First-principles study on persistent luminescence mechanism of LiYGeO4:Eu3+

Abstract

LiYGeO4:Eu3+ is a red persistent luminescent material with a duration of more than 21 h. Although its persistent luminescence phenomenon has been fully studied, its detailed mechanism is still the subject of debates. Herein, we performed first-principles study on the intrinsic point defects and the charge transfer processes in LiYGeO4:Eu3+ to reveal the mechanism of persistent luminescence. The results show that, under charge transfer excitation, the electron is promoted from the valence band (oxygen ion ligand) to the central Eu3+ ion to form Eu2+ ion in LiYGeO4:Eu3+, leaving a hole behind. The charge transfer excitation can relax quickly to the excited state of the Eu3+ ion, which produces the characteristic 5D0 → 7FJ (J = 0–6) emission. The Li vacancies (VLi) and the antisite defects of Li replacing Y site (LiY) and Y replacing Li site (YLi) are main defects, while O vacancies (VO) are less important in concentration due to high formation energy. VLi and LiY can serve as hole-type traps, with the trap depths suitable for trapping the holes produced by illumination. The delayed release holes can combine with the Eu2+ left behind by the illumination, leading to persistent luminescence. The VLi trap is shallower than LiY, and the latter is responsible for the long duration of persistent luminescence. A schematic based on the calculation results is constructed to illustrate the mechanism of persistent luminescence.