Eu ²⁺ Luminescence Stabilization at Ln ³⁺ Sites Promoted by Interstitial Alkali‐Metal Defects

Eu 2+ ‐activated inorganic phosphors are widely used in optoelectronic fields due to their superior luminescence properties. However, stabilizing Eu 2+ dopants at trivalent Ln 3+ sites in Ln(III)‐based phosphors (Ln = Y 3+ , La 3+ , and Lu 3+ ) remains challenging because of charge imbalance and lattice instability, which has limited related luminescence mechanistic insights and materials discovery. Here, we propose a universal strategy that introduces interstitial alkali‐metal defects to break through the thermodynamic and kinetic constraints of Eu 3+ →Eu 2+ reduction in Ln(III)‐based hosts. High‐throughput experiments validate the effectiveness of this approach across multiple host systems. As a proof of concept in classical Lu 2 SiO 5 , the introduced interstitial Na drives the emission from a sharp red Eu 3+ line to a broad green Eu 2+ band at ∼500 nm with persistent luminescence exceeding 10 h. The interstitial Na defects act dually by promoting Eu 3+ →Eu 2+ reduction and serving as an afterglow energy reservoir. The remarkable afterglow property enables multifunctional applications in advanced information encryption and fingerprint recognition. This work demonstrates a promising approach to exploring Eu 2+ luminescence properties in established materials via defect‐assisted reductive engineering, opening avenues for developing new optofunctional materials.