On basal slip and basal twinning in sapphire (α-Al 2O 3)—III. HRTEM of the twin/matrix interface

A high resolution transmission electron microscopy (HRTEM) investigation of basal twins in sapphire (α-Al 2O 3) indicates that they are either Type-II or Type-I “glide” twins with a shear of 1/3 [101¯0〉 (the latter type twin is also equivalent to a Type-II screw twin with reverse shear, 1/3 [1¯010〉). These two types cannot be distinguished with the resolution of the HRTEM used in the present study. However, a Type-II twin is consistent with the model of basal twinning presented in the second paper of this series, with an AC′BA′CB′/a′bc′ab′c stacking sequence for the Al O Al layers along a direction perpendicular to the twin/matrix interface, and pair potential calculations suggest that Type-II interfaces have the lowest energy of all possible twin interfaces in sapphire. An overlap of the twin/matrix leads to Moire´-like contrast features. This can also be explained with the proposed twinning mechanism.