Valence State Control of Manganese in MgAl2O4:Mn4+ Phosphor by Varying the Al2O3 Crystal Form

Mn4+ activated red phosphor is one of the current research hot-spots in the field of white light emitting diodes (wLEDs). The shortest emission of Mn4+2E -> (4)A(2) transition in aluminate is 651 nm realized in MgAl2O4, but the doped manganese ions exists in multiple valence states (+2/+4/+3, etc.) due to the fact that there exist two cationic sites (Mg2+/Al3+) forming tetrahedron or octahedron coordination in the spinel structure. In this study, variation of the Al2O3 polymorphs (gamma/alpha ratio) in the starting materials and post-annealing were used to control the doping sites and valence state of manganese ions in the MgAl2O4 structure. The results show that a high alpha/(alpha+gamma) ratio of starting Al2O3 favors the formation of Mn2+ while a low alpha/(alpha+gamma) ratio of starting Al2O3 favors the formation of Mn4+ dopant. By using highly active nano-gamma-Al2O3 as the Al3+-bearing source, the occupancy of manganese ions in the Mg2+ site and the formation of Mn2+ valence state were effectively suppressed. Bright and pure MgAl2O4:Mn4+ phosphors in which only the red luminescence from Mn4+ was observed in the visible spectral region were successfully prepared via once heat treatment at 1550 degrees C for 5 h in air. The intrinsic reason for the dependence of manganese doping valence state on the Al2O3 polymorph lies in that the reactivity of Al2O3 determined the sequences of doping reactions and then the doping site/valence of manganese ions in MgAl2O4:Mn. All the above data demonstrated that the control of reaction sequences was a new method to regulate the valence state of manganese in aluminate phosphors.