Unraveling the Relationship Between Layer Stacking and Magnetic Order in Nb₃X₈ Systems

Niobium halides of form Nb₃X₈ (X=Cl or Br) are cluster-based, 2D materials that exhibit an antiferromagnetic to non-magnetic transition. In Nb₃Cl₈, the loss of magnetic order occurs below 90 K and has been coupled to a layer re-stacking from a 2-layer (α-phase) to 6-layer (β-phase) unit cell. The transition temperature, however, depends strongly on composition with Nb₃Br₈ transitioning at 293 K. While tuning magnetic ordering temperature through composition is appealing, the layer re-stacking mechanism is not understood. Here, we used controlled-temperature cryogenic scanning transmission electron microscopy (cryo-STEM) to study the re-stacking in Nb₃Br₈ with atomic-resolution. Our results reveal a reversible transformation from the α-phase to β-phase at ~250 K upon cooling and the reverse at ~425 K upon heating through a series of intermediate phases. Tracking the emergence of intermediates with electron diffraction and Multislice image simulations provides a clearer picture of favorable stacking configurations for van der Waals Nb₃X₈. Understanding these stacking changes and their effect on magnetic ordering will afford handles for materials with tailored transition temperatures.