Origin of the Unusual Narrowband Near-Infrared Emission from Cr³⁺-Doped Oxides

Near-infrared (NIR) light is the basis of many optical and photonic applications. Trivalent chromium ions (Cr³⁺) are excellent NIR emitters that can be efficiently excited by broadband blue light. Although the broadband NIR emission of the ⁴T₂ → ⁴A₂ transition can be largely regulated via crystal field engineering, the narrowband (R-line) emission of the ²E → ⁴A₂ 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 Cr³⁺-doped SrM₁₂O₁₉ (M = Al and Ga) magnetoplumbites, which was previously attributed to the exchange-coupled Cr³⁺-Cr³⁺ pairs, arises from a unique isolated Cr³⁺ center with exceptionally strong covalency of Cr³⁺-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 Cr³⁺-O^2- covalent interaction and thus the relative energy position of the ²E level that determines the radiative pathways of the excited Cr³⁺. These findings not only provide a new dimension for manipulating the excited-state processes and thus the luminescence properties of Cr³⁺ but also highlight its great potential for use as an activator for broadband visible-light excitable, narrowband NIR-emitting materials.