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

Highlights: • In air atmosphere, the co-existence of Eu³⁺ and Eu²⁺ in CaAl₂Si₂O₈: Eu is confirmed. • A bond-energy method is used to determine the site occupancy preference (Ca4) of Eu³⁺ or Eu²⁺. • The deviation of the bond energy determines the Eu³⁺→Eu²⁺ reduction. • The bond-energy result uncovers the reason why Sm³⁺ can't be reduced into Sm²⁺. 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.