Microscopic structure and bonding at the rhombohedral twin interface in alpha-Al2O3

The local interfacial structure and bonding at the rhombohedral twin interf ace in alpha -Al2O3 (corundum) was studied by means of first-principles ele ctronic-structure calculations based on the local density functional theory . Two sets of geometrical interface models were selected, one with a termin ating oxygen layer at the interface, the other with a termination by inters titial vacancies of the corundum structure. Optimized interface configurati ons 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 en ergy, a well-defined relative translation state with screw-rotation symmetr y and a highly ordered atomic structure, which minimizes the mutual repulsi on between the neighboring like-charged ions. A second metastable configura tion with vacancy termination and a higher interface energy was also obtain ed as well as a metastable oxygen-terminated structure with an interface en ergy between those of the two vacancy-terminated configurations. The theore tical results for the interfacial structures and translation states are dis cussed with respect to recent experimental investigations of the twin inter face by high-resolution transmission electron microscopy. Furthermore, the calculated interfacial site-projected densities of electron states (PDOS) d isplay significant differences from the bulk-crystal PDOS in the conduction bands. (C) 2000 Acta Merallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.