White-light emission from a single-emitting-component Ca9Gd(PO4)7:Eu2+,Mn2+ phosphor with tunable luminescent properties for near-UV light-emitting diodes

Ca9Gd(PO4)7 and Ca9Gd(PO4)7:xEu2+,yMn2+ were synthesized by solid-state reaction. The refinement confirmed that Ca9Gd(PO4)7 belongs to space group R3c (No. 161) with unit cell parameters a = 10.4526 Å, c = 37.3769 Å, V = 3536.61 Å3, and Z = 6. Upon the excitation from 250 to 430 nm, the Ca9Gd(PO4)7:xEu2+,yMn2+ phosphors exhibit a broad blue-green emission band at 490 nm and a red emission band at 645 nm, which originate from the Eu2+ and Mn2+ ions, respectively. The varied emitted color from blue-green through white and eventually to red can be achieved by properly tuning the relative ratio of the Eu2+ to Mn2+ ions in the phosphors through the energy transfer from the Eu2+ and Mn2+ ions. The energy transfer from Eu2+ to Mn2+ was demonstrated to be a resonant type via a dipole-quadrupole reaction by the luminescence spectra, energy transfer efficiency, and decay curves of the phosphors. Furthermore, the critical distance of the Eu2+ and Mn2+ ions has also been calculated. More importantly, compared to the multiple emitting components of white LEDs system, single-emitting-component phosphors would enable easy fabrication with perfect stability and color reproducibility.