Enhanced Photoluminescence Quantum Efficiency and Thermal Stability of Cyan‐Emitting Li 2 CaSiO 4 :Eu 2+ Phosphor via a Heterovalent Substitution Strategy

Highly efficient Eu 2+ ‐activated phosphors are indispensable for solid‐state lighting, yet the parasitic Eu 3+ emission remains a significant efficiency killer. Herein, we report a robust strategy to suppress Eu 3+ formation in Li 2 CaSiO 4 :Eu 2+ via a heterovalent substitution strategy by combining a two‐step sintering process with charge compensation. We effectively promote the Eu 3+ →Eu 2+ reduction, boosting the external quantum efficiency (EQE) from 22% to 60%, and the deep defect levels induced by Si 4+ ‐P 5+ heterovalent substitution impart superior thermal stability to Li 2 CaSi 0.96 P 0.04 O 4 :Eu 2+ (94%@423 K), while retaining a remarkable 98% of its initial intensity after 30 days of water immersion. The as‐fabricated phosphor‐converted light‐emitting diodes achieve a high color rendering index ( R a = 90.6) and a 50% reduction in blue‐light hazard, demonstrating great potential for healthy illumination. Notably, this Eu 2+ stabilization strategy also effectively enhances the emission intensity of Li 2 CaGeO 4 :Eu 2+ and Li 2 MSiO 4 :Eu 2+ (M = Sr, Ba), thus establishing a general paradigm for the rational design of high‐performance phosphors for next‐generation light sources.