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

Abstract

The emission of Cr-doped Sc 2 O 3 (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 Sc 2 O 3 has two 6-coordinated Sc sites�the nearly octahedral site with S 6 point-group symmetry (S 6 site) and the highly distorted site with C 2 symmetry (C 2 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 Sc 2 O 3 , 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 Cr 3+ at the S 6 site. However, the broad NIR-II emission peaking at around 1280 nm cannot be attributed to Cr q (q = +2, +3, +4) at either site, suggesting the presence of possible trace impurities and phases. Ni 2+ ions at octahedral sites exhibit a narrow peak width and long lifetime, which contradict reported experimental observations. The Cr 4+ ions at a tetrahedral site, similar to that in the phase of Sc 2 O 3 with the space group Pna2 1 (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.