Trace Impurity\nMatters: Origin and Modulation of Near-Infrared\nLuminescence in Stannates

High-efficiency near-infrared (NIR) emitting materials\nplay a crucial\nrole in biomedicine, agriculture, spectroscopy, <i>etc</i>., and the stannates emerge due to their broadband and millisecond-lifetime\nNIR luminescence. However, their origin has confused the community\nfor 20 years. In this work, we combine first-principles calculations\nwith experiments to explicitly unveil that the unique NIR emission\noriginates from the electronic transitions of trace impurities rather\nthan the previously proposed Sn<sup>2+</sup>-related self-trapping\nexcitons (STEs) transition in the Sn<sup>4+</sup>-based compounds,\nsuch as <i>AE</i>SnO<sub>3</sub> (<i>AE</i> =\nBa, Sr, and Ca) perovskites, Ca<sub>2</sub><i>RE</i>Sn<sub>2</sub><i>M</i><sub>3</sub>O<sub>12</sub> (<i>RE</i> = Lu, Y, and Gd; M = Al; <i>RE</i> = La; and M = Ga) and\nNa<sub>2</sub>CaSn<sub>2</sub>Ge<sub>3</sub>O<sub>12</sub> garnets,\nMg<sub>2</sub>SnO<sub>4</sub> inverse spinel, and La<sub>2</sub>Sn<sub>2</sub>O<sub>7</sub> pyrochlore. First-principles calculations provide\ndetailed insights, ruling out the contribution of Sn-related activators\nto the NIR emission due to their high formation energy and strong\nelectron–phonon coupling but confirming the contribution of\nFe<sup>3+</sup> or Cr<sup>3+</sup> centers by analyses of site occupancies\nand 3d–3d optical transitions, which effectively explain the\nexperimental emissions and their shift trends in various hosts, as\nwell as the characteristics of luminescence decays. Furthermore, Fe<sup>3+</sup>-activated BaSnO<sub>3</sub> perovskite exhibits decent NIR\nradioluminescence (∼905 nm) and thus demonstrates it has potential\nas a new type of NIR scintillator. The fresh physical picture in our\nwork thoroughly resolves the origin of stannates’ NIR luminescence\nand extends the radioluminescence to the broadband NIR wavelengths\nwith millisecond-scale lifetimes for time-resolved bioimaging and\nbiosensing.