Thermal Stability Improvement of Cr3+‐Activated Broadband Near‐Infrared Phosphors via State Population Optimization

Abstract Technological progress has accelerated the researches into broadband near‐infrared (NIR) luminescent materials as next‐generation intelligent NIR light sources; however, poor thermal stability restricts the applications of NIR phosphors. Herein, new insights into Cr 3+ ‐activated NIR phosphors with improved thermal stability are provided. The photoluminescence intensity originating from the 4T2→4A2 broadband emission of CaLu 2 Al 4 SiO 12 :Cr 3+ (CLAS:Cr) with optimal electron occupation ( 4 T 2 / 2 E) increases up to 118% at 475 K compared with that at room temperature, which is the best for Cr 3+ ‐activated broadband NIR phosphor. Accordingly, new ideas for the design of thermally stable phosphors are proposed by optimizing the thermal population between 4 T 2 and 2 E states and electron migration from 2 E to 4 T 2 state. Finally, multifunctional applications of CLAS:Cr in NIR fluorescence intensity ratio thermometry, X‐ray detection, NIR phosphor‐converted light‐emitting diodes, and bio‐imaging are demonstrated.