Investigations on valence-change behaviors of europium ions in Eu-doped aluminate and silicate phosphors

Compounds of Sr₃Al₂O₆Eu, Sr₄Al₁₄O₂₅:Eu, and BaZnSiO₄:Eu were synthesized by high-temperature solid state reactions. The doping Eu³⁺ ions were partially reduced to Eu²⁺ in Sr₄Al₁₄O₂₅:Eu and BaZnSiO₄:Eu prepared in an oxidizing atmosphere, N₂ + O₂. However, such an abnormal reduction process could not be performed in Sr₃Al₂O₆: Eu, which was also prepared in an atmosphere of N₂ + O₂. Moreover, even though Sr₃Al₂O₆:Eu was synthesized in a reducing condition CO, only part of the Eu³⁺ ions was reduced to Eu²⁺. The existence of trivalent and divalent europium ions was confirmed by photoluminescent spectra. The different valence-change behaviors of europium ions in the hosts were attributed to the difference in host crystal structures. The higher the crystal structure stiffness, the easier the reduction process from Eu³⁺ to Eu²⁺.