Luminescence Investigation of Eu-Activated Sr_5(PO_4)_2SiO_4 Phosphor by Combustion Synthesis

摘要

Eu-activated Sr_5(PO_4)2SiO_4 with apatite structure was synthesized by the combustion synthesis. The phase formation and morphology were investigated by X-ray powder diffraction and scanning electron microscopy measurements, respectively. The phosphors were characterized by photoluminescence excitation and emission spectra, and decays. The band gap of Sr_5(PO_4)_2 (SiO_4) host is found to be about 7.3 eV from the VUV excitation spectra. The replacement of the (PO_4)~3 anionic network by the more covalent (SiO_4)~(4-) in apatite results in red-shift of the Eu~(2+) emission giving rise to the two broad emission bands with peaks at 495 and 560 nm corresponding to the 4f~65d~1 →4f~7(~8S_(7/2)) transition of Eu~(2+). Together with the Eu~(2+) luminescence, the unusual emission line (572 nm) due to the 5D_0→~7F_0 transition of Eu~(3+) is observed in Sr_s(PO_4)_2SiO_4 prepared by the combustion synthesis. The temperature-dependent luminescence and decay curves of the Eu~(2+) and Eu~(3+) ions are investigated from 10 to 300 K. The crystallographic sites and reduction mechanism for Eu~(2+) and Eu~(3+) in the Sr_5(PO_4)_2SiO_4 lattice are discussed. The Eu~(3+) ions doped in Sr_5(PO_4)_2(SiO_4) are suggested to occupy Mil sites with the charge compensation related to the interstitial oxygen O_i: 2Sr~(2+)→2Eu~(3+)+O"j. The unusual emission line due to the ~5D_0→~7F_0 transition of Eu~(3+) in Sr_5(PO_4)_2(SiO_4) is attributable to the charge-compensating oxide ion and an abnormal crystal field strength experienced by the Eu~(3+) ions.