The atomistic structure and energetics of the Σ13 (10•4)[1¯2•0] symmetrical tilt grain boundary in α-Al2O3 were 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, were 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 were described, and the local electronic structure and bonding were analyzed. The Al-terminated variant was 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 was proposed based on the different relative energetics of the internal interfaces with respect to the free surfaces.

Σ13 (10•4) Twin in α-Al2O3 - a Model for a General Grain Boundary. S.Fabris, C.Elsässer: Physical Review B, 2001, 64[24], 245117 (11pp)