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We use density functional theory (DFT) to study the molecular structure and electronic band structure of Sr 2 Si 5 N 8 :Eu 2+ doped with trivalent lanthanides (Ln 3+ = Ce 3+ , Tb 3+ , Pr 3+ ). Li + was used as a charge compensator for the charge imbalance caused by the partial replacement of Sr 2+ by Ln 3+ . The doping of Ln lanthanide atom causes the structure of Sr 2 Si 5 N 8 lattice to shrink due to the smaller atomic radius of Ln 3+ and Li + compared to Sr 2+ . The doped structure's formation energy indicates that the formation energy of Li + , which is used to compensate for the charge imbalance, is the lowest when the Sr2 site is doped. Thus, a suitable Li + doping site for double-doped lanthanide ions can be provided. In Sr 2 Si 5 N 8 :Eu 2+ , the doped Ce 3+ can occupy partly the site of Sr 1  2+ ([SrN 8 ]), while Eu 2+ accounts for Sr 1  2+ and Sr 2  2+ ([SrN 10 ]). When the Pr 3+ ion is selected as the dopant in Sr 2 Si 5 N 8 :Eu 2+ , Pr 3+ and Eu 2+ would replace Sr 2  2+ simultaneously. In this theoretical model, the replacement of Sr 2+ by Tb 3+ cannot exist reasonably. For the electronic structure, the energy level of Sr 2 Si 5 N 8 :Eu 2+ /Li + doped with Ce 3+ and Pr 3+ appears at the bottom of the conduction band or in the forbidden band, which reduces the energy bandgap of Sr 2 Si 5 N 8 . We use DFT+U to adjust the lanthanide ion 4f energy level. The adjusted 4f-CBM of Ce Sr1 Li Sr1 -Sr 2 Si 5 N 8 is from 2.42 to 2.85 eV. The energy range of 4f-CBM in Pr Sr1 Li Sr1 -Sr 2 Si 5 N 8 is 2.75-2.99 eV and its peak is 2.90 eV; the addition of Ce 3+ in Eu Sr1 Ce Sr1 Li Sr1 made the 4f energy level of Eu 2+ blue shift. The addition of Pr 3+ in Eu Sr2 Pr Sr2 Li Sr1 makes part of the Eu 2+ 4f energy level blue shift. Eu 2+ 4f energy level in Eu Sr2 Ce Sr1 Li Sr1 is not in the forbidden band, so Eu 2+ is not used as the emission center.

Research on Molecular Structure and Electronic Properties of Ln3+ (Ce3+, Tb3+, Pr3+)/Li+ and Eu2+ Co-Doped Sr2Si5N8 via DFT Calculation