Activating Deep‐Red Emission in Borate Phosphors via Low‐Coordination Engineering for Efficient Plant Lighting

Abstract The development of blue‐excitable deep‐red phosphors represents a key advancement in next‐generation agricultural lighting systems. Herein, a low‐coordination polyhedral engineering strategy is proposed using novel K 3 Y 1‐ x Lu x (BO 3 ) 2 :Eu 2+ to enable tunable broadband emission from 650 to 690 nm via Y 3+ →Lu 3+ substitution, achieving blue‐light‐excitable deep‐red luminescence that is uncommon among borate phosphors. Structural analysis reveals that Lu‐induced lattice compression shortens the bond lengths in KO 6 and Lu/YO 6 octahedra, enhancing crystal field splitting and enabling deep‐red emission with a full width at half maximum (FWHM) of 147 nm. Simultaneously, the photoluminescence quantum yield and thermal stability are significantly improved due to optimized Eu 2+ site occupancy and suppression of non‐radiative transitions. Density functional theory calculations and Hirshfeld surface analysis confirm that the shortened bond lengths and distorted polyhedra contribute to the enhanced luminescence properties. Prototype phosphor‐converted LEDs based on K 3 Lu(BO 3 ) 2 :Eu 2+ demonstrate excellent spectral overlap with chlorophyll and phytochrome absorption, accelerating wheat growth by 20% under tailored illumination. This work not only addresses the long‐standing challenge of achieving deep‐red emission in borate phosphors but also introduces a promising material for next‐generation agricultural lighting applications.