Atomic structures and energies of Σ7 symmetrical tilt grain boundaries in alumina bicrystals

Symmetrical Σ7 tilt grain boundaries of alumina (Al₂O₃) were studied using bicrystals. Three types of Σ7 boundaries were successfully fabricated, that is, rhombohedral twin (Σ7{11̄02}) and two types of [0001] symmetrical tilt grain boundaries with grain-boundary planes {45̄10} and {23̄10} (Σ7(45̄10} and Σ7(23̄10}). Their atomic structures and grain-boundary energies were investigated using high-resolution transmission electron microscopy (HRTEM) and a thermal grooving technique, respectively. HRTEM observations showed that the Σ7(11̄02} boundary had a completely symmetrical atomic arrangement with respect to the grain-boundary plane. In contrast, Σ7(23̄10} and Σ7(45̄10} boundaries exhibited asymmetrical atomic structures, which were confirmed by analyzing the atomic configurations using static lattice calculations. Thermal grooving experiments showed that the grain-boundary energies strongly depended on the properties of the grain-boundary planes.