The local interfacial structure and bonding at the rhombohedral twin interface in α-Al 2 O 3 (corundum) was studied by means of first-principles electronic-structure calculations based on the local density functional theory. Two sets of geometrical interface models were selected, one with a terminating oxygen layer at the interface, the other with a termination by interstitial vacancies of the corundum structure. Optimized interface configurations were obtained by minimization of the total internal energy with respect to relative translation states of the adjoining grains and relaxations of all atomic positions.
One vacancy-terminated configuration was found with a very low interface energy, a well-defined relative translation state with screw-rotation symmetry and a highly ordered atomic structure, which minimizes the mutual repulsion between the neighboring like-charged ions. A second metastable configuration with vacancy termination and a higher interface energy was also obtained as well as a metastable oxygen-terminated structure with an interface energy between those of the two vacancy-terminated configurations. The theoretical results for the interfacial structures and translation states are discussed with respect to recent experimental investigations of the twin interface by high-resolution transmission electron microscopy. Furthermore, the calculated interfacial site-projected densities of electron states (PDOS) display significant differences from the bulk-crystal PDOS in the conduction bands.