Stabilizing Mn 5+ Valence States via Site Engineering and Oxidizing Sintering for High‐Efficiency Near‐Infrared II Light Sources

Abstract The development of highly efficient near‐infrared (NIR) luminescent materials is essential for advancing next‐generation compact light sources. Nevertheless, achieving efficient emission in the second NIR spectral window (NIR‐II, 1000–1700 nm) remains a considerable challenge. In this study, a series of apatite‐structured compounds R 5 (PO 4 ) 3 Cl:Mn 5+ (R = Ca, Sr, Ba) is design and synthesize to systematically investigate the effect of host cation variation on the luminescence behavior of Mn 5+ . Density functional theory (DFT) calculations and electron paramagnetic resonance (EPR) spectroscopy reveal that the incorporation of Mn 2+ into Ba 2+ sites is suppressed due to the substantial ionic radius mismatch between Mn 2+ and Ba 2+ . As a result, Mn 5+ ions preferentially occupy the P 5+ sites, leading to the highest luminescence efficiency observed in Ba 5 (PO 4 ) 3 Cl:Mn 5+ . Furthermore, sintering in an oxidizing atmosphere notably boosts the luminescence intensity of Ba 5 (PO 4 ) 3 Cl:Mn 5+ , achieving a high internal/external quantum efficiency (IQE/EQE) of 86.9%/51.5%. Utilizing this optimized phosphor, a NIR‐II phosphor‐converted light emitting diode (pc‐LED) is fabricated by coating it onto a red‐light emitter (blue LED + red phosphor (Sr, Ca)AlSiN 3 :Eu 2+ ), resulting in a record NIR output power of 326.6 mW at 300 mA. As a compact NIR light source, this device demonstrates high potential for applications in infrared optical imaging.