Ab-initio theory of grain-boundary segregation in alpha-alumina: Energetics, atomistic and electronic structures

The modifications in atomistic structure, chemical bonding, and energetics induced by substitutional cation impurities isolated in bulk and segregated at grain boundaries of -Al2O3 were investigated by first-principles electronic-structure calculations. The dependency of these modifications on the boundary type, species and concentration of defects, was studied by selecting the following variety of systems: two different twin boundaries (the prismatic 3 (1010) and the pyramidal 13 (1014) twins), five cation impurities X (X=Ti, Sc, Y, Ca, and La), and two concentration regimes for the segregant (3 and 6 atoms/nm2). A partial covalent character is found to be a distinctive feature of the X-O bond 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.