Enhanced energy transfer from Bi³⁺ to Eu³⁺ ions relying on the criss-cross cluster structure in MgMoO₄ phosphor

The discovery of efficient red-emitting-phosphor materials is critical to the next-generation white light-emitting diodes (white LEDs). In this work, we prepared a promising red-phosphor candidate: MgMoO₄:Eu³⁺,Bi³⁺,M (M=Li⁺, Na⁺, K⁺) with a standard solid-state reaction method. This material had high luminescence intensity upon ultraviolet light (UV), near ultraviolet light (near-UV), and blue excitation. We discovered that every four Mg sites formed a criss-cross unit site in the MgMoO₄ structure. These sites were used to remotely control the relative distance between Bi³⁺ and Eu³⁺ ions through changing the doping concentrations and charge compensation. A distinct new excitation band from the MgMoO₄:Eu³⁺,Bi³⁺,M (M=Li⁺, Na⁺, K⁺) phosphors coming from the ¹S₀→³P₁ transition of Bi³⁺ ions was clearly observed at about 352 nm. Moreover, the super energy transfer coming from Bi³⁺ to Eu³⁺ ions was designed and realized. Bi³⁺ and Eu³⁺ begin to sit in two adjacent Mg(1) sites when their total molar concentration is beyond 1/4. Thus, the new super energy-transfer emerges due to the adjacent Bi³⁺ and Eu³⁺ ions. The process and mechanism of the super energy transfer were further discussed. Our results indicated potential industrial applications of MgMoO₄:Eu³⁺,Bi³⁺,M in white LEDs.