Robust Ferroelasticity and Carrier Dynamics Across the Domain Wall in Perovskite-Like van der Waals WO₂I₂

As a new group of van der Waals (vdWs) ferroic materials, transition metal dioxydihalides MO₂X₂ (M: Mo, W; X: halogen) with a perovskite-like structure are theoretically predicted to exhibit intriguing physics and versatile ferroic characteristics, which is not achieved experimentally as far as it is known. In this work, the robust ferroelasticity in vdWs WO₂I₂ with the switching strain as low as ≈0.3%, accompanied with the striped optical contrast between adjacent domains, spot splitting of selected area electron diffraction (SAED) patterns at domain wall, and 90° domain wall is demonstrated. With the aid of ab-initio calculations, the origin of ferroelasticity in WO₂I₂ is unveiled, where the imaginary phonon mode in the high-symmetry paraelastic phase leads to the spontaneous displacement of W atom away from the center of the [WO₄I₂] octahedron, resulting in the switchable spontaneous strain under an external strain field. Moreover, transient absorption microscopy (TAM) measurements demonstrate that the diffusion of photogenerated carriers is significantly hindered by the ferroelastic domain walls. This study provides deep insights into the ferroic order and domain wall in perovskite-like vdWs MO₂X₂ for new physics and functionalities.