Ab initiocalculation of near-edge structures in electron-energy-loss spectra for metal-oxide crystals

The local electronic structure and its relation to the atomic environment of the ions in MgO, ${\mathrm{MgAl}}_2{\mathrm{O}}_4,$ and $\alpha -{\mathrm{Al}}_2{\mathrm{O}}_3$ are analyzed by means of ab initio band-structure calculations based on the density-functional theory (DFT). The theoretical results for local densities of states are compared to electron-energy-loss near-edge structures (ELNES) obtained by analytical transmission electron microscopy. For the peak positions of the anionic centers, the validities of the effective one-electron approximation for the crystal potential in the DFT and of the perturbative treatment of single-electron excitations are demonstrated at least semiquantitatively. Limitations in the theoretical treatment for ELNES at the cationic centers are investigated. Improvements are illustrated and discussed in comparison to experimental ELNES data. In particular, this concerns the explicit account for core-hole relaxation effects in the excitation process, and the inclusion of symmetry constraints on the probabilities for the excitations from a core state to conduction-band states.