Noncollinear ferrielectricity in a van der Waals crystal.

Inspired by noncollinear magnetic dipole order, noncollinear electric dipole order is anticipated to open up rich ferroelectric physics and new opportunities for device applications. However, establishing such order in single crystals remains challenging, as electric polarization is typically locked to crystallographic axes. Here we report noncollinear ferrielectricity in the van der Waals crystal WO2Br2, arising from competition between ferroelectric and antiferroelectric phonon modes. The noncollinear dipole order of WO2Br2 is revealed by decoupling the antipolar and the polar displacement components, both of which are directly visualized by scanning transmission electron microscopy. This noncollinear dipole order enables 90° polarization flip under hydrostatic pressure through two energetically degenerate transition pathways. Moreover, ultrafast electron diffraction measurements show that optical excitation drives two distinct coherent phonon modes associated with the ferroelectric and antiferroelectric orders. This work enriches the physics of noncollinear electric dipole order for emergent ferroelectric device applications.