Photon Factory Activity Report 2016 #34 (2017)

BL-9A/ 2014G600

Investigation of Valence State Change in Y3Al2Ga3O12: Ce3+-Cr3+ Persistent Phosphors
Jumpei Ueda1,* Misaki Katayama,2 Kazuki Asami,1 Jian Xu,1 Yasuhiro Inada,2 Setsuhisa Tanabe1
Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan2
Department of Applied Chemistry, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan

11 Introduction
Persistent phosphors show continuous luminescence
for a long duration ranging from several minutes to a few
hours even after the excitation light source is removed. In
2014, we successfully developed new persistent ceramic
phosphors with a new combination of lanthanide
luminescent centers and transition metal trap centers in
Y3Al5-xGaxO12: Ce3+-Cr3+ (x=2.5, 3, 3.5). The phosphorsshow Ce3+:5d-4f green persistent luminescence (em=510nm) for several hours after blue-light excitation[1,2].
This phosphor can absorb blue light via Ce3+:4f-5d1
transition with a large cross section and the excitedelectron can transfer to the conduction band (CB) easily
due to the small energy gap between the 5d1 level and thebottom of CB. In our previous work, we suggested that
the Ce3+ ion loses an electron and the Cr3+ ion captures
the electron under excitation, which is the main
mechanism of persistent luminescence[1,2]. However,
the origin of the electron donors and acceptors in Y3Al53+3+
persistent phosphors have never been
xGaxO12:Ce -Cr
directly demonstrated. In this study, the valence statechange of Ce3+ was investigated by X-ray absorption nearedge structure (XANES) spectroscopy.
2 ExperimentPolycrystalline ceramics of Ce3+(0.5%) singly-doped,Ce3+(0.5%)-Cr3+(0.05%)-codoped and Cr3+(0.2%) singlydoped Y3Al2Ga3O12 (YAGG) were synthesized by solidstate reaction. The chemicals Y2O3 (99.99 %), Al2O3(99.99 %), Ga2O3 (99.99 %), CeO2 (99.99 %), Cr2O3(99.9%) were used as starting materials. The powders
were mixed by ball milling (Fritsch, Premium Line P-7)
with ethanol and the obtained slurry was dried andpulverized. The dried powder was pressed into pellets
sized 10 mm- X 2 mm thickness. The pellets weresintered at 1600 °C in air. The X-ray absorption
spectroscopy (XAS) was performed at the beamline
BL9A of Photon Factory (KEK, Japan). The photon flux
density of X-ray at 7 keV is 4X1012 photons/sec/mm2.
The Ce LIII and Cr K X-ray absorption near edge structure(XANES) were recorded for all the samples influorescence mode with/without UV (250nm-400nm)
illumination of Xe lamp (Asahi Spectra, Max 302). The
reason why the sample was illuminated by UV is to cause
the ionization of Ce3+. As references, standard Ce(NO3)3,CeO2, Cr2O3 samples were also measured.
3 Results and Discussion
Fig. 1 shows the XANES spectra for the Ce LIII edge ofthe YAGG samples under different conditions. The redand blue solid lines show the XANES spectra for theYAGG:Ce3+-Cr3+ sample and YAGG:Ce3+ with UVillumination. In these XANES spectra, two main peaks
were observed at 5725eV and 5735 eV. Compared withthe reference XANES spectra, the peaks at 5725 eV and5735 eV are mainly contributed by the X-ray absorptionof Ce3+ and Ce4+, respectively. From the previous resultsof thermoluminescence excitation spectrum for
YAGG:Ce3+-Cr3+, it was confirmed that this persistentphosphor can be charged by UV illumination[2]. Fromthe XANES spectrum, it is found that the valence state of
Ce ion in YAGG:Ce3+ is almost trivalent even under UV
illumination. However, in YAGG:Ce3+-Cr3+, a part ofCe3+ changes into Ce4+ from the XANES spectra with UV
illumination. The valence state change from Ce3+ to Ce4+
can be caused by the existence of Cr3+ electron traps
which attract the ionized electrons from Ce3+.

Fig. 1: XANES spectra of Ce LIII edge in
Y3Al2Ga3O12:Ce3+-Cr3+ illuminated with UV light andY3Al2Ga3O12:Ce3+ illuminated with UV light.
References[1]
J. Ueda, K. Kuroishi, S. Tanabe, Appl. Phys. Lett.
104, 101904, 2014.
[2]
J. Ueda, P. Dorenbos, A. J. J. Bos, K. Kuroishi,
S. Tanabe, J. Mater. Chem. C 3, 5642, 2015.
[3]
J. Ueda, M. Katayama, K. Asami, J. Xu, Y.
Inada, S. Tanabe, Optical Materials Express 7,2471, 2017.
* ueda.jumpei.5r@kyoto-u.ac.jp