Overcoming Rigidity-Flexibility Constraints in Garnet Phosphors for Broadband and Thermally Stable NIR Emission

Achieving near-infrared (NIR) phosphors with broadband emission, robust thermal stability, high internal quantum efficiency (IQE), and excellent chemical durability remains a longstanding challenge, largely due to the fundamental trade-off between structural rigidity and flexibility. Here, we overcome this constraint by designing a novel series of garnet-type phosphors, BaY₂Ga_4-xGeO₁₂:xCr³⁺ (BYGG:xCr³⁺). The optimized composition BYGG:0.06Cr³⁺ delivers an IQE of 93%, remarkable anti-thermal quenching behavior (103% emission retention at 423 K), and outstanding chemical stability (retaining 97% emission after 35 days in water). Notably, fine-tuning the Cr³⁺ concentration broadens the emission bandwidth from 77 to 194 nm while preserving strong thermal stability (93%@423 K). This exceptional performance arises from the synergistic combination of ultra-high structural rigidity (Debye temperature, Θ_D = 987 K), wide bandgap (5.10 eV), weak crystal field strength, and optimized ⁴T₂ excited-state population. A fabricated NIR phosphor-converted LED (pc-LED) device demonstrates an NIR radiated power of 189.35 mW at 100 mA and a photoelectric conversion efficiency up to 12%. These results establish NIR phosphor BYGG:xCr³⁺ as a promising candidate for next-generation NIR pc-LED applications in night vision, non-destructive inspection, and biomedical imaging.