Atomistic observation of electron irradiation-induced defects in CeO 2
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Atomistic observation of electron irradiation-induced defects in CeO 2 Seiya Takaki, Tomokazu Yamamoto, Masanori Kutsuwada, Kazuhiro Yasuda, Syo Matsumura Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Fukuoka 819-0395, Japan ABSTRACT We have investigated the atomistic structure of radiation-induced defects in CeO 2 formed under 200 keV electron irradiation. Dislocation loops on {111} habit planes are observed, and they grow accompanying strong strain-field. Atomic resolution scanning transmission electron microscopy (STEM) observations with high angle annular dark-field (HAADF) and annular bright-field (ABF) imaging techniques showed that no additional Ce layers are inserted at the position of the dislocation loop, and that strong distortion and expansion is induced around the dislocation loops. These results are discussed that dislocation loops formed under electron irradiation are non-stoichiometric defects consist of oxygen interstitials. INTRODUCTION It has been shown that oxide ceramics with fluorite structure, such as yttria-stabilized cubic zirconia (YSZ) and ceria (CeO 2 ), exhibit exceptional resistance to radiation damage. For example, previous investigations on YSZ have revealed its excellent resistance to amorphization [1] and volumetric swelling [2] under irradiations with energetic particles. Fluorite-type structure oxides, therefore, have potential applications to inert matrix fuels and transmutation targets [3,4]. In fluorite-type oxides, there exists a significant difference in mass between cations and anions. In addition, the displacement energy under energetic particle irradiation has been reported to be larger for cation-sublattice than anion-one [5], leading to the displacement damage rate (dpa/s) for anion sublattice being larger than cation one. A calculation of displacement cross-section in CeO 2 for O- and Ce-sublattice by using McKinly-Feshbach formula [6] revealed that elastic displacements are induced only in O-sublattice with electron energies less than about 1500 keV, whereas both O- and Ce-lattices are displaced with electrons above 1500 keV (Figure 1). Recent studies on electron irradiated CeO 2 reported the formation of non-stoichiometric dislocation loops below 1250 keV, and the defect clusters formed on {111} planes accompanying strong strain-field are discussed to be dislocation loops consist of oxygen ions through transmission electron microscopy (TEM) analysis [7,8]. A molecular dynamic simulation including several oxygen Frenkel pairs has shown that interstitial oxygen ions aggregate to form plate-like clusters on a (111) plane after the relaxation [9], which supports the interpretation based on the TEM analysis. The present study aims to gain insights into the atomistic structure of radiation-induced defects formed under electron irradiation. Atomic resolution scanning transmission electron microscopy (STEM) with high angle annular dark-field (HAADF) and annular bright-field (ABF) technique was utilized to clarify the atomic structure of dis
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