Noncollinear ferrielectricity in a van der Waals crystal

Inspired by noncollinear magnetic dipole order, the noncollinear electric dipole order is anticipated to induce rich ferroelectric physics for fascinating device applications. However, establishing noncollinear electric dipole ordering in single crystals is challenging due to electric polarization locking to the crystallographic axes. Here, we report the noncollinear ferrielectricity in a van der Waals crystal WO2Br2 by introducing the competition of ferroelectric and antiferroelectric modes, and uncover its critical role on the polarization switching under hydrostatic pressure and coherent phonon dynamics. The noncollinear dipole order of WO2Br2 was revealed by decoupling the antipolar displacement component along the b-axis and the polar component along the c-axis, both of which were directly visualized by scanning transmission electron microscopy (STEM). Moreover, this noncollinear dipole order not only drives the robust in-plane uniaxial macroscopic ferroelectric behavior, but also enables 90° polarization flip under hydrostatic pressure with two energetically degenerate transition pathways: one via an antipolar intermediate phase, the other through a 45° polar phase respectively. Specially, ultrafast electron diffraction measurements confirm that optical excitation drives two distinct coherent phonon modes associated with the ferroelectric and antiferroelectric orders respectively, shedding light on ultrafast control of the exotic noncollinear ferroelectric states. Our work definitely unlocks the underlying physics of noncollinear ferroelectricity for emergent ferroelectric device applications.