Effect of host compositions on the afterglow properties of phosphorescent strontium aluminate phosphors derived from the
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The effect of compositions of host precursors on the afterglow and phosphorescent decay properties of Eu2+- and Dy3+-coactivated strontium aluminates (SAED) synthesized by a sol-gel process has been investigated. A variety of strontium aluminates such as Sr3Al2O6, SrAl2O4, and SrAl12O19 have been identified in the samples prepared from starting precursors with Al/Sr ratios ranging from 1 to 12, respectively. The initial afterglow intensity (I0) for SAED phases was found to vary with Al/Sr ratio of the sol-gel precursor, with a maximal I0 appearing in the sample with Al/Sr of 2 in which SrAl2O4 dominated. The afterglow decay rate was found to be fastest for sample with Al/Sr ratio of 1:1 in which Sr3Al2O6 phase dominated. Moreover, the afterglow decay rates for those with Al/Sr ratio of 2:1 to 12:1, in which SrAl2O4 and SrAl12O19 dominated, were found to be slow and similar.
I. INTRODUCTION
The SrAl2O4:Eu2+,Dy3+ phase with -tridymite type structure1 was reported to exhibit long persistent bluishgreen phosphorescence and has been considered as a useful green phosphor in the application of luminous watches and clocks.2–5 Katsumata et al. have reported that among the four Eu2+- and Dy3+-coactivated strontium aluminates (SAED) crystals of SrAl2O4, SrAl4O7, SrAl12O19, and Sr3Al2O6, only the first three were found to exhibit long-lasting phosphorescence with emission peaking at wavelengths of 520, 480, and 400 nm, respectively.4 On the basis of the investigation on the phosphorescence (or afterglow), thermoluminescence, and photoconductivity characteristics of polycrystalline SrAl2O4:Eu2+,Dy3+, Matsuzawa et al.3 suggested a mechanism indicating that the phosphorescence is ascribed to the presence of holes and to the trapping and thermal release of holes by Dy3+ ions in the system. Katsumata et al. were able to grow Eu2+- and Dy3+coactivated strontium aluminate crystals with different Sr:Al compositions by floating zone techniques.4,5 Katsumata et al. further investigated the effects of Al/Sr mole ratio (from 2.05 to 2.22) and Dy/Eu mole ratio (from 0 to 3.55) on the phosphorescent properties (i.e., spectra and afterglow characteristics) of SrAl2O4:Eu2+,Dy3+ crystals and concluded that em of
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 5, May 2001
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520 nm attributed to Eu2+ ions did not vary with the compositions.5 On the other hand, Yamamoto et al. investigated the mechanism of long phosphorescence of polycrystalline MAl 2 O 4 :Eu 2 + ,R 3 + (M ⳱ Sr, Ca; R ⳱ Dy, Nd) phases and reported that long-lasting phosphorescence in both phases is attributed to the 4f 6 5d → 4f 7 transition of Eu2+ and to the holes thermally released from the trap levels with optimal depth formed by Dy3+ or Nd3+ in the band gap of the hosts.6 Jia et al. reported the measurements of phosphorescence and decay dynamics for both SrAl 2 O 4 :Eu 2+ and SrAl2O4:Eu2+,Dy3+ single crystals.7 Their results indicated that the trapping rate o
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