Quantitative electron diffraction study of cation configuration and irradiation induced displacement in magnesium aluminate spinel crystals

The electron channeling enhanced X-ray microanalysis and the large angle convergent beam electron diffraction were simultaneously employed to determine cation configuration in magnesium aluminate spinel (MgO·nAl₂O₃) crystals as a function of composition n and irradiation dose with 1 MeV electrons or 300 keV O⁺ ions. In as-grown single crystals, about 70% of Mg²⁺ ions and about 20% of Al³⁺ ions reside on the tetrahedral sites, while the remaining ions are located on the octahedral sites. The occupation probabilities of ions are almost independent of the value n. Most of structural vacancies are located on the tetrahedral sites in non-stoichiometric compounds with n > 1. The cation distribution is hardly changed under irradiation with 1 MeV electrons to 1.9 dpa at 870 K. In contrast, significant displacements of Al³⁺ ions are induced under irradiation with 300 keV O⁺ ions to 1.4 dpa at 870 K. The anti-site Al³⁺ ions on the tetrahedral sites almost vanish under the latter condition of irradiation.