The existing mechanisms proposed to explain the phosphorescence of SrAl2O4:Eu2+,Dy3+ 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 Eu2+ are located near the conduction band bottom of SrAl2O4, that the Eu2+ concentration decreases during UV excitation, and that trace amounts of Eu3+ are always present in these phosphors. In our mechanism, some Eu2+ ions are oxidized to Eu3+ under UV, and the released electrons are trapped at the oxygen vacancy levels located in the vicinity of the photogenerated Eu3+ cations. The phosphorescence arises from the recombination of these trapped electrons around the photogenerated Eu3+ sites with emission at 520 nm. The codopant Dy3+ enhances the phosphorescence by increasing the number and the depth of electron traps, and the codopant B3+ enhances the phosphorescence by increasing the depth of electron traps. We also probed the origin of another emission at 450 nm of SrAl2O4:Eu2+ that occurs at low temperatures. Our analysis indicates that this emission is caused by a charge transfer from oxygen to Eu3+ cations and is associated with a hole trapping.

The phosphorescence mechanisms of various phosphors were explored by taking into consideration the presence of interactions between dopant cations and vacancies as well as the effect of co-dopant cations on these interactions. Our study indicates that the distribution of dopant and co-dopant cations around vacancies is not random, and luminescent properties are strongly influenced by this nonrandom distribution. To a first approximation, the ionization potentials of dopants, co-dopants, and host cations can be used to rationalize their tendencies for this nonrandom distribution. On the basis of our analysis, we explained the properties of known phosphorescent materials, examined several factors important for designing new improved phosphors, and discussed how point defects might affect phosphorescence as well as fluorescence.