Origin of the red luminescence in Sr3Al2O6:Eu phosphor—From the synergetic effects of Eu2+ and Eu3+

Abstract

In order to uncover the real origin of red luminescence from Sr3Al2O6:Eu and the physical mechanisms that were involved in the dynamical process of luminescence, variant amount of Eu and Dy activated Sr3Al2O6 phosphors were synthesized with the solid-state reaction and the combustion-assisted solid-state reaction, respectively, using the fine graphite powder or the mixed H2+N2 gases as a reducing agent. The phase was examined with XRD analysis and the photoluminescence properties were characterized by a fluorescence spectrometer. Although the phosphors possessed the same Sr3Al2O6 phase, different emission colors (red or green) were obtained, relying on synthesis conditions. The simultaneous existence of Eu2+ and Eu3+ was not only observed in the emission and excitation spectra, but also identified with the near edge X-ray absorption fine structure spectroscopy (NEXAFS). The mixed valence (higher than +2 and less than +3) of Eu may be related with the six different sites of Sr, whose effective valence ranged from +1.5058 to +2.2698, in the crystal lattice of Sr3Al2O6 that could accommodate Eu. Moreover, the reduction of Eu3+ to forming Eu2+ depended on the amount of Eu3+ or Dy3+ doped, due to the different energy barrier in each site of Sr that Eu had to overcome. The residual Eu3+, similar to the doped Dy3+, played an important role in supplying the hole for Eu2+ to form a bound trap (Eu2+)* after excitation. During electron returning to the 4f7 ground state of Eu2+, the red luminescence was radiated. Therefore, the synergetic effects of Eu2+ and Eu3+ (Dy3+) produce red luminescence.