Integrated Computational and Spectroscopic Investigation of Mn ⁵⁺ -Doped LaSr ₂ AlO ₅ with Intense Green Color and NIR-II Emission

The Mn 5+ -doped lanthanum strontium aluminate (LaSr 2 AlO 5 ) was synthesized using conventional solid-state processes. Structural/optical characterizations and DFT calculations were performed to enable a thorough understanding of the Mn doping effect on site occupancy and optical properties. The compounds were found to exhibit a double-band absorption characteristic, i.e., a dominant band in the visible to near-infrared (NIR) range due to Mn 5+ intra-atomic transitions, and a sub-band in the UV–visible range resulting in ligand-to-metal charge transfer (LMCT) from O 2– to Mn 5+ . This dual absorption renders the body color to be tuned from pale green to dark green with increasing Mn 5+ concentration. Crystal field analysis based on absorption spectra indicates that Mn 5+ is located in a tetrahedral site with Dq/ B = 6.5. Furthermore, LaSr 2 AlO 5:Mn 5+ displays a narrow-band NIR-II emission at 1170 nm, arising from the spin-forbidden 1 E → 3 A 2 transition. These results might reveal the potential of these compounds as nontoxic green pigments and NIR-II phosphors.