Crystal structure and luminescence properties of Na2MMg(PO4)2:Eu2+ (M = Ca/Sr/Ba) phosphors

Abstract A series of Na2MMg(PO4)2:Eu2+ (M = Ca/Sr/Ba) phosphors were synthesized by the high-temperature solid-state reaction. X-ray diffraction (XRD), photoluminescence (PL) and GSAS structural refinement were used to characterize the samples. Rietveld refinements of powder X-ray diffraction patterns of Na2MMg(PO4)2 confirm that the structure of Na2MMg(PO4)2 consist of stacking along the [001] direction. The M2+ cations are located within the interlayer space, while Na+ is found in a mount of cavities bounded to the layers. Upon excitation at 350 nm, the phosphors exhibit the asymmetric broadband emission peaked at about 392 nm attributed to the 4f65d1→4f7 transitions of Eu2+. It is found that the energy transfer occurred mainly by the electric dipole-dipole interaction according to the Eu2+ concentration quenching mechanism. Moreover, when Ca2+ sites are substituted by Sr2+ or Ba2+, the Eu2+ emission band shifts to a longer wavelength initially and then returns to a shorter wavelength. In particular, 89.5% of the room-temperature emission intensity is still maintained at 500 K, indicating a good thermal stability. The obtained results suggest that the phosphor could be a potential candidate for applications in white light-emitting diodes.