High Angular Resolution Electron Channeling X-ray Spectroscopy for Radiation-Induced Local Disordering in Magnesium Aluminate Spinel Compounds

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This project aimed to promote basic understanding of radiation-induced structural change in magnesium aluminate spinel compounds (MgO-nAl_2O_3), which are drawing much attention as candidates for radiation resistant materials in future nuclear programs. To this end, we developed a new experimental technique of transmission electron microscopy to analyze quantitatively local atomic configuration. Then the technique, which is called as high angular resolution electron channeling x-ray spectroscopy (HARECXS), was applied to MgO-nlAl_2O_3 irradiated with 1 MeV Ne^+ or 500 keV He^+ ions to disclose ion displacements involved in structural disordering. The project started in FY 2000 with examination of HARECXS as a practical tool to locate lattice ions in spinel compounds. In the procedure of HARECXS, characteristic x-ray emission from an electron-illuminated local specimen area is measured as a function of the electron beam direction with high angular resolution. The theoretical arguments b … More ased on the dynamical electron diffraction and inelastic scattering clearly showed that the HARECXS profiles of x-ray intensity variations thus obtained sensitively depends on not only ion-configuration but also specimen thickness and Debye-Waller factors. From the profiles, one can determine accurately the occupation probabilities of constituent ions on specific sites in the crystal lattice. In the next two years, the research work was mostly focused on partially ordered states in as-grown crystals and evolution of structural disordering under ion irradiation. In MgO-nAl_2O_3 with n=1.0 and 2.4, most Al^<3+> lattice-ions occupy the octahedral (VI) sites with 6-fold coordination, while Mg^<2+> lattice-ions reside on both the tetrahedral (IV) and the octahedral (VI) sites. The structural vacancies are enriched in the IV-sites. Irradiation with 1 MeV Ne^+ ions causes disordering of cation configuration in the pre-peak damaged range without any defect cluster formations, while it displaces not only cations but also anions from their crystallographic sites and forms small defect clusters in the peak-damaged area. It is suggested that the structural vacancies due to nonstoichiometry for n=2.4 take part in recombinations with displaced ions. Irradiation with 500 keV He^+ causes less disordering than 1 MeV Ne^+ irradiation does. A higher ionizing rate under the former condition is ascribed to the less disordering tendency, since recovery or reordering is more promoted with ionization-enhanced migration of displaced ions. Less