High-Temperature Negative-Thermal-Quenching in Broadband NIR Light-Emitting ScF₃:Cr³⁺ Phosphors

Broadband near-infrared (NIR) phosphors with superior thermal stability are critical enablers of high-power NIR pc-LEDs, emerging as essential light sources for NIR spectroscopy applications. In this work, an unprecedented high-temperature negative thermal quenching (NTQ) phenomenon persisting up to 500 K is reported in Cr³⁺-activated ScF₃ phosphors, which emit broadband NIR emission in the 700–1200 nm (λ_em = 850 nm, FWHM = 132 nm). The theoretical calculations employing the Exchange-Charge Model (EMC) for crystal field parameters of Cr³⁺ ions in [ScF₆] octahedral sites remain in perfect agreement with the experimental results. Critically, Yb³⁺-codoping enables efficient Cr³⁺ → Yb³⁺ energy transfer, achieving substantial spectral broadening (FWHM = 254 nm) and significantly enhanced thermal stability. The temperature-dependent X-ray diffraction (XRD) and theoretical calculations reveal that anisotropic F-atom vibrations drive negative thermal expansion (NTE) in cubic ScF₃, distorting [ScF₆] octahedra. It is demonstrated that synergistic electron-phonon coupling and NTE-driven structural dynamics underpin the superior thermal stability of NIR emission. These findings establish a design paradigm for new broadband NIR phosphors with outstanding thermal stability for next-generation high-power NIR pc-LED applications.