Unconventional out-of-plane domain inversionviain-plane ionic migration in a van der Waals ferroelectric

Two-dimensional ferroelectrics have gained attention because of their unique ferroelectric properties,e.g., giant negative electrostriction, odd/even layer effect, intercorrelation of in-plane and out-of-plane ferroelectricity, ferroelectric metal nature or tunable quadruple-well ferroelectric property. Here, the interplay between ionic kinetics and ferroelectric switching properties is reported in a model system, CuInP₂S₆, one of the few van der Waals room-temperature ferroelectric crystals. An unconventional out-of-plane domain inversion phenomenon induced by an in-plane electric field has been observed and clarified experimentally. The intralayer hopping motions of Cu-ions can play a role as media of ferroelectric domain inversion. These findings not only provide direct evidence for the microscopic origin of ferroelectric polarization in CuInP₂S₆, but also establish a new mechanism to reverse ferroelectric dipoles simply and effectively, offering opportunities for both fundamental studies and new device applications in information and energy storage.