ON BASAL SLIP AND BASAL TWINNING IN SAPPHIRE (ALPHA-AL2O3) .3. HRTEM OF THE TWIN MATRIX INTERFACE/

A high resolution transmission electron microscopy (HRTEM) investigati on of basal twins in sapphire (alpha-Al2O3) indicates that they are ei ther Type-II or Type-I ''glide'' twins with a shear of 1/3 [10 (1) ove r bar 0] (the latter type twin is also equivalent to a Type-II screw t win with reverse shear, 1/3[(1) over bar 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'C B'/a'bc'ab'c stacking sequence for the Al-O-Al layers along a directio n perpendicular to the twin/matrix interface, and pair potential calcu lations suggest that Type-II interfaces have the lowest energy of all possible twin interfaces in sapphire. An overlap of the twin/matrix le ads to Moire-like contrast features. This can also be explained with t he proposed twinning mechanism.