Twinning in θ Alumina Investigated with High Resolution Transmission Electron Microscopy

θ alumina obtained by dehydration of boehmite at about 1200°C for 3 h was investigated by HRTEM. The major characteristics of the microstructure are the multiple twinned lamellae arranged along the [001] direction. The width of these lamellae changes from half a θ alumina unit cell to several unit cells. The lamellae were observed either to form ledges along the [100] direction or to be terminated within the twinned lamellae. Polytypism resulting from repetitive twinning has been found leading to a variety of periodic arrays having a repeat along the c axis of n × 0.59 nm. The polytype with n = 2 was determined by electron diffraction to have an orthorhombic structure.