Random in-plane translational stacking faults in the van der Waals superlattice Ba₆Nb₁₁Se₂₈

The Ba₆M₁₁X₂₈ (M = Nb, Ta; X = S, Se) family of natural van der Waals superlattices hosts highly anisotropic superconductivity and exotic two-dimensional electronic behavior. However, the actual stacking sequences of these superconducting superlattices, crucial for property engineering, remains poorly understood. Here we present a detailed microstructural study of Ba₆Nb₁₁Se₂₈ using aberration-corrected scanning transmission electron microscopy (STEM), powder X-ray diffraction (PXRD) and DIFFaX simulations. We identify intrinsic in-plane translational stacking faults confined to the Ba₃NbSe₅ spacer layers and directly resolve three symmetry-equivalent stacking configurations at the atomic scale. These stacking variants preserve the local coordination environment and occur randomly without correlation between adjacent spacer layers. The stacking disorder leads to characteristic extinction of (10l) reflections in diffraction, which is quantitatively reproduced by simulations based on a random stacking model. Our results establish in-plane translational stacking disorder as an intrinsic structural feature of Ba₆Nb₁₁Se₂₈ and highlight the importance of spacer-layer degrees of freedom in layered van der Waals superlattices.