In-Plane Ferrielectric Order in van der Waals β'-In₂Se₃

van der Waals ferroic materials exhibit rich potential for implementing future generation functional devices. Among these, layered β'-In₂Se₃ has fascinated researchers with its complex superlattice and domain structures. As opposed to ferroelectric α-In₂Se₃, the understanding of β'-In₂Se₃ ferroic properties remains unclear because ferroelectric, antiferroelectric, and ferroelastic characteristics have been separately reported in this material. To develop useful applications, it is necessary to understand the microscopic structural properties and their correlation with macroscopic device characteristics. Herein, using scanning transmission electron microscopy (STEM), we observed that the arrangement of dipoles deviates from the ideal double antiparallel antiferroelectric character due to competition between antiferroelectric and ferroelectric structural ordering. By virtue of second-harmonic generation, four-dimensional STEM, and in-plane piezoresponse force microscopy, the long-range inversion-breaking symmetry, uncompensated local polarization, and net polarization domains are unambiguously verified, revealing β'-In₂Se₃ as an in-plane ferrielectric layered material. Additionally, our device study reveals analogous resistive switching behaviors of different types owing to polarization switching, defect migration, and defect-induced charge trapping/detrapping processes.