Chromium Ion Pair Luminescence: A Strategy in Broadband Near-Infrared Light-Emitting Diode Design

Portable near-infrared (NIR) light sources are in high demand for applications in spectroscopy, night vision, bioimaging, and many others. Typical phosphor designs feature isolated Cr³⁺ ion centers, and it is challenging to design broadband NIR phosphors based on Cr³⁺-Cr³⁺ pairs. Here, we explore the solid-solution series SrAl_11.88-GaO₁₉:0.12Cr³⁺ (x = 0, 2, 4, 6, 8, 10, and 12) as phosphors featuring Cr³⁺-Cr³⁺ pairs and evaluate structure-property relations within the series. We establish the incorporation of Ga within the magentoplumbite-type structure at five distinct crystallographic sites and evaluate the effect of this incorporation on the Cr³⁺-Cr³⁺ ion pair proximity. Electron paramagnetic measurements reveal the presence of both isolated Cr³⁺ and Cr³⁺-Cr³⁺ pairs, resulting in NIR luminescence at approximately 650-1050 nm. Unexpectedly, the origin of broadband NIR luminescence with a peak within the range 740-820 nm is related to the Cr³⁺-Cr³⁺ ion pair. We demonstrate the application of the SrAl_5.88Ga₆O₁₉:0.12Cr³⁺ phosphor, which possesses an internal quantum efficiency of ∼85%, a radiant flux of ∼95 mW, and zero thermal quenching up to 500 K. This work provides a further understanding of spectral shifts in phosphor solid solutions and in particular the application of the magentoplumbites as promising next-generation NIR phosphor host systems.