Strain-Driven Formation of Misfit van der Waals Gaps Via Topotactic Cation Exchange.

Lattice misfit strain is a fundamental tool for engineering the properties of epitaxial oxides and thin films. In van der Waals (vdW) layered materials, however, harnessing intrinsic misfit strain remains challenging because the weak interlayer bonding accommodates large lattice mismatches without imposing sufficient misfit constraint. Here, employing a topotactic reaction to unfold the formation process demonstrates that a misfit vdW gap is a dynamically active interface governed by misfit strain. Atomic-resolution in situ scanning transmission electron microscopy captures the real-time relaxation of the gap from two distinct energetic extremes. The interface evolves toward its equilibrium spacing through either rapid vertical relaxation when compressed or diffusion-mediated cation expulsion when expanded. This behavior reveals that misfit-enhanced interactions are sufficiently strong to drive the reconstruction of local chemistry and structure. Ultimately, harnessing this mechanism offers a generalizable pathway to precisely engineer the electronic and magnetic coupling across the broad family of vdW heterostructures.