Unique features of polarization in ferroelectric ionic conductors

TL;DR

This paper investigates the polarization behavior in ferroelectric ionic conductors, specifically CuInP2S6, and introduces an asynchronous migration scheme to accurately model polarization evolution during ionic conduction.

Contribution

It presents a novel asynchronous Cu-migration scheme that improves the modeling of polarization changes in ferroelectric ionic conductors, especially across van der Waals gaps.

Findings

The modern theory of polarization with concerted migration is inadequate for Cu crossing van der Waals gaps.

The asynchronous migration scheme accurately describes polarization evolution during ionic migration.

The framework unifies ferroelectric switching and ionic conduction processes.

Abstract

Ferroelectrics that are also ionic conductors offer possibilities for novel applications with high tunability, especially if the same atomic species causes both phenomena. In particular, at temperatures just below the Curie temperature, polarized states may be sustainable as the mobile species is driven in a controlled way over the energy barrier that governs ionic conduction, resulting in unique control of the polarization. This possibility was recently demonstrated in CuInP2S6, a layered ferroelectric ionic conductor in which Cu ions cause both ferroelectricity and ionic conduction. Here, we show that the commonly used approach to calculate the polarization of evolving atomic configurations in ferroelectrics using the modern theory of polarization, namely concerted (synchronous) migration of the displacing ions, is not well suited to describe the polarization evolution as the Cu ions cross the van der Waals gaps. We introduce an asynchronous Cu-migration scheme, which reflects the physical process by which Cu ions migrate, resolves the difficulties, and describes the polarization evolution both for normal ferroelectric switching and for transitions across the van der Waals gaps, providing a single framework to discuss ferroelectric ionic conductors.