Theoretical investigations of the origin of persistent luminescence in spinel oxides MgGa2O4 and MgAl2O4

Abstract: MgGa2O4 and MgAl2O4 have attracted significant interest due to their unique intrinsic persistent luminescence, offering promising potential for various applications. In this paper, from the perspective of defect physics, we systemically investigate the origin of persistent luminescence phenomena in pristine MgGa2O4 and MgAl2O4, employing accurate hybrid functional calculations. Our results show that vacancies and antisite defects involving the two cations are the dominant point defects in both materials. Our calculated optical excitation and emission peaks associated with the MgGa defect agree well with the experimentally observed blue luminescence peak at about 2.9 eV in MgGa2O4. In MgAl2O4, the intradefect optical transition within the VO-MgAl donor-acceptor defect complex is identified as a likely origin for the observed 2.7 eV emission peak. Furthermore, the calculated radiative recombination coefficients of MgGa and VO-MgAl are significantly higher than their nonradiative counterparts, supporting their roles as efficient luminescent centers. Our results regarding the optical processes of oxygen vacancy VO in MgGa2O4 and MgAl2O4 are also in good agreement with experimental results. Based on the calculated defect thermodynamic transition levels, the intrinsic persistent luminescence in MgGa2O4 and MgAl2O4 may be attributed to electron traps, GaMg and VO, in the former and a hole trap, MgAl, in the latter. Donor-acceptor defect complexes (VO+VMg and VO+VGa) are also found to serve as effective carrier trapping centers in MgGa2O4. The calculated trap depths are also consistent with thermoluminescence spectroscopy measurements.