Transition from Conventional Ferroelectricity to Ion-Conduction-Like Ferroelectricity

The cross-unit-cell long displacements in some recent emergent ferroelectrics have actually challenged the classical definition of ferroelectricity, while the explorations are still in the early stage and even controversial. Here, we provide a general model that gives the picture for the evolution and transition from typical ferroelectricity to long displacement ferroelectricity, which is classified into types I and II. In particular, type I with two switching modes of different barriers may switch between conventional ferroelectricity and ion-conduction-like ferroelectricity depending on various factors, including the electric field, boundaries, vacancies, temperature, etc., which is demonstrated by first-principles calculations on γ-AlOOH and CuInP₂S₆ as two paradigmatic cases. Intriguingly, their polarizations are non-local, since boundaries also determine the switching mode and polarization direction, which can be different for the same given crystal structure. Such type I can be evolved from conventional ferroelectricity as the migration barrier across the unit cell is reduced and will behave like type II at an elevated temperature.