Activating High‐Energy 5 D 1,2,3 Radiative Transitions of Eu 3+ via Defect and Phonon Engineering in Apatite Oxysilicate

ABSTRACT Achieving radiative emissions from high‐energy excited states 5 D 1,2,3 of Eu 3+ has remained a hard challenge in photoluminescence materials due to severe concentration quenching and multi‐phonon non‐radiative transitions. Herein, we demonstrate a novel stepwise annealing strategy (650°C–850°C) in a reducing atmosphere to activate and stabilize 5 D 1,2,3 → 7 F J (blue‐green) emissions alongside conventional 5 D 0 → 7 F J (red) transitions in apatite‐type NaLa 9 (SiO 4 ) 6 O 2 :Eu 3+ . Under a higher temperature (> 850°C), the distinct band emission from induced Eu 2+ appears, accompanying with progressive quenching of 5 D 1,2,3 and 5 D 0 transitions. This defect engineering approach, involving controlled oxygen redispersion, successfully suppresses multi‐phonon relaxation and creates a favorable “phonon‐glass” state within the lattice. Notably, emissions from the 5 D 1,2 states remain detectable even at a high Eu 3+ doping of 30 mol%, far exceeding the severe quenching limits (typically