VUV Excitation of YBO3 and (Y,Gd)BO3 Phosphors Doped with Eu3+ or Tb3+: Comparison of Efficiencies and Effect of Site-Selectivity
Résumé
Borate phosphors, with formulations Y0.95Eu0.05BO3 (YBEu), Y0.95Tb0.05BO3 (YBTb), Y0.57Gd0.38Eu0.05BO3 (Y6G4BEu), and Y0.57Gd0.38Tb0.05BO3 (Y6G4BTb) have been synthesized by spray pyrolysis at the maximum temperature of 700 °C, and then annealed at 1200 °C. The yttrium oxide-doped phosphor Y0.95Eu0.05O1.5 (YOEu) was obtained by the same experimental conditions. The UV-VUV excitation spectra of the samples were recorded using synchrotron radiation. The emission intensities of these phosphors have also been investigated as a function of plasma gas pressure in a dedicated experimental setup. The intensities in the emission spectra were integrated, to compare the relative efficiencies of the phosphors. All phosphors were more efficient with increasing gas pressure in the plasma chamber. The emission intensity of YOEu was similar to that of YBEu or Y6G4BEu at low Ne-Xe pressure, but stronger at high pressure. For both lanthanide activators, the mixed borates Y6G4B and the pure YBO3 (YB) have about same luminescence efficiency at low pressure, but the Y6G4B are better matrices than YB at high Ne-Xe pressure. With the introduction of Gd3+ in the borates, the emission intensity is enhanced due to a better spectral overlap with the plasma emission at 173 nm, and also the occurrence of multiple energy transfers between Gd3+ ions before transfer to the emitter Eu3+ or Tb3+. A \textquotedblleftcolor effect\textquotedblright with the variation of pressure in the plasma chamber is observed for the Eu3+-doped phosphors. The orange:red intensity ratio (i.e., 5D0 \textrightarrow 7F0,1 (560-600 nm)/5D0 \textrightarrow 7F2 (600-640 nm)) increases as the gas pressure is raised. This observation is related to the spectral changes occurring in the plasma, and it is corroborated by the observation of selective excitation in the VUV range of Eu3+ at Ci versus C1 crystallographic sites.