Prismatic Sigma 3 (10(1)over-bar-0) twin boundary in alpha-Al2O3 investigated by density functional theory and transmission electron microscopy

The microscopic structure of a prismatic Sigma3 (10 (1) over bar0) twin boundary in alpha-Al2O3 is characterized by combining ab initio local-density-functional theory, electron energy-loss spectroscopy measuring energy-loss near-edge structures (ELNES) of the oxygen K-ionization edge, and high-resolution transmission electron microscopy (HRTEM). Theoretically, two distinct microscopic boundary variants with very low interface energies are derived and analyzed. The interface-projected densities of states (PDOS) calculated for the two variants agree equally well with ELNES, therefore the comparison between experimental ELNES and theoretical PDOS cannot discriminate the one or the other boundary structure. The analysis reveals that the distinction between the metastable interfaces from ELNES is limited by the spatial resolution of the scanning transmission electron microscope used to measure ELNES, not by its energetical resolution. The quantitative analysis of experimental HRTEM images obtained with an atomic-resolution microscope yields that the experimentally observed interface corresponds to the boundary variant with the lowest energy.