F‐Center‐Mediated Valence Engineering of Europium in Fluoride Nanoscintillators Toward Diverse Optical Sensing

Lanthanide‐doped fluoride nanoscintillators (NSs) are promising candidates for x‐ray imaging, owing to their high environmental stability, controllable nanostructures, and tunable multicolor emissions. However, their performance is typically limited by the intrinsically low radiative efficiency of parity‐forbidden 4f–4f transitions in trivalent lanthanide activators, and effective strategies for controlling lanthanide valence in fluoride NSs remain elusive. Here, an F‐center‐mediated valence engineering strategy is developed to enable the controlled transformation of Eu 3+ to Eu 2+ in fluoride NSs. Density functional theory calculations reveal that Eu 2+ is thermodynamically more stable in mixed‐anion hosts (SrFCl and SrFBr) than in SrF 2 . During the anion‐exchange process from SrF 2 seeds, a high density of F‐centers is generated, which act as electron reservoirs to drive the reduction of Eu 3+ to Eu 2+ . As a result, SrF 2 :Eu exhibits only Eu 3+ emission, SrFCl:Eu shows coexisting Eu 2+ /Eu 3+ emissions, and SrFBr:Eu displays dominant Eu 2+ emission with significantly enhanced scintillation intensity. This valence tunability not only improves x‐ray imaging performance but also enables ratiometric temperature sensing based on dual‐emission characteristics. This work establishes a general F‐center‐mediated strategy for lanthanide valence control, providing new opportunities for designing high‐performance NSs toward diverse optical sensing applications.