Mechanism of Phosphorescence Appropriate for the Long-Lasting Phosphors Eu2+-Doped SrAl2O4 with Codopants Dy3+ and B3+

The existing mechanisms proposed to explain the phosphorescence of SrAl 2 O 4:Eu 2+,Dy 3+ and related phosphors were found to be inconsistent with a number of important experimental and theoretical observations. We formulated a new mechanism of phosphorescence on the basis of the facts that the d orbitals of Eu 2+ are located near the conduction band bottom of SrAl 2 O 4, that the Eu 2+ concentration decreases during UV excitation, and that trace amounts of Eu 3+ are always present in these phosphors. In our mechanism, some Eu 2+ ions are oxidized to Eu 3+ under UV, and the released electrons are trapped at the oxygen vacancy levels located in the vicinity of the photogenerated Eu 3+ cations. The phosphorescence arises from the recombination of these trapped electrons around the photogenerated Eu 3+ sites with emission at 520 nm. The codopant Dy 3+ enhances the phosphorescence by increasing the number and the depth of electron traps, and the codopant B 3+ enhances the phosphorescence by increasing the depth of electron traps. We also probed the origin of another emission at 450 nm of SrAl 2 O 4:Eu 2+ that occurs at low temperatures. Our analysis indicates that this emission is caused by a charge transfer from oxygen to Eu 3+ cations and is associated with a hole trapping.