Structural Disordering in Magnesium Aluminate Spinel Compounds under Ion-Beam Irradiation
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Structural Disordering in Magnesium Aluminate Spinel Compounds under Ion-Beam Irradiation Syo Matsumura, Mikio Shimada, Kazuhiro Yasuda, and Chiken Kinoshita Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Fukuoka 812-8581, Japan ABSTRACT High Angular Resolution Electron Channeling X-ray Spectroscopy (HARECXS) of analytical electron microscopy was employed in quantitative analysis of irradiation-induced structural disordering in MgO·nAl2O3 spinel compounds irradiated with 1 MeV Ne+ ions and 200 MeV Xe14+ ions. The results successfully show the disordering behavior in MgO·nAl2O3 as a function of composition and irradiation condition. It has been revealed by HARECXS that 1 MeV Ne+ irradiation at 873 K causes significant cation-disordering in the pre-peak damaged area where no indication is recognized in conventional TEM observation. Displacement of O2- ions is also recognized in heavily damaged areas. Irradiation with 200 MeV Xe14+ swift ions, on the other hand, forms structurally-disordered cylinders of 4 nm in diameter along their tracks. HARECXS profiles indicate that the disordering takes place over extended areas of about 10 nm in radius centered on the ion tracks. INTRODUCTION Magnesium aluminate spinel compounds (MgO·nAl2O3) have been nominated as candidate materials for insulators in fusion reactors, as well as inert matrix in nuclear transmutation processes for radioactive minor actinides, since they exhibit strong resistance to formation of defect clusters under irradiation with high energy particles [1,2]. For example, no significant swelling occurs in MgO·nAl2O3 under neutron irradiation up to 2.5 x 1027 n/m2 (E>1 MeV) at 658–1100 K [3]. The reliable stability should be attributed to the crystal structure (i.e., spinel type), since it has a number of vacant sites in the lattice. The spinel structure consists of anions (O2-) arranged in a nearly fcc lattice and cations (Mg2+ and Al3+) residing on the 4-fold coordinated tetrahedral (IV) and the 6-fold coordinated octahedral (VI) interstices [4]. In the ideal or "normal" structure of stoichiometric case (n=1), Mg2+ and Al3+ cations occupy 1/8 of the IV interstices and 1/2 of the VI positions, respectively. The rest of the interstices remain unoccupied as empty holes in the lattice. In a non-stoichiometric compound with n>1, which is regarded as a solid solution of MgO·Al2O3 and γ-Al2O3, a significant number of structural vacancies V are introduced into the cation sites, according to MgO ⋅ nAl 2 O 3 = MgAl 2 O 4 + 34 (n − 1)Al 8/3V1 / 3 O 4 .
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The structural vacancies and/or the empty holes are expected to play a role as recombination centers or sinks for displaced interstitial ions, suppressing aggregation of defects into clusters under irradiation conditions. However, the behavior of displaced ions and vacancies in MgO·nAl2O3 under irradiation has not been conclusively confirmed by experimental measurements. So far, a number of experiments with neutron diffraction [5], nuclear magnetic
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resonance [6], R
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