Mechanism for the long afterglow in Eu₂O₃ and Nd₂O₃ co-doped SrAl₂O₄ single crystals

Persistent luminescence is reported for a series of SrAl2O4 single crystals co-doped with 1.0 mol% Eu2O3 and (0.5-3.0 mol%) Nd2O3. Crystals were successfully grown under an Ar atmosphere in an optical floating zone furnace and consisted of a single phase with a monoclinic structure. The maximum intensity of the photoexcitation spectra (PLE) of all samples peaked at 421 nm. Under excitation at 421 nm, the photoluminescence (PL) spectra showed a broad emission in the visible region centered on 522 nm, which corresponds to the Eu2+ 4f(6)5d(1) -> 4f(7) transition, and three additional emissions in the near infrared region, corresponding to the F-4(3/2) -> I-4(9/2) (880/907 nm), F-4(3/2) -> I-4(11/2) (1062/1114 nm) and F-4(3/2) -> I-4(13/2) (1332 nm) transitions from Nd3+. Thermoluminescence (TL) spectra showed that traps in the conduction band are shallower than that of the singly Eu-doped sample, and their concentration gradually increased with increasing Nd3+ content, reaching a maximum in the crystal prepared with 2.5 mol% Nd2O3. The lifetime of the afterglow decay curve also increased with increasing Nd3+ content of the crystals, reaching a maximum value of almost 35 seconds with 2.5 mol% Nd2O3. A mechanism for the long afterglow is proposed in which Eu2+ electrons are first excited, then transported through the conduction band to defect centers which function as traps (produced by Nd3+ doping), and the thermal energy at room temperature is sufficient to reverse this process, so that equilibrium is established and a long residual luminescence results.