, CaAl2O4, contains trapped electrons, probably in anion vacancies. Similar EPR properties are shown by the Eu2+ doped and R3+ co-doped materials. Other paramagnetic defects are present, too. The Moessbauer results indicated complex distribution of the Eu2+ ions in the CaAl2O4 host. Different R3+ ions affect the thermoluminescence and persistent luminescence properties of the CaAl2O4:Eu2+, R3+ materials in a very different manner, from enhancing to total suppression