NUV-pumped red-emitting Ca₉MnK(PO₄)₇ phosphor: energy transfer and charge compensation

The development of novel Mn-based phosphor hosts has received increasing interest in the search for highly efficient red emitting phosphors for white LED applications. In this study, Ca₉MnK(PO₄)₇, a compound with the β-Ca₃(PO₄)₂-type structure, was successfully synthesized by a high-temperature solid-state reaction process. The Eu²⁺-doped Ca₉MnK(PO₄)₇ phosphor exhibits a broadband red emission peaking at 650 nm. The optimal excitation wavelength is 395 nm, which matches that of commercial ultraviolet (NUV) chips. Codoping Ce³⁺ ions into the Ca₉MnK(PO₄)₇:Eu²⁺ phosphor efficiently improves Mn²⁺ luminescence. Here, Ce³⁺ acts as a charge compensator rather than a sensitizer and substantially increases the effective number of Eu²⁺ and finally improves the red emission of Mn²⁺. The charge compensation mechanism is also verified by codoping some optically inert rare earth ions (Ln³⁺) including Y³⁺, La³⁺ and Gd³⁺. The results demonstrate that these developed Ca₉MnK(PO₄)₇:Eu²⁺, Ln³⁺ phosphors have great potential for application in NUV-based white LEDs. The energy transfer approach combined with the charge compensation technique is valuable for improving the performance of the red-emitting Ca₉MnK(PO₄)₇:Eu²⁺ phosphor, which can further be used in developing other Mn-based phosphors.