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Transition from conventional ferroelectricity to ion-conduction-like ferroelectricity

The cross-unitcell long displacements in some recent emergent ferroelectrics have actually challenged the classical definition of ferroelectricity, while the relative explorations are still in the early stage and even controversial. In this paper 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 type-I and type-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 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 nonlocal since the 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 unitcell is reduced, and will behave like type-II at elevated temperature as the conventional part becomes paraelectric. These unconventional behaviors can be applicable to various systems, and many previously unclarified phenomena can be well explained.