Multistate Ferroelectricity Enabled by Electrically Controlled Phase Transition of Two-Dimensional Ices

Multistate ferroelectric polarization holds promise for realizing high-density nonvolatile memory devices, but so far is restricted to a few traditional ferroelectrics. Here, we show that nanoconfined two-dimensional (2D) ferroelectric ice can achieve phase-dependent multistate polarization through extensive classical and ab initio molecular dynamics simulations. An in-plane electric field is found to induce the reversible transition between a low-polarization AA-stacked hexagonal ice phase and an unprecedented high-polarization AB-stacked ice phase, resulting in a four-state ferroelectric switching pathway. The findings highlight the potential of 2D ice as a functional material for the development of multistate ferroelectric memory devices.