A high-resolution transmission electron microscopic investigation of basal twins in sapphire indicated that they were either type-II or type-I so-called glide twins with a shear of [10•0>. The latter type of twin was also equivalent to a type-II screw twin with reverse shear. These 2 types could not be distinguished within the resolution permitted by this study. A type-II twin was consistent with the a new model for basal twinning; with an AC’BA’CB’|a’bc’ab’c stacking sequence for Al-O-Al layers along a direction that was perpendicular to the twin/matrix interface. Pair potential calculations suggested that type-II interfaces had the lowest energy of all possible twin interfaces in sapphire. An overlap of the twin and matrix produced moiré-like contrast effect. This could also be explained in terms of the new twinning mechanism.

T.Geipel, J.B.Bilde-Sørensen, B.F.Lawlor, P.Pirouz, K.P.D.Lagerlöf, A.H.Heuer: Acta Materialia, 1996, 44[5], 2165-74