Intense Luminescence and Good Thermal Stability in a Mn²⁺-Activated Mg-Based Phosphor with Self-Reduction

White light-emitting diodes provide widespread applications in lighting, electronic equipment, and high-tech displays. However, thermal quenching effect severely limits their practical application. Here, we developed an orange-red phosphor β-KMg(PO₃)₃:Mn²⁺, which emits bright orange-red light when excited by ultraviolet light without the energy transfer of sensitizer, owing to the strong crystal field provided by β-KMg(PO₃)₃ for Mn²⁺. The self-reduction of Mn⁴⁺ → Mn²⁺ and good thermal stability have been realized in an ambient atmosphere. The defect types were verified by X-ray photoelectron spectroscopy, and cationic vacancy plays a significant role in the self-reduction of Mn⁴⁺ → Mn²⁺. Furthermore, the properties of the trap energy levels were studied by thermoluminescence. The recombination luminescence of the detrapped carriers released from the deep trap levels at high temperatures suppresses the luminescence loss of thermal quenching. Moreover, the trap energy levels play an important role in the mechanoluminescence of β-KMg(PO₃)₃:Mn²⁺. This work emphasizes the significance of the defects in the luminescent characteristics and opens up a new approach for the development of advanced optical functional materials.