Minimizing Bond Angle Distortion to Improve Thermal Stability of Cr³⁺ Doped Near-Infrared Phosphor

Broadband near-infrared (NIR) phosphor-converted light-emitting diodes are next-generation smart NIR light sources. However, the NIR phosphor suffers from serious thermal quenching (TQ), resulting in efficiency reduction and spectral shift. Here, a novel strategy is realized to suppress TQ by minimizing bond angle distortion, completely different from the conventional TQ suppression approach through bond length variation. Li(Sr_1−xCa_x)AlF₆:Cr³⁺ NIR phosphor is taken as an example in which rotation between the two parallel fluorine planes perpendicular to the C₃ axis in the [AlF₆] octahedron is found to dominate TQ. Increasing x from 0 to 1 reduces the amplitude of the rotation from 16.17° to 5.06°, weakening the electron–phonon coupling and, consequently, raising the TQ temperature significantly from 320 to 570 K. This mechanism is elucidated from both theoretical calculations and spectroscopic studies. The findings open a new horizon for the exploration of thermally stable NIR phosphors.