Eu2+→Eu3+ reduction and Tb3+→Tb4+ transformation in SrAl2Si2O8:Eu/Tb system

Abstract

The existing valence state (Eu2+/Eu3+ and Tb3+/Tb4+) in Eu/Tb ions doped in SrAl2Si2O8 were investigated through experimental and theoretical method. Their photoluminescence properties indicated that parts of Eu3+ ions were reduced to Eu2+ in SrAl2Si2O8. Based on the bond energy method, the reduction process of the Eu3+→Eu2+ and the preferentially occupancy of Eu3+/Eu2+ in SrAl2Si2O8 samples have been discussed. The values of deviation of its bond energy were calculated when Eu ions located at different crystal sites. Theoretical results indicated that the Eu3+ and Eu2+ would preferentially occupy the Sr sites due to the smaller energy variation. In addition, the variation of bond energy of Eu3+(ΔEEu3+-O2-Sr=11.367 kcal/mol) is close to Eu2+ (ΔEEu2+-O2-Sr= 15.112 kcal/mol), which indicated that the both of Eu2+ and Eu3+can stably exist in SrAl2Si2O8 host. When Eu, Tb ions are co-doped in SrAl2Si2O8, the photoluminescence shows that only the characteristic emission peaks of Eu2+ and Eu3+ can be found and the emission of Tb3+ disappeared. It is suggested that the Tb3+ is oxidized to Tb4+ and the Eu3+ is reduced to Eu2+. Compared with Eu ion, the variation of bond energy for Tb3+(ΔETb3+-O2-Sr=10.084 kcal/mol) is more closer to Tb4+ (ΔETb4+-O2-Sr=9.628 kcal/mol), so in Tb3+, Eu3+ co-doped in SrAl2Si2O8, the Tb4+ ion has the highest priority to occupy the Sr sites, Tb3+ loses one electron to form the tetravalent Tb4+, and the excess electrons are transferred to the surrounding neighboring Eu3+ site, Eu3+ gets an electron and is reduced to Eu2+. The theoretical results are in accord with the experimental measurement.