Unusual manganese luminescence channels in low doped MgAl2O4

We have done the rigorous characterizing of the Mn-doped MgAl₂O₄ (Mn = 0, 0.02, 0.04, 0.1 wt%) single crystals and the comprehensive analysis of their optical properties theoretically supported by the Modified Crystal Field Theory to obtain the most complete picture, to date, of the diverse absorption and luminescence behavior of MgAl₂O₄:Mn spinels. The study has focused on exploring the formation and coexistence of multivalent states of Mn and identifying the conditions needed for activating luminescence channels. We found that Mn takes the charge state +2, +3 and + 4 and occupies both tetrahedral and octahedral positions. Six luminescence channels related to Mn ions were identified for the first time. Two channels activate the luminescence at 520 nm: intra-atomic transitions in tetrahedral coordinated Mn²⁺ ions under VIS-excitation and a recombination Mn³⁺→Mn²⁺→Mn³⁺ process involving Mn³⁺ in tetrahedral positions under X-ray. Mn³⁺ in tetrahedrons and octahedrons activate luminescence channels at 733 and 926 nm under UV-C and VIS excitations. Two distinct processes involving Mn³⁺ and Mn⁴⁺ ions in octahedrons result in the luminescence at 651 nm. The first is an intra-atomic transition in the Mn⁴⁺ occurring under VIS-excitation. The second appears through a charge transfer Mn³⁺→Mn⁴⁺→Mn³⁺ in the octahedrons under UV-B-excitation.