Domain-wall engineering and topological defects in ferroelectric and ferroelastic materials

Abstract

Ferroelectric and ferroelastic domain walls are two-dimensional (2D) topological defects with thicknesses approaching the unit cell level. When this spatial confinement is combined with observations of emergent functional properties, such as polarity in non-polar systems or electrical conductivity in otherwise insulating materials, it becomes clear that domain walls represent a new and exciting state of matter. In this review, we discuss the exotic polarisation profiles that can arise at domain walls with multiple order parameters and the different mechanisms that lead to domain wall polarity in non-polar ferroelastic materials. The emergence of energetically degenerate variants of the domain walls themselves suggests the existence of interesting quasi-1D topological defects within such walls. We also provide an overview of the general notions which have been postulated as fundamental mechanisms responsible for domain wall conduction in ferroelectrics. We then discuss the prospect of combining domain walls with transition regions observed at phase boundaries, homo-and heterointerfaces, and other quasi-2D objects, enabling emergent properties beyond those available in today's topological systems. Key points• In ferroelectrics, the emergence of a second polarisation component leads to analogues of Bloch and Néel walls. The stabilization of these walls opens the possibility for quasi-1D topological defects separating wall regions of opposite polarities.• Polar domain walls in ferroelastics rely on two mechanisms: a polarity imposed by the natural symmetry of strain-compatible domain walls, which can often be described by flexoelectric gradient coupling, and the emergence of a potentially switchable polarity when their natural symmetry is broken.• Several mechanisms are responsible for domain wall conduction in ferroelectrics: extrinsic intra-bandgap defect states, intrinsic suppression of the conduction band and intrinsic shift of the band structure induced by local electric fields.• Transition regions occurring at phase boundaries, homo-and heterointerfaces, and other quasi-2D objects probably exist at a smaller length scale, in the vicinity of domain walls, and could lead to exceptional properties and coupling phenomena.

Avalanche criticality during ferroelectric/ferroelastic switching

“…Their dynamical behaviour is subject to intense fundamental and applied research as it is at the core of ferroelectric switching [1][2][3][4] , which is the key design parameter, e.g., in piezoelectric 5,6 , magnetoelectric [7][8][9] and memristive [10][11][12] devices, as well as in devices operating above GHz frequencies 13 . Furthermore, the recent research on two dimensional functionalities offered by ferroelectric domain walls [14][15][16][17] will find further applications once strategies to dynamically deploy the functionalities, through the motion of domain walls, have been found.…”

Crackling noise and avalanches in minerals

“…As an example, if the movement relates to low-dimensional dynamical patterns, the relationship is linear S ~ A while in magnetic systems with high fractal dimensions we find S ~ A 2 . This already highlights that model calculations are often required to determine this S(A) scaling and that scaling depends sensitively on the fractal dimension of the domain patterns (Casals et al 2019(Casals et al , 2021aNataf et al 2020;Xu et al 2020).…”

“…Microstructures cover a huge range of length scales from coarse twinning (mm scale), fine twins (typically on a micrometer scale) and tweed structures with repetition scales between 10 and 100 nm. On an even smaller scale we have structural disruptions, like kinks and domain wall bendings, so-called wobbles, inside these microstructures (Salje et al 2017a;He et al 2018;Wang et al 2018;Nataf et al 2020). These small disruptions appear as shifts of atomic positions and are typically measured on a pm scale (e.g.…”

Layer number dependent ferroelasticity in 2D Ruddlesden–Popper organic-inorganic hybrid perovskites

“…These domains can then be switched by the application of external stress, which is analogous to ferroelectricity and the spontaneous switching of polarization in response to an external electric field 3 – 5 . The switching behavior triggered by external strain or stress enables dynamic tuning of material properties by inelastic strain engineering, particularly for applications in flexible electronics 6 – 8 . The study of ferroelasticity in organic-inorganic halide perovskites began with the observation of domain-like structures by various characterization techniques 9 – 13 .…”