Enhancement of white-light-emission from single-phase Sr5(PO4)3F:Eu2+,Mn2+ phosphors for near-UV white LEDs
A series of single-phase broadband white-light-emitting Sr5(PO4)3F:Eu2+,Mn2+ phosphors were prepared by a solid state reaction. The luminescence property, and the crystal and electronic structures of the fluorophosphates were studied by photoluminescence analysis, XRD Rietveld refinement and density functional theory calculation (DFT), respectively. Under near ultraviolet excitation in the 250 to 430 nm wavelength range, the phosphors exhibit two emission bands centered at 440 and 556 nm, caused by the Eu2+ and Mn2+ ions. By altering the relative ratios of Eu2+ and Mn2+ in the compounds, the emission color could be modulated from blue to white. The efficient energy transfer from the Eu2+ to Mn2+ ions could be ascribed to the well crystallized host lattice and the facile substitution of Eu2+ and Mn2+ for Sr2+ sites due to similar ionic radii. A series of fluxes were investigated to improve the photoluminescence intensity. When KCl was used as flux in the synthesis, the photoluminescence intensity of Sr5(PO4)3F:Eu2+,Mn2+ was enhanced by 85% compared with no fluxes added. These results demonstrate that the single-phase Sr5(PO4)3F:Eu2+,Mn2+ with enhanced luminescence efficiency could be promising as a near UV-convertible direct white-light-emitting phosphor for WLED applications.