Photon Factory Activity Report 2019 #37 (2020)

BL-27A/2012G081
3+Stability of Ce valence state and lattice expansion of CeO2 induced by hightemperature annealing in vacuum
Akihiro IWASE,1,2* Norito ISHIKAWA3 and Takanori KISHINO1
1Osaka Prefecture University, Sakai, Osaka, 319-1195, Japan
2 The Wakasa Wan Energy Research Center (WERC), Tsuruga, Fukui, 914-0192, Japan
3 Japan Atomic Energy Agency, Tokai-mura, Ibaraki 319-1195, Japan
1 Introduction
In our previous papers, we have shown that energeticion irradiation produces oxygen vacancies in CeO2[1]. Oxygen vacancies in CeO2 cause the change of thevalence state of Ce atoms near the oxygen vacancies from4+ to 3+ as a result of charge balance [2] and the lattice
expansion due to the mutual repulsion of Ce cation
atoms [3]. We have also shown that the existence ofCe3+ valence state induces the ferromagnetism in theCeO2 specimens [3,4]. It is well known, however thatoxygen vacancies can be produced in CeO2 more easily
under the reduction process by the high temperatureannealing in vacuum. In the present experiment, thestability of Ce3+ state and the lattice expansion
induced by the high temperature annealing invacuum was investigated by using the x-ray
photoelectron spectroscopy (XPS) at the BL-27A
beamline of KEK-PF and a conventional x-ray
diffraction method (XRD).
2 Experiment
CeO2 pellet was synthesized by sintering CeO2
powders at 1873K for 12 hours in air. It was annealed at1273 K for 1 hour in vacuum, and just after the annealing
in vacuum, it was stored in an evacuated glass tube
until the XPS spectra were measured. Just before thespecimen was set in an XPS chamber, it was taken outof the glass tube. The specimen was left in theatmosphere only for 15 minutes before evacuating theXPS chamber.
After the XPS measurement, thespecimen was left in the atmosphere for 50 minutes andthen the XPS spectra were measured again. The energy ofx-ray for the XPS measurements was 2.2 keV. For theXRD measurement with Cu K x-ray, a CeO2 thin film,
which was deposited on an Al2O3 substrate by using aRF magnetron sputtering, was used.
.
3 Results and Discussion
The XPS spectrum (a) in Fig. 1, which was measured
for the CeO2 specimen without high temperatureannealing in vacuum, shows typical Ce-3d peaks for Ce4+valence state. In the figure, the reference spectrum for theCe3+ valence state is also plotted ( spectrum (d)) [2]. Thespectrum (b) is for the CeO2 specimen which wasexposed to the atmosphere for 15 minutes before theXPS measurement. XPS peaks for Ce3+ valence state canbe observed as well as those for Ce4+ state in thespectrum. The Ce3+ state and the oxygen vacancies in thespecimen, therefore, survive the exposure to theatmosphere for 15 minutes.
As can be seen in thespectrum (c), however, Ce3+ peaks completelydisappear, and only Ce4+ peaks can be observed after theexposure to the atmosphere for 50 minutes. The present
experimental result clearly shows that the Ce3+ valence
state and oxygen vacancies produced by the hightemperature annealing in vacuum is very unstable, andoxygen atoms in the atmosphere re-oxidize the CeO2
specimen including oxygen vacancies in a very short time.

Fig. 1: (a) XPS spectrum for CeO2 specimen withouthigh temperature annealing, (b) XPS spectrum measured
after exposure to atmosphere for 15minutes, (c) XPSspectrum measured after exposure to atmosphere for 50
minutes, and (d) reference spectrum for Ce3+ valence
state.
The instability of oxygen vacancies produced by hightemperature annealing in vacuum has also been confirmedby the XRD measurement [5]. Fig. 2 shows the timedependence of XRD spectra for CeO2 thin foil annealed at1273 K in vacuum. The peak of the XRD spectrum for theCeO2 thin foil, which was exposed to the atmosphere for21 minutes before the XRD measurement, shifts to lowerangles, meaning that the lattice expansion occurs by theoxygen vacancy production. With increasing the exposurePhoton Factory Activity Report 2019 #37 (2020)

time to the atmosphere, the XRD peaks gradually shift tohigher angles and after the two-day exposure, the peak
position is about the same as for the spectrum of thespecimen before the high temperature annealing. Therefore,the exposure to the atmosphere for 2 days completely reoxidizes the specimen.

Fig. 2: Dependence of XRD spectrum of CeO2 thin film
annealed at 1273 K on exposure time to atmosphere [5].
Our previous reports show that oxygen vacancies, theaccompanying Ce3+ state and the resulting lattice
expansion induced by energetic ion irradiation are much
more stable than those produced by high temperatureannealing in vacuum. Even after the exposure to theatmosphere for more than a few months, we could find thelattice expansion and the ferromagnetic state of CeO2
irradiated with energetic ions [3,4]. The high temperatureannealing in vacuum produces only oxygen vacancies, but
in the case of energetic ion irradiation, not only oxygenvacancies but also oxygen interstitial atoms and thedisordering of Ce atom arrangements are produced. As
such complicated defects prevent oxygen atoms frommoving freely in the irradiated CeO2 specimens. theoxygen vacancies in the ion-irradiated CeO2 can survive
the long-time exposure to the atmosphere.
Acknowledgement
The authors thank Dr. Y. Baba for his great help.
References[1] H. Ohno, A. Iwase, D. Matsumura, Y. Nishihata,J. Mizuki, N. Ishikawa, Y. Baba, N. Hirao, T. Sonoda,M. Kinoshita, Nucl. Instrum. Methods, B266, 3013(2008).
[2] A. Iwase, H. Ohno, N. Ishikawa, Y. baba, N. Hirao,
T. Sonoda, M. Kinoshita, Nucl. Instrum. Methods,
B267, 969(2009).
[3] K. Shimizu, S. Kosugi, Y. Tahara, Y. Kaneta,
N. Ishikawa, F. Hori, T. Matsusi, A. Iwase, Nucl.
Instrum. Methods, B286, 291(2012).
[4] T. Kishino, K. Shimizu, Y. Saitoh, N. Ishikawa,Y. Okamoto, F. Hori, T. Matsui, A. Iwase, Jpn. J. Appl.
Phys. 53, 05FC07(2014).
[5] N. Ishikawa, private communication (2020).
* iwase@mtr.osakafu-u.ac.jp, aiwase@werc.or.jp