Valence and environment of rare earth ions in CaAl2O4:Eu2+,R3+ persistent luminescence materials

The existence of the different R2+/R3+/RIV (R: rare earth) ions as well as the modifications in the structural environment around the dopant and co-dopants in CaAl2O4:Eu2+,R3+ persistent luminescence materials was studied by LIII edge X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) measurements at Hamburger Synchrotronstrahlungslabor (HASYLAB) at Deutsches Elektronen-Synchrotron (DESY) (Hamburg, Germany). The measurements were carried out at 10 and 296 K for selected rare earth (co-)dopants (Eu2+; Ce3+, Nd3+, Sm3+, and Yb3+). NES results indicated the co-existence of both divalent and trivalent europium in all co-doped CaAl2O4:Eu2+,R3+ materials, but only divalent europium in CaAl2O4:Eu2+. The measurement temperature did not affect the XANES results. The interatomic distances extracted from the EXAFS data of the CaAl2O4:Eu2+,R3+ materials indicated that co-doping creates distortions around the Eu2+ ions suggesting dopant aggregation.