Full determination of individual reconstructed atomic columns in intermixed heterojunctions

Heterojunctions offer a tremendous opportunity for fundamental as well as applied research, ranging from the unique electronic phases in between oxides to the contact issues in semiconductor devices. Despite their pivotal roles, determining individual building atom of matter in heterojunctions is still challenging, especially for those between highly dissimilar structures, in which breaking of symmetry, chemistry, and bonds may give rise to complex reconstruction and intermixing at the junction. Here, we combine electron microscopy, spectroscopy, and first-principles calculations to determine individual reconstructed atomic columns and their charge states in a complex, multicomponent heterojunction between the delafossite CuScO₂ and spinel MgAl₂O₄. The high resolution enables us to demonstrate that the reconstructed region can accommodate a highly selective intermixing of Cu cations at specific Sc cation sites with half atomic density, forming a complex ordered superstructure. Such ability to resolve reconstructed heterojunctions to the atomic dimensions helps elucidate atomistic mechanisms and discover novel properties with applications in a diverse range of scientific disciplines.