Dual-Role Ion Dynamics in Ferroionic CuInP2S6: Insights into the Transition and Boundary from Ferroelectric to Ionic Switching Mechanisms

Abstract Due to its "ferroionic" nature, the van der Waals CuInP2S6 combines switchable ferroelectric polarization with highly mobile Cu ions, allowing for multiple resistance states and multifunctionalities. Its conductive mechanism typically involves the ferroelectric switching, ion migration, and corresponding intercoupling, which are highly sensitive to the external electric field. However, it is still a challenge to distinguish the dominated contribution of either ferroelectric switching or ion migration to dynamic conductivity, and the underlying mechanism is not clear yet currently. Here, we demonstrate the Cu ions migration pathways and their interactions with ferroelectric switching, based on PFM analyses and first-principles calculations. It is confirmed that the Cu ion migrations within the intralayer and interlayer enable the formation of a quadruple-well state, thus determining the conductive mechanism. Accordingly, it favors the manipulation of Cu ion transport in the quadruple state in a controlled manner, and makes a transition from ferroelectric-dominated to ion-migration-dominated conductivity, by tailoring the electric fields. This work deepens the understanding of ion migration dynamics and conductive switching in ferroionic systems, which is critical for the advancement of memristor-based neuromorphic computing.