Synthesis, Structure and Luminescence Properties of Mn-doped MgAl₂O₄ Red-Emitting Phosphors with Varying Sintering Temperature

Oxide matrix red-emitting phosphors are deemed as excellent color converters for white light emitting diodes (WLEDs) and laser diodes (LDs). Manganese-doped MgAl₂O₄ powder was synthesized by a solid-state reaction method at different sintering temperatures. Microstructure shows that grain size is mainly in the range of 0.2–5 μm, and grain agglomeration occurs with increased sintering temperature. XPS analysis indicates that the doped Mn ion exhibits a valence state of + 4 within the MgAl₂O₄ matrix. The diffraction peak of the phosphors is shifted by the sintering temperature, which affects lattice constant. Upon excitation by 300 nm ultraviolet light, the samples emit asymmetric broadband red light within the range of 620–720 nm, attributed to Mn⁴⁺ ion’s transition from ²E_g to ⁴A_2g states. With the increasing temperature, the main emission peak shifts from 677 nm to 650 nm, ascribed to the change in energy level (²E_g) resulting from the reduction of Al₂O₃ phase. Crystal field theory confirmed that Mn⁴⁺ ions are within a strong crystal field environment created by MgAl₂O₄ matrix. By affecting particle size and crystallinity, the sintering temperature influences the fluorescence lifetime of the Mn⁴⁺ ion. Notably, these red-emitting phosphors exhibits remarkable thermal stability as their emission intensity remains approximately at 58% of initial intensity even at elevated temperature (435 K). Consequently, Mn⁴⁺: MgAl₂O₄ red-emitting phosphors with high thermal stability render them promising candidates for WLED applications.