Sigma 13 (10(1)over-bar4) twin in alpha-Al2O3: A model for a general grain boundary

The atomistic structure and energetics of the Sigma13 (10(1) over bar4)[1(2) over bar 10] symmetrical tilt grain boundary in alpha-Al2O3 are studied by first-principles calculations based on the local-density-functional theory with a mixed-basis pseudopotential method. Three configurations, stable with respect to intergranular cleavage, are identified: one Al-terminated glide-mirror twin boundary and two O-terminated twin boundaries, with glide-mirror and twofold screw-rotation symmetries, respectively. Their relative energetics as a function of axial grain separation are described, and the local electronic structure and bonding are analyzed. The Al-terminated variant is predicted to be the most stable one, confirming previous empirical calculations, but in contrast with high-resolution transmission electron microscopy observations on high-purity diffusion-bonded bicrystals. which resulted in an O-terminated structure. An explanation of this discrepancy is proposed based on the different relative energetics of the internal interfaces with respect to the free surfaces.