Uncover the Unusual Near‐Infrared Luminescence in Stannates – Interplay of Cr^{3 +} with Codopants

Abstract The unintentional incorporation of various unknown trace amounts of impurities poses challenges and controversies in identifying luminescent mechanisms of certain systems. A common approach to confirm or exclude the involvement of a certain activation center is to monitor the changes in luminescence by intentionally doping the inferred species into the crystal. Here, an intriguing luminescent phenomenon resulting from the hidden interplay between the dopants and trace impurities, is presented. In alkaline‐earth stannate perovskites, near‐infrared luminescence is observed. By combining density‐functional calculations with the well‐designed doping and codoping experimental strategies, the mechanisms behind this anomaly is successfully uncovered. It originates from the stabilization of Cr^{3 +} by codopants such as Bi and rare‐earth (RE) ions, acting as electron donors. The luminescence is almost unobservable when chromium is doped alone, and thus the traditional identification method does't work. This work emphasizes the important role of trace impurity in materials, especially the subtle change of valence state of trace impurity upon doping, which provides enlightening insights and prompts reassessments of numerous reported anomalous luminescent phenomena.