Lowering Cr3+-dimer excited states via In3+-mediated crystal field weakening in spinel: A route to long-wavelength broadband NIR luminescence

Broadband near-infrared (NIR) phosphors with long-wavelength emission are essential for next-generation NIR light sources. However, concurrently achieving substantial redshift, high efficiency, and thermal stability remains challenging. Here, we report a strategy to engineer highly redshifted Cr3+ emission in Mg(Ga/In)2O4 spinel via synergistic crystal field weakening and Cr3+-dimer formation. Partial substitution of Ga3+ with larger In3+ effectively reduces the crystal field strength, while heavy Cr3+ doping promotes exchange-coupled Cr3+-dimer centers, collectively depressing the dimer excited-state energy. As a result, the phosphor delivers a tunable broadband NIR emission centered at 940 nm with a full width at half maximum of 240 nm, an internal quantum efficiency of 71.56%, and thermal stability of 65% at 423 K. The dual role of In3+ in modifying the local coordination environment and stabilizing coupled Cr3+ dimers is comprehensively elucidated. A fabricated phosphor-converted LED device yields an NIR output power of 55.71 mW at 300 mA, validating its utility in spectroscopic testing, nondestructive inspection, bio-imaging, and night-vision applications.