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

本项目旨在促进对镁铝酸盐尖晶石化合物(MgO-Ni2O_3)辐射引起的结构变化的基本了解，这些化合物作为未来核计划中抗辐射材料的候选材料受到了极大的关注。为此，我们发展了一种新的电子显微镜实验技术来定量分析局域原子构型。然后利用高角分辨电子沟道X射线能谱(HARECXS)技术，研究了1 MeV Ne~(++)和500 keV He~(2+)离子辐照下的MgO-n1Al_2O_3的结构无序性。该项目始于2000财年，研究了HARECXS作为在尖晶石化合物中定位晶格离子的实用工具。在HARECXS过程中，测量了电子束照射的局部样品区域的特征x射线发射随电子束方向的高角度分辨率。…的理论论证此外，基于动态电子衍射和非弹性散射的研究表明，得到的X射线强度变化的HARECXS分布不仅敏感地依赖于离子组态，而且还依赖于样品厚度和德拜-沃勒因子。根据这些分布，可以准确地确定组成离子在晶格中特定位置上的占据几率。在接下来的两年里，研究工作主要集中在原生晶体中的偏序状态和离子辐照下结构无序的演化。在n=1.0和2.4时，大多数Al^&lt；3+&gt；晶格离子占据六配位的八面体(VI)位，而Mg^&lt；2+&gt；晶格离子同时存在于四面体(IV)位和八面体(VI)位。结构性空缺在IV-站点中得到了丰富。1 MeV Ne~(++)离子辐照使峰前损伤区的阳离子构型无序，不形成缺陷团簇，同时使阳离子和阴离子从其晶位上移位，在峰损伤区形成小的缺陷团簇。结果表明，n=2.4时非化学计量比产生的结构空位参与了与置换离子的复合。500keV He~(++)辐照引起的无序度小于1 MeV Ne~(++)辐照。前一种情况下较高的电离速率归因于较少的无序倾向，因为置换离子的电离增强迁移更有利于恢复或重新有序化。较少

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