Mechanistic Insights into Dual NIR Emission from Cr-Doped Sc₂O₃ via First-Principles Calculations

The emission of Cr-doped Sc₂O₃ (space group Ia3̅, No. 206) phosphor features a broad band in NIR-I (700-1000 nm) and another in NIR-II (1000-1700 nm), which is significant for spectral analysis and medical applications. Although Sc₂O₃ has two 6-coordinated Sc sites─the nearly octahedral site with S₆ point-group symmetry (S₆ site) and the highly distorted site with C₂ symmetry (C₂ site)─the origin of the dual-band emission remains widely debated. In this study, we performed first-principles calculations to investigate the properties of Cr and Ni dopants in Sc₂O₃, including preference in site occupation, valence state, ligand field strength, Stokes shift, and line shape. Our calibrated calculations conclusively determined that the NIR-I emission peak at 840 nm is due to Cr³⁺ at the S₆ site. However, the broad NIR-II emission peaking at around 1280 nm cannot be attributed to Cr (q = +2, +3, +4) at either site, suggesting the presence of possible trace impurities and phases. Ni²⁺ ions at octahedral sites exhibit a narrow peak width and long lifetime, which contradict reported experimental observations. The Cr⁴⁺ ions at a tetrahedral site, similar to that in the phase of Sc₂O₃ with the space group Pna2₁ (No. 33), show the most consistent line shape and emission decay rates with experimental data. A systematic first-principles approach incorporating the line shape calculation can be useful to resolve the issues in identifying luminescent centers in systems involving intrinsic defects and dopants.