Modulation of Sharp‐Line Red Emissions to Efficient Broadband Near‐Infrared Luminescence in Cr ³⁺ ‐Activated Spinel Phosphors via Defect Engineering

Abstract Spectral tuning toward longer emission wavelengths, while maintaining high quantum efficiency (QE) and thermal stability, remains a formidable challenge for near‐infrared (NIR) luminescent materials. Herein, a Mg‐deficiency strategy is proposed to achieve the redshift and broadening of Cr 3+ emission in MgAl 2 O 4 spinel without sacrificing QE and thermal stability. The emission spectrum shifts from sharp lines around 700 nm for stochiometric MgAl 2 O 4 :Cr 3+ to an ultra‐broadband centered at 860 nm for Mg‐deficient Mg 0.9 Al 2 O 3.9 :Cr 3+ , with a profound increase in full width at half maximum (FWHM) from ≈85 to 303 nm. Meanwhile, the Mg 0.9 Al 2 O 3.9 :0.05Cr 3+ phosphor exhibits an internal QE of 87% and can maintain 80% of initial emission intensity at 150 °C. Moreover, tunable emission bands peaking from 685 to 908 nm are achieved for Mg 0.9 Al 2 O 3.9 : x Cr 3+ by varying the Cr 3+ concentration. The Cr 3+ broadband emission can be attributed to the formation of Al Mg anti‐site defects, while the overall lattice contraction caused by the Mg‐deficiency contributes to the maintenance of high QE and low thermal quenching. Finally, a NIR phosphor‐converted light‐emitting diode (pc‐LED) is fabricated and its application in nondestructive testing is demonstrated. This study initiates a new way to improve the spectral performance of NIR phosphors while preserving high QE and thermal stability.