Research on Molecular Structure and Electronic Properties of Ln (Ce, Tb, Pr)/Li and Eu Co-Doped SrSiN via DFT Calculation.

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