Underestimated Color Centers: Defects as Useful Reducing Agents in Lanthanide‐Activated Luminescent Materials

Inorganic hosts, such as SrB(4)O(7) or certain nitrides, intrinsically stabilize Eu(2+) even when the dopant is an Eu(3+)‐based precursor and reducing conditions are not employed in the synthesis. Although this concept is well known in the synthesis of phosphorescent materials, the mechanistic details are scarcely understood. Herein, we demonstrate that trapped charge carriers, such as color centers, can also act as redox partners to stabilize certain oxidation states of activators. Eu‐activated CsMgCl(3) and CsMgBr(3) are used as examples. Upon doping with EuCl(3) and in the absence of reducing conditions during the synthesis, dominant cyan or green luminescence from Eu(2+) ions was observed. Photoluminescence spectroscopy at 10 K revealed that the reduction is correlated to color centers localized at defects. Although defects are typically undesired in phosphors, we have shown that their role may be underestimated and they could be used on purpose in the preparation of selected inorganic phosphors.