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 a rguments suggest that the basal slip plane should be a plane passing t hrough the normally-vacant octahedral sites defined by the anion subla ttice. This definition of the basal slip plane implies that half of th e cations lying between two adjacent anion layers are above and the ot her half are below the slip plane. The stacking faults that would be c reated by the motion of a dissociated basal dislocation differs in str ucture and energy, depending on whether the 1/3 [10 (1) over bar 0] pa rtial is leading or trailing. Thus, the motion of a 1/3[10 (1) over ba r 0] partial in its favorable (lower energy) direction creates a stack ing fault with a stacking sequence in which the cations are in a local ized twin symmetry, whereas the motion of a 1/3[10 (1) over bar 0] par tial in its unfavorable (higher energy) direction is ''forbidden''. Fu rthermore, the motion of a 1/3[10 (1) over bar 0] partial in its favor able direction can take place without the need for synchroshear.