Sensitive detection of mirror symmetry by CBED applied to LaAlO3 and GdAlO3

Convergent-beam electron diffraction (CBED) patterns tilted away from major zone axes have been used to re-examine the space groups of LaAlO3 and GdAlO3 perovskites. The higher-order Laue zones (HOLZ) in CBED patterns at such tilted orientations have been found to have high sensitivity to the presence of mirror symmetry. This sensitivity has enabled the space group of LaAlO3, previously claimed to be cubic (Fm3c), to be confirmed as rhombohedral R3c by CBED. It is clear that patterns of sufficient tilt will be more sensitive for distinguishing between screw axes and glide planes than the observation of HOLZ deficit lines in zero-order Laue zone reflections. No symmetry degrading effects due to the influence of tilted crystal surfaces could be observed.