The development of new efficient afterglow phosphors is currently hampered by a limited understanding of the persistent luminescence mechanism. Radioluminescence (RL) and x-ray absorption measurements on the persistent phosphor SrAl${}_2$O${}_4$:Eu,Dy were combined to reveal possible valence state changes for the rare earth (co)dopants. Traps in the phosphor material are quickly filled when exposing thermally emptied SrAl${}_2$O${}_4$:Eu,Dy powder to x rays. On the same time scale a partial oxidation of Eu${}^{2+}$ to Eu${}^{3+}$ is observed by x-ray absorption near-edge spectroscopy (XANES), while for the trivalent dysprosium the valence state remains unchanged. The impact of these observations on the recently proposed models for persistent luminescence is discussed.

• Received 23 May 2011

DOI:https://doi.org/10.1103/PhysRevB.84.085140