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Abstract The existing valence state (Eu2+/Eu3+ and Tb3+/Tb4+) in Eu/Tb ions doped in SrAl 2 Si 2 O 8 were investigated through experimental and theoretical method. Their photoluminescence properties indicated that parts of Eu3+ ions were reduced to Eu2+ in SrAl 2 Si 2 O 8. Based on the bond energy method, the reduction process of the Eu3+→Eu2+ and the preferentially occupancy of Eu3+/Eu2+ in SrAl 2 Si 2 O 8 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+(Δ E Eu 3 + - O 2 - Sr =11.367 kcal/mol) is close to Eu2+ (Δ E Eu 2 + - O 2 - Sr = 15.112 kcal/mol), which indicated that the both of Eu2+ and Eu3+can stably exist in SrAl 2 Si 2 O 8 host. When Eu, Tb ions are co-doped in SrAl 2 Si 2 O 8 , 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+(Δ E Tb 3 + - O 2 - Sr =10.084 kcal/mol) is more closer to Tb4+ (Δ E Tb 4 + - O 2 - Sr =9.628 kcal/mol), so in Tb3+, Eu3+ co-doped in SrAl 2 Si 2 O 8 , 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. Highlights • Eu/Tb doped in SrAl 2 Si 2 O 8 were synthesized by high-temperature solid state method. • Rietveld refinement result demonstrate that Eu2+/3+, Tb3+/4+ ions occupy the Sr site. • Eu3+ can be reduced to Eu2+ and Tb3+ can be oxidized by Eu3+ in SrAl 2 Si 2 O 8 at air. • Calculation results of bond energy method prove Eu2+/3+, Tb3+/4+ occupy the Sr site. [ABSTRACT FROM AUTHOR]

A bond-energy method is used to determine the site occupancy preference of Eu 3+ or Eu 2+ and further firstly extended to study the spontaneous reduction of Eu 3+ doped CaAl 2 Si 2 O 8 based on the bond valence model. The phosphor CaAl 2 Si 2 O 8 : Eu was synthesized under air condition. The co-existence of Eu 3+ and Eu 2+ cations was detected through photoluminescence measurement. The occupancy of Eu 3+ or Eu 2+ in CaAl 2 Si 2 O 8 can be determined by comparing the deviation of its bond energy in different locations at Ca 2+ , Si 4+ and Al 3+ sites. Theoretical calculation result indicates that the Eu 3+ and Eu 2+ would preferentially occupy the smaller energy variation sites Ca4, which can be assigned to Eu4 site. The more important result is that the energy variation of Eu 2+ at Eu4 sites is smaller than that of Eu 3+ at the same sites, which indicates that Eu 2+ is more stable than Eu 3+ at Eu4 sites, so the spontaneous Eu 3+ →Eu 2+ reduction can be found in the CaAl 2 Si 2 O 8 . The same bond energy method can be extended to explain the reason why the Sm 3+ doped CaAl 2 Si 2 O 8 can't be reduced into Sm 2+ under reduction condition. The theoretical result is consistent with the experimental phenomenon. This should help us to have a further understanding of spontaneous Eu 3+ →Eu 2+ reduction in the inorganic material. [ABSTRACT FROM AUTHOR]