POLYMORPHIC BASAL TWIN BOUNDARIES AND ANISOTROPIC GROWTH IN ALPHA-AL2O3

High resolution transmission electron microscopy (HRTEM) was used to study the structure of annealing twins in alpha-Al2O3. A basal twin was found where each of the two twin boundaries with the matrix had a different two-dimensional structure. The energies of alternative twin boundary structures were calculated using an ionic model. The relative difference in the energy of the boundary structures is appreciable, and the relative energies of steps at the boundary required for twin formation and termination show large differences for opposite growth directions in the basal plane. This suggests that the different twin boundary structures are the result of anisotropic growth of alumina, which is related to the lack of centro-symmetry in the trigonal corundum structure. The model is important for an understanding of annealing twin formation and growth mechanisms in sapphire, and could be extended to predict anisotropic growth phenomena for other non-centro-symmetric structures.