An Insight into the Various Defects-Induced Emission in MgAl2O4 and Their Tunability with Phase Behavior: Combined Experimental and Theoretical Approach

The present work describes various defects-induced tunable emission behavior of MgAl2O4 compounds obtained after annealing at different temperatures through a sol–gel combustion route. Multiple defect centers, such as F, F2, F+, and F2 2+ and different shallow and deep defects were found to be present inside the band gap, as confirmed by the lifetime and time-resolved emission spectroscopy (TRES) studies. The tunable emission characteristic at different annealing temperatures could be linked with the phase behavior of the spinel. Excitation wavelength variation suggested that a photoconversion process of F to F+ centers was involved with λex = 250 nm, followed by a trapping–de-trapping mechanism of the released electrons within different trap states. An exchange mechanism of electrons in between conduction band and shallow states was also observed at room temperature, which was absent at low temperature, as indicated by the emission profile. These observations render it to be a potential optical-based thermal sensor material. DFT-based calculations were carried out for both pure and various oxygen-vacancy-introduced spinel phases in order to characterize the different defect states inside the band gap. Finally, on the basis of theoretical and experimental results, a model has been proposed to explain the mechanisms related to emission tunability.