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.

Tunable luminescence in co-doped Zn3Al2Ge2O10:Cr3+by controlling crystal field splitting and nephelauxetic effect

Ce3+/Dy3+/Tb3+/Eu3+/Mn2+and Cr3+ions co-doped Zn3 Al2 Ge2 O10 phosphor were prepared by a hightemperature solid-state method.X-ray diffraction patterns prove the cubic phase structure of prepared Zn3 Al2 Ge2 O10 phosphor,Emission,excitation spectra and decay curves confirm the tunable luminescence.Different degrees of the decrease of emission FWHM in Zn3 Al2 Ge2 O10:0.02 Cr3+,RE(RE=Ce3+,Dy3+,Tb3+,Eu3+)and Zn3 Al2 Ge2 O10:0.02 Cr3+,Mn2+are observed.The reason of variable FWHM is the effect of crystal field splitting and nephelauxetic effect,and the nephelauxetic effect is dominant.Therefore,the emission FWHM decreases with the increasing concentration of Mn2+/Tb3+/Eu3+in Zn3 Al2 Ge2 O10:0.02 Cr3+,and for Zn3 Al2 Ge2 O10:0.02 Cr3+,Ce3+and Zn3 Al2 Ge2 O10:0.02 Cr3+,Dy3+,it is a constant.The variation of Zn3 Al2 Ge2 O10:0.02 Cr3+,Tb3+is more obvious than that of Zn3 Al2 Ge2 O10:0.02 Cr3+,Eu3+,because Tb3+ion has smaller electronegativity.Thus,the tunable luminescence of Cr3+can be realized by co-doping different ions.And these phosphors have potential applications in light-emitting diodes for plant growth.