Polycrystalline ZnO was sintered with 5 mol.% Ga2O3 at 1350°C for lhour in air. Samples for transmission electron microscopy (TEM) were prepared by cold pressing, grinding, dimpling and Argon ion-milling.

ZnO crystallizes in the hexagonal wurtzite structure. Bright field and dark field images reveal lamellar defect structures which are not observed in undoped ZnO. Electron diffraction shows that the lamellar regions consist of heavily twinned ZnO. The twin boundaries (TB) of both twin variants are parallel to lattice planes of type { }. Lattice images along reveal narrow planar twin boundaries as well as a diffuse image contrast within the twin lamellae (Fig. la). to characterize the nature of the boundaries we recorded dark field images of the twinned regions using reflections of type ±{0002}. in combination with microdiffraction it can be shown that the diffuse boundaries are inversion domain boundaries (IDB) which alternate with the sharp TB. This microstructure is related to the polarity of ZnO.