Tracing the origin of near-infrared emissions emanating from manganese (II)

Abstract The enduring enigma surrounding the near-infrared (NIR) emission of Mn 2+ continues to ignite intense academic discussions. Numerous hypotheses have emerged from extensive research endeavors to explain this phenomenon, such as the formation of Mn 2+ –Mn 2+ ion pairs, Mn 2+ occupying cubically coordinated sites, as well as conjectures positing the involvement of Mn 3+ oxidized from Mn 2+ or defects. Despite these diverse and valuable insights, none of the hypotheses have yet achieved broad consensus. In this study, we have observed prolonged fluorescence lifetimes (~10 ms) for the NIR emissions of Mn 2+ ions, hinting at these ions occupying the high-symmetry octahedral sites inherent to the garnet lattice. This inference is supported by the corroborating results from X-ray absorption fine structure analysis and first-principles calculations. The intense crystal field of octahedral sites, similar to that of AlO 6 , facilitates the splitting of d – d energy levels, thereby inducing a red-shift in the emission spectrum to the NIR region due to the transition 4 T 1 ( 4 G) → 6 A 1 ( 6 S) of isolated Mn 2+ . Our findings not only offer a plausible rationale for the NIR emission exhibited by other Mn 2+ -activated garnet phosphors but also pave a definitive route towards understanding the fundamental mechanisms responsible for the NIR emission of Mn 2+ ions.