Luminescence Investigation of Eu-Activated Sr5(PO4)2SiO4 Phosphor by Combustion Synthesis

Eu-activated Sr-5(PO4)(2)SiO4 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(PO4)(2)(SiO4) host is found to be about 7.3 eV from the VUV excitation spectra. The replacement of the (PO4)3- anionic network by the more covalent (SiO4)4- in apatite results in red-shift of the Eu2+ emission giving rise to the two broad emission bands with peaks at 495 and 560 nm corresponding to the 4f65d1 -> 4f7(8S(7/2)) transition of Eu2+. Together with the Eu2+ luminescence, the unusual emission line (572 nm) due to the 5D(0)-> 7F(0) transition of Eu3+ is observed in Sr-5(PO4)(2)SiO4 prepared by the combustion synthesis. The temperature-dependent luminescence and decay curves of the Eu2+ and Eu3+ ions are investigated from 10 to 300 K. The crystallographic sites and reduction mechanism for Eu2+ and Eu3+ in the Sr-5(PO4)(2)SiO4 lattice are discussed. The Eu3+ ions doped in Sr-5(PO4)(2)(SiO4) are suggested to occupy MII sites with the charge compensation related to the interstitial oxygen O-i: 2Sr2+-> 2Eu3++O '(i). The unusual emission line due to the 5D(0)-> 7F(0) transition of Eu3+ in Sr-5(PO4)(2)(SiO4) is attributable to the charge-compensating oxide ion and an abnormal crystal field strength experienced by the Eu3+ ions.