On the intrinsic defect nature of persistent luminescence in undoped and Eu-doped CaAl2O4

In this paper, we present a comparative study of the persistent luminescence or afterglow (AG), photoluminescence (PL) and thermoluminescence (TL) in reduced undoped and Eu-doped CaAl₂O₄, in order to establish the roles played by Eu and the oxygen vacancy in the processes responsible for the persistent luminescence of this material. When illuminated by light with a photon energy between 3.18 and 5.39 eV, the PL intensity gradually increases over hundreds of seconds before reaching a steady level. This phenomenon is known as photo-charging (PC). The AG decays according to the Becquerel law with a characteristic time of tens of seconds. Subsequent heating of the phosphors results in TL, which peaks at about 70 °C and has an activation energy of about 0.65 eV. Although the intensity of PL, AG and TL are significantly higher for the Eu-doped sample than for the un-doped sample, the values of the parameters that govern the PC, TL and AG are very similar for both samples. In particularly the emission spectra of PL, AG, and TL for both samples are very similar in shape, exhibiting a single emission band that peaks at approximately 2.8 eV with a width of 0.3 eV. This implies that the recombination or luminescence centers are intrinsic defects, presumably oxygen vacancies. The PL excitation spectra for the doped and undoped samples exhibit an excitation region around 3.9 eV and one around 4.9 eV. However, the excitation regions for the Eu-doped sample are broader and exhibit additional features indicating the participation of Eu in the absorption of photon energy. The absence of the Eu emission in the PL emission spectra suggest that the excitation energy is transferred from the Eu to the oxygen vacancy. The AG and TL creation spectra for both samples grow monotonously with increasing excitation photon energy and seem unrelated to the PL excitation spectra. This suggests that the role of Eu in enhancing the AG and TL intensity is mainly to facilitate the formation oxygen vacancies.