On basal slip and basal twinning in sapphire (alpha-Al2O3) .1. Basal slip revisited

The motion of partial dislocations (b = 1/3[01 (1) over bar 0] and 1/3[10 (1) over bar 0]) parallel to during basal slip ((0001) 1/3[11 (2) over bar 0]) in sapphire (alpha-Al2O3) is analysed. Crystallographic arguments suggest that the basal slip plane should be a plane passing through the normally-vacant octahedral sites defined by the anion sublattice. This definition of the basal slip plane implies that half of the cations lying between two adjacent anion layers are above and the other half are below the slip plane. The stacking faults that would be created by the motion of a dissociated basal dislocation differs in structure and energy, depending on whether the 1/3 [10 (1) over bar 0] partial is leading or trailing. Thus, the motion of a 1/3[10 (1) over bar 0] partial in its favorable (lower energy) direction creates a stacking fault with a stacking sequence in which the cations are in a localized twin symmetry, whereas the motion of a 1/3[10 (1) over bar 0] partial in its unfavorable (higher energy) direction is ''forbidden''. Furthermore, the motion of a 1/3[10 (1) over bar 0] partial in its favorable direction can take place without the need for synchroshear.