Near coincidence grain boundary in alumina

The rhombohedral twin orientation in alumina has been obtained by diffusion bonding and studied by transmission electron microscopy. It corresponds to a Sigma 7 near coincidence orientation with a grain boundary plane parallel to (01 (1) over bar 2). The grain boundary contains two arrays of dislocations with the same line direction and alternately distributed Burgers vectors which are translation vectors of the DSC lattice. The associated step vectors are large, up to 1.75 nm. Starting from symmetry considerations on the various dichromatic complexes, all possible unrelaxed atomic configurations are constructed. Oxygen terminated grain boundary structures as well as those passing through non occupied aluminium sites are selected on the basis of optimal compacity at the interface. Correlations between simulated and experimental high resolution images favor the structures based on empty octahedral sites at the interface. The most probable structure has a two fold screw axis at the interface and leaves Al-O bonds with very little distorsions from the perfect coordination in alumina.