Eu2+ singly and Eu2+/Mn2+ co-doped Na2SrMg(PO4)2 (NSMP) phosphors have been prepared via a high-temperature solid-state reaction process. Upon UV excitation of 260–360 nm, the NSMP:xEu2+ phosphors exhibit a violet band located at 399 nm and a blue band centered at 445 nm, which originate from Eu2+ ions occupying two different crystallographic sites: Eu2+(I) and Eu2+(II), respectively. Excitation wavelengths longer than 380 nm can selectively excite Eu2+(II) to emit blue light. Energy transfer processes in the Eu2+(I)–Eu2+(II) and Eu2+–Mn2+ pairs have been observed and investigated by luminescence spectra and decay curves. The emission color of as-prepared samples can be tuned by changing the relative concentrations of Eu2+ and Mn2+ ions and adjusting the excitation wavelength. Under UV excitation of 323 nm, the absolute quantum yield of NSMP:0.005Eu2+ is 91%, which is higher than most of the other Eu2+-doped phosphors reported previously. The temperature dependent luminescence properties and decay curves (4.3–450 K) of NSMP:Eu2+ and NSMP:Eu2+, Mn2+ phosphors have been studied in detail. Thermal quenching of Eu2+ has been observed while the emission band of Mn2+ shows a blue-shift and an abnormal increase of intensity with increasing temperature. The unusual thermal quenching behavior indicates that the NSMP compound can serve as a good lattice host for Mn2+ ions which can be used as a red-emitting phosphor. Additionally, the lifetimes for Eu2+(I) and Eu2+(II) increase with increasing temperatures.
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