A ZONAL DISLOCATION MECHANISM FOR RHOMBOHEDRAL TWINNING IN SAPPHIRE (ALPHA-AL2O3)

HRTEM investigations of rhombohedral twins in sapphire (alpha-Al2O3) i ndicate that they are ''screw twins''; the twin axis eta1, is [0111BAR ] and involves a 1/6[0111BAR] translation in addition to the 2-fold ro tational symmetry. Based on this data, a zonal dislocation mechanism f or rhombohedral twinning in sapphire is proposed which predicts the co rrect shear strain s =0.202. This mechanism only requires movements of the Al and 0 ions of less than 50% of the closest Al-Al or O-O distan ces in the ideal crystal, although some ions have to move against the shear.