Unexpected Ionic Contribution Driving Polarization Switching in Hexagonal Interfacial Ferroelectrics

Hexagonal interfacial ferroelectrics have challenged conventional understanding of ferroelectricity by exhibiting ultrafast switching and ultralow energy barriers, which have been widely interpreted within a purely electronic framework. Here, we demonstrate that this picture overlooks an essential ionic degree of freedom. First-principles calculations reveal that pronounced atom-resolved out-of-plane (OOP) Hellmann-Feynman forces in the flattened polar configuration drive intrinsic ionic displacements, generating layer-asymmetric OOP corrugations and an additional OOP polarization associated with ionic relaxation that aligned with the electronic polarization. Rather than slowing down switching, this ionic response lowers the energy barrier and accelerates polarization reversal. Combining force analysis with large-scale molecular dynamics powered by an AI potential with first-principles accuracy, we uncover the multiscale nature of polarization switching. This approach enables direct disentanglement of diagonal and off-diagonal BEC contributions, revealing that intrinsic OOP ionic corrugation couples to interlayer in-plane motion and enhances domain wall propagation beyond the conventional off-diagonal Born effective charge (BEC) mechanism. Remarkably, when OOP corrugation is incorporated, the diagonal BEC term (Z₃₃), previously considered ineffective, becomes sufficient to drive polarization switching. Furthermore, biaxial strain can effectively modulate the corrugation amplitude, offering an experimentally accessible strategy for accelerating polarization switching. These findings reveal that ionic corrugation, rather than being a negligible structural response, is an essential component of the polarization switching mechanism in hexagonal interfacial ferroelectrics. Beyond this specific system, the identified ionic mechanisms closely resemble those in conventional ferroelectrics, establishing a unified picture of ferroelectric polarization and switching across distinct material platforms.