Time- and Temperature-Dependent Luminescence of Manganese Ions in Ceramic Magnesium Aluminum Spinels

Samples of magnesium aluminum spinel ceramics doped with manganese ions were prepared by a high-temperature solid-state reaction method; their potential as red-emitting phosphors was analyzed using a time-resolved luminescence spectroscopy technique, from room temperature to 10 K. It was found that in the red spectral range, the luminescence spectra of manganese ions in the MgAl₂O₄ spinel showed a narrow band peaking at 651 nm due to the emission of Mn⁴⁺ and a broader emission band in the region of 675 ÷ 720 nm; the ratio of intensities for these bands depends on the synthesis conditions. By applying a special multi-step annealing procedure, the MgAl₂O₄:Mn⁴⁺ phosphor containing only tetravalent manganese ions, Mn⁴⁺, was synthesized. Broad-band far-red emission observed from MgAl₂O₄:Mn and Mg_1.25Al_1.75O_3.75F_0.25:Mn phosphors, prepared by a conventional method of a solid-state reaction, was interpreted as coming from Mn³⁺ ions.