TWINNING IN THETA-ALUMINA INVESTIGATED WITH HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY

theta alumina obtained by dehydration of boehmite at about 1200 degree s C for 3h was investigated by HRTEM. The major characteristics of the microstructure ave the multiple twinned lamellae arranged along the [ 001] direction. The width of these lamellae changes from half a theta alurrtina unit cell to several unit cells. The lamellae were observed either to form ledges along the [100] direction or to be terminated wi thin the twinned lamellae. Polytypism resulting from repetitive twinni ng has been found lending to a variety of periodic arrays having a rep eat along the c axis of n x 0.59 nm. The polytype with n = 2 was deter mined by electron diffraction to have an orthorhombic structure. (C) 1 998 Published by Elsevier Science Limited.