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

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 K₃Y_1-xLu_x(BO₃)₂:Eu²⁺ to enable tunable broadband emission from 650 to 690 nm via Y³⁺→Lu³⁺ 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 KO₆ and Lu/YO₆ 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 Eu²⁺ 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 K₃Lu(BO₃)₂:Eu²⁺ 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.