ON BASAL SLIP AND BASAL TWINNING IN SAPPHIRE (α-Al_20_3)--III. HRTEM OF THE TWIN/MATRIX INTERFACE

摘要

A high resolution transmission electron microscopy (HRTEM) investigation of basal twins in sapphire (α-Al_20_3) indicates that they are either Type-II or Type-I "glide" twins with a shear of 1/3[1010> (the latter type twin is also equivalent to a Type-II screw twin with reverse shear, 1/3[1010>). 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.