Precisely Manipulating the Self-Reduction of Manganese in MgGa₂O₄ through Lithium Incorporation for Optical Thermometry and Anti-Counterfeiting

Manganese (Mn) doped MgGa2O4 (MGO) phosphors are prepared by a solid-state reaction technique in air. Upon UV light excitation, the featured green and red emissions of Mn2+ and Mn4+, respectively, are detected in the Mn-doped MGO phosphors, indicating the incomplete self-reduction behavior of Mn in the MGO host lattices. To clarify the incomplete self-reduction behavior, theoretical calculations based on density functional theory and bond energy theory are performed. Moreover, the self-reduction behavior of Mn can be manipulated by the incorporation of Li+, leading to the precise regulation of the Mn2+ and Mn4+ contents in the MGO host lattices. Based on the ionic radii, the inhibited self-reduction mechanism is triggered by Li+ occupying the tetrahedral Mg2+ site, leading to the reduced occupancy of tetrahedral Mg2+ sites by Mn ions, and forcing Mn to take up the octahedral Ga3+ sites. Furthermore, the afterglow properties of phosphors with wide and continuous multiple traps are studied in detail. Finally, based on unique luminescence properties of the phosphors, their potential application in ratiometric optical thermometer and optical anti-counterfeiting is also systematically exploited.