First-principles study on persistent luminescence mechanism of LiYGeO_(4):Eu^(3+)

LiYGeO_(4):Eu^(3+)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 LiYGeO_(4):Eu^(3+)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 Eu^(3+)ion to form Eu^(2+)ion in LiYGeO_(4):Eu^(3+),leaving a hole behind.The charge transfer excitation can relax quickly to the excited state of the Eu^(3+)ion,which produces the characteristic^(5)D_(0)→^(7)F_(J)(J=0-6)emission.The Li vacancies(V_(Li))and the antisite defects of Li replacing Y site(Li_(Y))and Y replacing Li site(YLi)are main defects,while O vacancies(V_(O))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 Eu^(2+)left behind by the illumination,leading to persistent luminescence.The VLi trap is shallower than Li_(Y),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.