A new type of phosphate-based phosphor Na₃CsMg₇(PO₄)₆:Eu²⁺/Mn²⁺ with high quantum efficiency and high thermostability

The Eu²⁺ ion, with the 4f⁷5d¹ electronic configuration, is one of the most important activated ions due to its symmetry-allowed 4f → 5d electron transition. Herein, we successfully prepared a new type of blue phosphor, Na₃CsMg₇(PO₄)₆:xEu²⁺ (NCMP:xEu²⁺), in which Eu²⁺ occupies the Na⁺ sites of the host lattice. Under 395 nm light excitation, a broad emission band from 410 to 550 nm can be seen, peaking at 458 nm, due to the 5d → 4f transition of Eu²⁺. Moreover, Eu²⁺ can be used as a sensitizer ion in an NCMP host. For co-doping phosphor NCMP:0.18Eu²⁺/0.4Mn²⁺, both Eu²⁺ and Mn²⁺ emission bands can be seen using 395 nm as the excitation wavelength for the occurrence of Eu²⁺ → Mn²⁺ charge transfer. The optimized concentration of Eu²⁺ for NCMP:xEu²⁺ is x = 0.18, with high internal (IQE) and external quantum efficiencies (EQE) of 66.4% and 22.8%. The luminescence of NCMP:0.18Eu²⁺ has high thermostability with 77.3% the intensity at 450 K compared to that at 300 K. Finally, a white illuminating lamp was produced using a 395 nm UV chip, blue phosphor NCMP:xEu²⁺, green phosphor (Sr,Ba)₂SiO₄:Eu²⁺ and red phosphor CaAlSiN₃:Eu²⁺ with CIE coordinates of (0.378, 0.369), color temperature (CCT) of 3971 K, and color rendering index (R_a) of 79.8.