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

该项目旨在促进对铝酸镁尖晶石化合物（MgO-nAl_2O_3）辐射引起的结构变化的基本认识，该化合物作为未来核项目中抗辐射材料的候选材料受到广泛关注。为此，我们开发了一种新的透射电子显微镜实验技术来定量分析局部原子构型。然后，将高角分辨率电子通道x射线能谱（HARECXS）技术应用于1 MeV Ne^+或500 keV He^+离子辐照的MgO-nlAl_2O_3，以揭示与结构紊乱有关的离子位移。该项目于2000财政年度启动，将HARECXS作为定位尖晶石化合物中晶格离子的实用工具。在HARECXS过程中，以高角分辨率测量了电子照射局部样品区域的特征x射线发射作为电子束方向的函数。基于动态电子衍射和非弹性散射的理论论证清楚地表明，由此获得的x射线强度变化的HARECXS曲线不仅与离子构型有关，还与样品厚度和Debye-Waller因子有关。从谱图中，可以准确地确定组成离子在晶格中特定位置的占据概率。在接下来的两年里，研究工作主要集中在生长晶体的部分有序态和离子辐照下结构无序的演变。在n=1.0和2.4的MgO-nAl_2O_3中，大多数Al^<3+>晶格离子以6倍配位占据八面体（VI）位，而Mg^<2+>晶格离子同时位于四面体（IV）和八面体（VI）位。结构空位在iv -位点富集。1 MeV Ne^+离子辐照导致峰前损伤区阳离子构型紊乱，但未形成缺陷团簇，同时使正离子和阴离子从其晶体位置移位，并在峰损伤区形成小缺陷团簇。结果表明，由于n=2.4的非化学计量引起的结构空位参与了与置换离子的复合。500kev He^+辐照比1mev Ne^+辐照引起的无序性更小。在前一种条件下，较高的电离率归因于较少的无序倾向，因为电离增强的位移离子迁移更促进了恢复或重新排序。少

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