Red luminescent Eu2+ in K2MgH4 and comparison with KMgH3

Abstract

The engineering of covalency and crystal field is a key point for the control of the 5d energy level of Eu²⁺. The hydride ion exhibits strong covalency, which results in a large 5d energy shift by the nephelauxetic effect compared to their fluoride analogues. In this study, the Eu²⁺-doped K₂MgH₄ Ruddlesden–Popper (n = 1) perovskite was prepared and its optical properties were investigated. Red luminescence due to the 4f⁶5d¹ → 4f⁷ transition was observed at 670 nm by UV excitation of the K₂MgH₄:Eu²⁺ sample. Compared with yellow Eu²⁺-luminescence at 565 nm of the KMgH₃ cubic perovskite, which has almost the same chemical composition as K₂MgH₄, the additional red-shift of the luminescence in K₂MgH₄:Eu²⁺ is caused by the strong crystal field splitting in the unique 9-fold coordinated site in addition to the large nephelauxetic effect by hydride ions. Using a hydride model system that allows for a clear distinction between crystal field splitting and nephelauxetic effect, we successfully demonstrated the design concept for the 5d energy shifting by covalency and crystal field in Eu²⁺-doped K₂MgH₄. Also, the thermal quenching process of K₂MgH₄:Eu²⁺ by the ionization process was discussed in comparison to KMgH₃:Eu²⁺ on the basis of the energy diagram.