Ultraviolet afterglow of undoped MgAl2O4 derived via solution combustion

Combining experiments to density functional calculations, photoluminescence (PL) and afterglow properties of solution-combustion derived MgAl₂O₄ were investigated. Under the 255 nm photoexcitation, the PL spectrum of undoped MgAl₂O₄ exhibits two ultraviolet subbands peaking at 3.41 eV (363.6 nm) and 3.18 eV (389.9 nm), respectively. After the irradiation of a high-pressure mercury lamp is terminated, undoped MgAl₂O₄ exhibits ultraviolet afterglow that lasts for about 20.5 min. The afterglow spectrum of undoped MgAl₂O₄ consists of dual ultraviolet subbands peaking at about 360 and 390 nm, correspondingly. Within the scheme of meta generalized gradient approximation, defect energy levels of oxygen vacancies (F⁰, F⁺, F²⁺), aluminum vacancy and magnesium vacancy are determined numerically via density functional calculations. In terms of the defect energy levels of oxygen and aluminum vacancies, optical transitions arising from these defects are constructed for undoped MgAl₂O₄, both the dual-subband PL and the dual-subband afterglow can be interpreted. The ultraviolet afterglow with dual subbands validates our hypothesis of the oxygen and cation vacancies in host acting as the origins of the afterglow of undoped MgAl₂O₄. Apart from opening a window for exploring the afterglow mechanisms of rare-earth doped MgAl₂O₄, the validity of the hypothesis unlocks the power of harnessing intrinsic defects as luminescence center of afterglow to develop novel afterglow materials.