Temperature dependent luminescence and energy transfer properties of Na2SrMg(PO4)2:Eu2+, Mn2+ phosphors

Abstract

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.