Substitutional cation impurities in α-Al2O3: ab-initio case study of segregation to the rhombohedral twin boundary Original Research Article

The energetics of substitutional cation impurities in the bulk volume and at the Σ7 (1̄012) rhombohedral twin boundary in alumina (α-Al2O3) were studied by means of ab-initio mixed-basis pseudopotential calculations based on the local density functional theory. For fifteen different substitutional cation impurity species, the positions of the impurity energy levels relative to the valence bands of the oxide were determined, and the type of bonding versus the atomic number of the impurity cation was analysed. For the rhombohedral twin boundary, increased propensity for segregation was found for impurity cations of large ionic radii for which an enhanced overlap between impurity and oxygen electronic states was observed. The effect of structural relaxation at the boundary was studied for the isovalent group-IIIB impurities Sc, Y and La. Segregation broke the screw-rotation symmetry of the pure, unsegregated boundary and altered the interface-plane termination.