Mechanism of the reduction and energy transfer between Eu2+ and Eu3+ in Eu-doped CaAl2Si2O8 materials prepared in air

Abstract

Eu²⁺ doped CaAl₂Si₂O₈ materials prepared under a reducing atmosphere show a white-bluish luminescence under UV excitation. Eu³⁺ cations naturally give rise to line emission spectra for which the colour of emissions is between orange and red. To achieve a warm color or to change the CIE(x, y) position of the Ca_1−xEu_xAl₂Si₂O₈ phosphors, we decided to investigate the synthesis of these compounds under air conditions to favour the co-existence of Eu²⁺ and Eu³⁺ cations and investigate the mechanism of the “abnormal” reduction from Eu³⁺ to Eu²⁺ and the energy transfer from Eu²⁺ to Eu³⁺ in the “co-doped” CaAl₂Si₂O₈ host lattice. The results indicate that both Eu²⁺ and Eu³⁺ co-exist in the aluminosilicates, even when synthesized under air conditions, due to the substitution of Eu³⁺ cations for alkaline earth metal (Ca²⁺) triggering the formation of vacancies that play the role of electron donors towards V^//_Ca defects. The higher the concentration of V^//_Ca, the easier the reduction of Eu³⁺ into Eu²⁺. Consequently, the “co-doping” with higher state chemical elements (e.g. Zr⁴⁺) may help a lot in increasing the concentration of Eu²⁺ cations at the expense of Eu³⁺. We will take advantage of this observation in the future for sure to help prepare materials.