Competing roles of defects in SrAl 2 O 4 :Eu 2+ ,Dy 3+ phosphors detected by luminescence techniques
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Z.G. Xia School of Materials Science & Engineering, University of Science and Technology, Beijing, Beijing, 100083, China
A.A. Finch Department of Earth & Environmental Sciences, University of St Andrews, Fife, KY16 9AL, UK
P.D. Townsend Physics Building, University of Sussex, Brighton, BN1, 9QH, UK (Received 4 January 2016; accepted 8 April 2016)
Thermoluminescence (TL) and radioluminescence (RL) spectra of the long-lasting phosphorescence of SrA12O4:Eu21,Dy31 with A1N addition and commercially used SrA12O4:Eu21,Dy31 were compared. Their spectra were slowly recorded over the temperature range from 25 to 673 K (400 °C). A1N offers a higher temperature TL peak, which should lengthen the phosphor lifetime. However, both TL and RL, especially that below room temperature, reveal that there are additional decay paths for the samples of SrA12O4:Eu21,Dy31 with A1N additions. These new defect sites reduce the phosphor efficiency. Some speculative models of potential sites are proposed and discussed. In addition, discontinuous intensity changes have been observed for both sample types in TL and RL spectra, which are assigned to the transitions of embedded impurity phases. The justification for this model is explained. Suggestions for future experimentation are also considered.
I. INTRODUCTION 21
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Eu doped alkaline earth aluminates, MAl2O4:Eu (M 5 Ca and Sr), a new generation of persistent luminescence phosphors, have been widely developed since the mid 1990s to replace ZnS:Cu phosphors.1–4 Among these alkaline earth aluminates phosphors, SrAl2O4:Eu21,Dy31 phosphors are the best developed so far, and they have been commercially used in safety, emergency, transportation and other fields because of their excellent long persistence.5 The phosphors show a strong broad green emission characteristic of the Eu21 ion, and the lifetime of these phosphors can be as long as 10 h at ambient temperatures. By contrast, ordinary Eu21 doped material more typically has lifetimes of the order of a few hundred nanoseconds.6 To understand the mechanism on the long afterglow phosphorescence research work has been carried out by several groups.7–9 Different explanations about the mechanism on the long afterglow phosphorescence have been proposed, but a consistent view is that the introduction of Dy31, leads to a defect trapping level with a trap that is slowly thermally
Contributing Editor: Winston V. Schoenfeld a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.177 J. Mater. Res., Vol. 31, No. 10, May 28, 2016
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excited at room temperature. Recombination and luminescence then proceeds at the Eu site. The presence of dopants controls the volume of the distortions around the emission site. We also assume that this is a preferential charge transfer from the trap to the recombination site as the distortion offers directional charge motion; hence there is high luminescence efficiency, and the implication that the Eu and Dy are in close proximity. T
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