Research on Molecular Structure and Electronic Properties of Ln(3+) (Ce(3+), Tb(3+), Pr(3+))/Li(+) and Eu(2+) Co-Doped Sr(2)Si(5)N(8) via DFT Calculation

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.