Observation of multiple sites for trivalent europium ions in SrAl2O4

We have measured room temperature and low temperature luminescence spectra of Eu3+ in SrAl2O4:Eu3+/2+. The powder samples were synthesized in reducing atmosphere above 1200 °C, cooled down to room temperature in the same atmosphere, and heated again to 750 °C in air. The analysis of the emission spectra of Eu3+ showed the presence of multiple sites for this ion, revealing that many different charge compensating defects are located in close vicinity to the europium ions. These defects can be positioned in each of the two void sites identified in the crystal structure of SrAl2O4. Lifetime measurements reveal the presence of very short-lived excited Eu3+ species with lifetimes around 0.7 ms, in addition to species with lifetimes around 1.6 ms. These very short lifetimes are postulated to be related to intervalence charge transfer with few remaining divalent Eu2+ ions which had not been oxidized by the thermal treatment in air. This type of intervalence charge transfer between divalent to the trivalent rare earth ion could be crucially associated to processes involved in the afterglow of the persistent phosphor SrAl2O4:Eu2+,Dy3+.