Influence of Light Atoms on Quantification of Atomic Column Positions in Distorted Perovskites with HAADF-STEM

Measurement of picometer-scale atomicdisplacements byaberration-correctedSTEM has become invaluable in the study of crystalline materials,where it can elucidate ordering mechanisms and local heterogeneities.HAADF-STEM imaging, often used for such measurements due to its atomicnumber contrast, is generally considered insensitive to light atomssuch as oxygen. Light atoms, however, still affect the propagationof the electron beam in the sample and, therefore, the collected signal.Here, we demonstrate experimentally and through simulations that cationsites in distorted perovskites can appear to be displaced by severalpicometers from their true positions in shared cation-anioncolumns. The effect can be decreased through careful choice of samplethickness and beam voltage or can be entirely avoided if the experimentallows reorientation of the crystal along a more favorable zone axis.Therefore, it is crucial to consider the possible effects of lightatoms and crystal symmetry and orientation when measuring atomicpositions.

Automated summary

Measurement of picometer-scale atomic displacements by aberration-corrected STEM is crucial in studying crystalline materials, as it can provide insights into ordering mechanisms and local heterogeneities. However, HAADF-STEM imaging, commonly used for such measurements, is considered insensitive to light atoms like oxygen. This study demonstrates that cation sites in distorted perovskites can appear displaced from their true positions due to the effect of light atoms and crystal symmetry, which can be minimized by adjusting sample thickness and beam voltage or by reorienting the crystal.