Mechanistic\nInsights into Dual NIR Emission from Cr-Doped\nSc<sub>2</sub>O<sub>3</sub> via First-Principles Calculations

The emission of Cr-doped Sc<sub>2</sub>O<sub>3</sub> (space\ngroup\nIa3̅, No. 206) phosphor features a broad band in NIR-I (700–1000\nnm) and another in NIR-II (1000–1700 nm), which is significant\nfor spectral analysis and medical applications. Although Sc<sub>2</sub>O<sub>3</sub> has two 6-coordinated Sc sitesthe nearly octahedral\nsite with <i>S</i><sub>6</sub> point-group symmetry (<i>S</i><sub>6</sub> site) and the highly distorted site with <i>C</i><sub>2</sub> symmetry (<i>C</i><sub>2</sub> site)the\norigin of the dual-band emission remains widely debated. In this study,\nwe performed first-principles calculations to investigate the properties\nof Cr and Ni dopants in Sc<sub>2</sub>O<sub>3</sub>, including preference\nin site occupation, valence state, ligand field strength, Stokes shift,\nand line shape. Our calibrated calculations conclusively determined\nthat the NIR-I emission peak at 840 nm is due to Cr<sup>3+</sup> at\nthe <i>S</i><sub>6</sub> site. However, the broad NIR-II\nemission peaking at around 1280 nm cannot be attributed to Cr<sup><i>q</i></sup> (<i>q</i> = +2, +3, +4) at either\nsite, suggesting the presence of possible trace impurities and phases.\nNi<sup>2+</sup> ions at octahedral sites exhibit a narrow peak width\nand long lifetime, which contradict reported experimental observations.\nThe Cr<sup>4+</sup> ions at a tetrahedral site, similar to that in\nthe phase of Sc<sub>2</sub>O<sub>3</sub> with the space group Pna2<sub>1</sub> (No. 33), show the most consistent line shape and emission\ndecay rates with experimental data. A systematic first-principles\napproach incorporating the line shape calculation can be useful to\nresolve the issues in identifying luminescent centers in systems involving\nintrinsic defects and dopants.