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 Eu³⁺ to Eu²⁺ in fluoride NSs. Density functional theory calculations reveal that Eu²⁺ is thermodynamically more stable in mixed-anion hosts (SrFCl and SrFBr) than in SrF₂. During the anion-exchange process from SrF₂ seeds, a high density of F-centers is generated, which act as electron reservoirs to drive the reduction of Eu³⁺ to Eu²⁺. As a result, SrF₂:Eu exhibits only Eu³⁺ emission, SrFCl:Eu shows coexisting Eu²⁺/Eu³⁺ emissions, and SrFBr:Eu displays dominant Eu²⁺ 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.