Atomic-Scale Insights into the Polar Surface of KTaO₃(001)

Measuring the atomic-scale surface structure of KTaO₃ (KTO) is important and challenging due to its broken translational symmetry. Here, we employed integrated differential phase contrast imaging to resolve the KTO surface at atomic resolution. Through precise measurements of lattice constants, bond lengths, atomic displacements, and strain gradients, we determined the polarization characteristics at the subunit-cell level. Our results reveal a significant increase in lattice constants and strain gradients within the top ∼4 unit cells as the out-of-plane polarization enhances. Electron energy loss spectroscopy further uncovered the electronic origins of surface reconstruction, showing pronounced distortions of the Ta-O octahedra. In combination with density functional theory calculations, we demonstrate that these effects arise from surface-driven orbital reconstruction and Ta-O hybridization. This work offers insights into subunit-cell surface polarization and is expected to provide guidance for surface engineering in functional applications.