Luminescence of MgAl2O4 and ZnAl2O4 spinel ceramics containing some 3d ions

The luminescence properties of ceramic phosphors based on two spinel hosts MgAl2O4 and ZnAl2O4 doped with manganese ions have been studied. It has been found that the spectral properties of these phosphors can be strongly varied by changing synthesis conditions. Both types of doped ceramic spinel can serve as efficient Mn2+ green-emitting phosphors having peak emissions at 525 and 510 nm, respectively. Mn-doped MgAl2O4 spinel can also be prepared as an efficient Mn4+ red-emitting phosphor having peak emission at similar to 651 nm by using specific temperatures of heat treatment in air. It has also been shown that the conversion of Mn2+ to Mn4+ and vice versa, as well as the coexistence of Mn2+ green and Mn4+ red emissions, can be accomplished by properly chosen annealing conditions of the same initially synthesized MgAl2O4:Mn sample. Manganese doped MgAl2O4 spinel with an optimal intensity ratio of green and red emissions can be a promising single-phase bicolor phosphor suitable for the development of warm white phosphor-converted LED lamps. On the other hand, it has been determined that perfectly normal MgAl2O4 spinel cannot be doped with Mn4+ ions in contrast to partially inverse MgAl2O4 spinel. However, ZnAl2O4 samples unintentionally doped with impurity Cr3+ ions show emission spectra in the far-red region with well pronounced R, N and vibronic lines of Cr3+ luminescence due to the perfect normal spinel structure of synthesized ZnAl2O4 ceramics. Also, by partially substituting Al3+ cations for Mn2+ in ZnAl2O4 there is an opportunity to obtain Mn4+ doped or Mn4+/Cr3+ codoped far-red emitting phosphors which can be suitable for indoor plant growth lighting sources.