The distinct displacement of atomic column position induced by crystal symmetry effects in ABF-STEM and iDPC-STEM...

High-resolution imaging and accurate measurement of atomic column positions are essential for achieving an atomistic understanding of crystalline structures. Annular bright-field scanning transmission electron microscopy (ABF-STEM) and integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) are powerful techniques for achieving atomic resolution characterization, offering the advantage of direct image interpretation for the positions of atomic columns and enabling simultaneous detection of both heavy and light atomic columns. Nevertheless, the image contrast and determination of atomic column positions in ABF-STEM and iDPC-STEM are susceptible to variations in specimen conditions and experimental conditions, posing significant challenges for accurately determining atomic configuration. Compared to the image contrast deviations observed in ABF-STEM and iDPC-STEM imaging, atomic column displacements are less perceptible, as they typically occur at the picometer scale, presenting a greater challenge to the reliability of ABF-STEM and iDPC-STEM imaging techniques. However, the underlying mechanisms governing atomic column displacements have not yet been thoroughly investigated. In this study, SrTiO₃ and LaSrAlO₄ are selected as representative examples of highly symmetric and lower symmetric crystalline structures, respectively, to systematically investigate the variations in atomic column displacement observed in ABF-STEM and iDPC-STEM images through a combination of experimental observations and simulation analyses. The oxygen atomic columns in SrTiO₃ remain undisplaced. In contrast, the displacement of apical oxygen atomic columns in LaSrAlO₄ from their ideal positions is observed to vary with specimen thickness under both ABF-STEM and iDPC-STEM imaging; however, the two techniques yield markedly different observations. The propagation dynamics of the electron probe within SrTiO₃ and LaSrAlO₄ indicate that crystallographic symmetry plays a dominant role in the formation of deviations in oxygen atomic column positions. This study highlights the necessity of exercising caution when quantifying atomic column displacements from ABF-STEM and iDPC-STEM images, particularly in cases involving lower crystal symmetry and variations in sample thickness.