Domain boundary-dominated systems: adaptive structures and functional twin boundaries

Domain boundaries typically constitute only a minute fraction of the total volume of a crystal. However, a special (but not unusual) situation can occur in which the domain boundary energy becomes very small. Specifically, the domain size is miniaturized to near-atomic scales and the domain boundary density becomes extremely high. In such cases, the properties of the crystal become dominated by a combination of both the domains and the domain boundaries. This phenomenon differs from most ferromagnetic or ferroelectric materials wherein the motion of the domain boundaries dominates the response. As reported herein, novel emergent phenomena that differ from the properties of either the domains or the domain boundaries may be expected. In this article, we focus on one specific state found in ferroic materials – namely, the adaptive ferroic state. This state can be found, for example, in tweed-like structures in morphotropic phase boundary piezoelectric crystals, ferromagnetic shape memory alloys, and pre-martensitic states. In these materials, the properties of the twin boundaries represent the principal contributors to the functionality of a given system. In fact, further investigations of domain boundary-dominated phenomena could result in novel potential for tailoring functional properties for a desired outcome. It should also be noted that new properties can be designed into twin boundaries that are not the properties of the domains. In this paper, adaptive structures and functional twin boundaries are reviewed, and examples of various observed functionalities (e.g. superconductivity, polarity, and ferroelectricity) and corresponding twin boundary structures are provided. In addition, this review confirms that various theoretically predicted, structurally bridging low-symmetry phases do, in fact, exist. Moreover, the values of the lattice constants of the adaptive state are adjustable parameters that are determined by combinations of cubic, rhombohedral/tetragonal phases, and geometrical invariant conditions. Finally, we show that, in such cases, macroscopic properties are controlled by the unique functionality of the twin walls. Looking forward, domain boundary-dominated phenomena offer an important approach for enhancing the properties of the bulk, and to unique local properties where the “twin is the device”. We encourage the community to rethink their approaches to materials by design that have treated the structure as homogeneous and to consider the alternative paradigm where the local structure is different from the apparent average symmetry.