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

The enduring enigma surrounding the near-infrared (NIR) emission of Mn²⁺ continues to ignite intense academic discussions. Numerous hypotheses have emerged from extensive research endeavors to explain this phenomenon, such as the formation of Mn²⁺-Mn²⁺ ion pairs, Mn²⁺ occupying cubically coordinated sites, as well as conjectures positing the involvement of Mn³⁺ oxidized from Mn²⁺ 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 Mn²⁺ 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 AlO₆, 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 ⁴T₁(⁴G) → ⁶A₁(⁶S) of isolated Mn²⁺. Our findings not only offer a plausible rationale for the NIR emission exhibited by other Mn²⁺-activated garnet phosphors but also pave a definitive route towards understanding the fundamental mechanisms responsible for the NIR emission of Mn²⁺ ions.