The modifications in atomistic structure, chemical bonding, and energetics induced by substitutional cation impurities isolated in the bulk volume and segregated at grain boundaries of α-Al2O3 were investigated by combining empirical ionic-model and first-principles electronic-structure calculations. The dependence of these modifications upon the boundary type, species and concentration of impurities, was studied by selecting the following variety of systems: 3 twin boundaries (the prismatic Σ3 (10•0), the rhombohedral Σ7 (10•2), and the pyramidal Σ13 (10•4) twins), three impurities X (X = Sc, Y, and La), and two concentrations for the segregant (about 3 or 6 atoms/nm2). A partial covalent character was found to be a distinctive feature of the X-O bonds in both bulk and interfacial atomic environments, and to drive the structural distortions of the octahedral XO6 clusters. The energetics of segregation reveals a linear relationship between segregation energy and impurity size. This was interpreted as resulting from a stress field localized at the interface.

First-Principles Analysis of Cation Segregation at Grain Boundaries in α-Al2O3. S.Fabris, C.Elsässer: Acta Materialia, 2003, 51[1], 71-86