First-principles analysis of cation segregation at grain boundaries in α-Al2O3

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 dependency of these modifications on the boundary type, species and concentration of impurities, was studied by selecting the following variety of systems: three twin boundaries (the prismatic Σ3 (101-bar0), the rhombohedral Σ7 (101-bar2), and the pyramidal Σ13 (101-bar4) twins), three impurities X (X=Sc, Y, and La), and two concentrations for the segregant (∼3 and ∼6 atoms/nm2). A partial covalent character is 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 is interpreted as resulting from a stress field localized at the interface