Dual energy transfers and color tunable emission in Eu²⁺/Tb³⁺/Mn²⁺-triactivated Mg₂₁Ca₄Na₄(PO₄)₁₈ phosphors

A series of single-composition Mg₂₁Ca₄Na₄(PO₄)₁₈: Eu²⁺,Tb³⁺,Mn²⁺ phosphors have been synthesized via a combustion-assisted synthesis technique. The crystal structure, luminescence properties, and energy transfers (ET) from Eu²⁺ to Tb³⁺ and/or Mn²⁺ ions are discussed in detail. The ET_Eu–Tb and ET_Eu–Mn processes in the Mg₂₁Ca₄Na₄(PO₄)₁₈ host are demonstrated to be resonant types via dipole–dipole interaction and quadrupole–quadrupole interaction, respectively. Additionally, the emitting colors of Mg₂₁Ca₄Na₄(PO₄)₁₈: Eu²⁺, Tb³⁺, Mn²⁺ samples can be tuned from blue to blue–green via the ET_Eu–Tb process and from blue to reddish purple via the ET_Eu–Mn process. Finally, the tunable white light emission with high quantum yields (>50%) was realized by precise control of the concentrations of Eu²⁺, Tb³⁺, and Mn²⁺ ions. The results suggest the developed phosphors can find applications in the field of ultraviolet irradiation pumped white-LEDs.