Hydrofluoric Acid-Free\nBroadband Near-Infrared Phosphors\nK<sub>2</sub>LiMF<sub>6</sub>:Cr<sup>3+</sup> with Zero-Thermal Quenching:\nStructure, Luminescence, and Application

Near-infrared (NIR) phosphor-converted light-emitting\ndiodes (pc-LEDs)\nare considered promising light sources for night vision, food analysis,\nbiomedicine, and plant growth. Yet, the application potential of this\ntechnology is vulnerable to the function degradation of the phosphors\nused, such as thermal quenching, which needs to be addressed urgently.\nHerein, the NIR phosphors K<sub>2</sub>LiMF<sub>6</sub>:Cr<sup>3+</sup> (M = Al, Ga, In) with a cubic double-perovskite structure synthesized\nby a green hydrofluoric acid-free hydrothermal method exhibit outstanding\nthermal stability. Under 450 nm excitation, the as-synthesized K<sub>2</sub>LiMF<sub>6</sub>:Cr<sup>3+</sup> phosphors all exhibited broadband\nNIR emission covering 650–1000 nm peaking at 755–780\nnm. The prepared K<sub>2</sub>LiAlF<sub>6</sub>:Cr<sup>3+</sup> phosphor\nshows a unique zero-thermal quenching performance (I<sub>423 K</sub>/I<sub>298 K</sub> = 102%). The comprehensive effects of a wide\nband gap, large thermal energy barrier, weak electron–phonon\ncoupling effect, and high structural rigidity are responsible for\nthe suppression of thermal quenching in this material. The output\npower of the NIR pc-LED device reached 285 mW at 100 mA. This series\nof phosphors has promise in night vision and bioimaging applications.