Awakening the Dumb Site to Realize Ultra-Broadband NIR Phosphor

Near-infrared (NIR) light sources have gained immense popularity in recent years due to their wide range of applications in various fields, including spectroscopy and biomedical imaging. However, the limited emission bandwidth of NIR phosphors is a significant bottleneck in their development. Here, a novel strategy is reported to broaden the emission bandwidth of NIR phosphors by awakening the dumb site. Na4M3Ta(PO4)(6): Cr3+ (M = Al3+, Ga3+, In3+) phosphor is synthesized, which exhibits a greatly broadened bandwidth from 134 to 232 nm. Structural and spectral analysis reveals that the NaO6 octahedral site has a severe t(2g)-type distortion, making it a dumb site for Cr3+. By introducing larger In3+ at the M site, the angular distortion at the NaO6 site decreases to the normal range, enabling the luminescence of Cr3+ again. Along with the broadened bandwidth, the emission peak also redshifts from 802 to 977 nm, giving advantages in NIR spectroscopy applications. Interestingly, the awakened luminescence at the NaO6 site shows even better luminescence properties than the original M/TaO6 octahedral site. These findings reveal a novel insight into the luminescence of Cr3+ at the octahedral site, which could potentially revolutionize the design of NIR phosphors.