Origin of the Unusual Narrowband Near-Infrared Emission from Cr ³⁺ -Doped Oxides

Near-infrared (NIR) light is the basis of many optical and photonic applications. Trivalent chromium ions (Cr 3+ ) are excellent NIR emitters that can be efficiently excited by broadband blue light. Although the broadband NIR emission of the 4 T 2 → 4 A 2 transition can be largely regulated via crystal field engineering, the narrowband (R-line) emission of the 2 E → 4 A 2 transition is usually limited to the far-red region near 690 nm and remains much less explored. Here, we reveal that the unusual NIR R-line emission (>750 nm) in Cr 3+ -doped Sr M 12 O 19 (M = Al and Ga) magnetoplumbites, which was previously attributed to the exchange-coupled Cr 3+ –Cr 3+ pairs, arises from a unique isolated Cr 3+ center with exceptionally strong covalency of Cr 3+ –O 2– bonds. Furthermore, the experimental and computational results collectively indicate that the pertinent bond angles, along with the inductive effect of the neighboring cations, play a pivotal role in regulating the Cr 3+ –O 2– covalent interaction and thus the relative energy position of the 2 E level that determines the radiative pathways of the excited Cr 3+ . These findings not only provide a new dimension for manipulating the excited-state processes and thus the luminescence properties of Cr 3+ but also highlight its great potential for use as an activator for broadband visible-light excitable, narrowband NIR-emitting materials.