Defect aggregates in the Sr2MgSi2O7 persistent luminescence material

The crystal and electronic structure of the Eu2+ doped and defect containing Sr2MgSi2O7 persistent luminescence material were studied using the density functional theory (DFT). The defects may act as energy storage or even luminescence quenching centres in these materials, however their role is very difficult to confirm experimentally. The probability of vacancy formation was studied using the total energy of the defect containing host. Significant structural modifications in the environment of the isolated defects, especially the strontium vacancy, as well as defect aggregates were found. The experimental band gap energy of Sr2MgSi2O7 was well reproduced by the calculations. The defect induced electron traps close to the host's conduction band were found to act as energy storage sites contributing to its efficient persistent luminescence. The interactions between the defects were found to modify both the Eu2+ 4f7 ground state energy as well as the trap structure. The effect of charge compensation induced by the rare earth co-doping on the defect structure and energy storage properties of the persistent luminescence materials was discussed.