Mechanism of the persistent phosphorescence in Sr4Al14O25:Eu and SrAl2O4:Eu codoped with rare earth ions

Abstract

Luminescence and phosphorescence properties are investigated in two series of strontium aluminates, Sr4Al14O25:Eu2+ and SrAl2O4:Eu2+, codoped with rare earth (lanthanoid, Ln) ions. Persistent phosphorescence is observed for the phosphors codoped with Pr, Nd, Dy, Ho, and Er. In the phosphorescence process of these phosphors, the excited electron in the Eu2+ ion is transferred between the Eu and the codopant Ln ions through the 5d states of these ions. When the trivalent codopant ion Ln3+ captures the excited electron in the 4f shell, the ion is reduced to the divalent state and, therefore, the energy necessary to thermally relieve the electron from the ion is determined by the energy difference between the 4fn and the 4fn−15d configurations of the divalent Ln ion. The trapping depths (activation energies) of the Ln ions, estimated theoretically on the basis of this mechanism, coincide with the observed depths (0.8±0.2eV) for the persistently phosphorescent phosphors, and those of the nonpersistent Ln ions are larger or smaller by 0.7eV than the former. It is also elucidated that the retrapping of the electron released from a trap plays an important role in the persistent phosphorescence process.