Burgers vector determination in deformed perovskite and post-perovskite of CaIrO(3) using thickness fringes in weak-beam dark-field images.

The thickness-fringe method [Ishida et al., Philosophical Magazine 42 (1980) 453] for complete determination of the character of a dislocation Burgers vector has been performed in CaIrO(3) perovskite and post-perovskite deformed at high pressures and high temperatures. By selecting several main zone axes and determining the number of terminating thickness fringes at the extremity of a dislocation from a wedge-shaped thin-foil specimen in weak-beam dark-field transmission electron microscope (TEM) images, the Burgers vectors were unambiguously determined. The results demonstrate that [100] screw and edge dislocations on the (010) slip plane are dominant in the post-perovskite phase. Curved [100] and [010] dislocations and straight 110 screw dislocations on a potential (001) slip plane were identified in the perovskite phase as well as a high density of {110} twins. Low-angle tilt boundaries consisting of different groups of parallel edge dislocations on the {110} and (001) planes indicate diffusion-assisted climb in perovskite at high temperatures. The differences in dislocation microstructures could be due to activations of limited numbers of slip systems for post-perovskite and of a large number of multiple slip systems for perovskite, which may result in the strong crystallographic preferred orientation (CPO) in post-perovskite and the lack of CPO in deformed perovskite.