Rectified Water Migration Behavior in the Noncentrosymmetric Channels of a Ferroelectric Proton Conductor

Ferroelectric ion conductors composed of noncentrosymmetric host structures and guest water molecules have recently garnered attention. These systems exhibit colossal polarization driven by long ion displacement facilitated by water molecules; however, the manner in which water molecules are perturbed by the polar backbone remains unclear. In this study, we investigated water migration behavior within the noncentrosymmetric channels of the ferroelectric proton conductor K₂MnN(CN)₄·H₂O using various first-principles computational methods, including climbing image nudged elastic band (CI-NEB) calculations, potential energy surface (PES) scans, and ab initio molecular dynamics (AIMD) simulations. The energetic and dynamic characteristics governing water migration, obtained through CI-NEB and PES scans, revealed a significant directional preference for migration. Specifically, a lower activation barrier for migration in the negative c-axis direction compared to the positive one suggested rectification characteristics. These direction-dependent transition state energies were attributed to anisotropic arrangements of CN ligands, whose π orbitals interact with the highest occupied molecular orbital of the water molecule. In addition, AIMD simulations demonstrated that water molecules exhibit dynamically biased fluctuations around their equilibrium positions, corroborating the role of the polar framework as an internal electric field that directs water flow.