Bond energy, preferential occupancy and spontaneous reduction ability of Eu3+ doped in CaAl2Si2O8

A bond-energy method is used to determine the site occupancy preference of Eu3+ or Eu2+ and further firstly extended to study the spontaneous reduction of Eu3+ doped CaAl2Si2O8 based on the bond valence model. The phosphor CaAl2Si2O8: Eu was synthesized under air condition. The co-existence of Eu3+ and Eu2+ cations was detected through photoluminescence measurement. The occupancy of Eu3+ or Eu2+ in CaAl2Si2O8 can be determined by comparing the deviation of its bond energy in different locations at Ca2+, Si4+ and Al3+ sites. Theoretical calculation result indicates that the Eu3+ and Eu2+ 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 Eu2+ at Eu4 sites is smaller than that of Eu3+ at the same sites, which indicates that Eu2+ is more stable than Eu3+ at Eu4 sites, so the spontaneous Eu3+-> Eu2+ reduction can be found in the CaAl2Si2O8. The same bond energy method can be extended to explain the reason why the Sm3+ doped CaAl2Si2O8 can't be reduced into Sm2+ under reduction condition. The theoretical result is consistent with the experimental phenomenon. This should help us to have a further understanding of spontaneous Eu3+-> Eu2+ reduction in the inorganic material. (C) 2017 Elsevier B.V. All rights reserved.