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.

中文翻译：

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领域边界主导系统：自适应结构和功能孪生边界

域边界通常仅占晶体总体积的一小部分。然而，可能会出现一种特殊（但并不罕见）的情况，其中域边界能量变得非常小。具体而言，域尺寸缩小到接近原子尺度，域边界密度变得非常高。在这种情况下，晶体的特性由域和域边界的组合决定。这种现象不同于大多数铁磁或铁电材料，其中畴边界的运动支配响应。如本文所报告的，可以预期与域或域边界的特性不同的新出现的现象。在本文中，我们关注在铁质材料中发现的一种特定状态——即适应性铁质状态。例如，这种状态可以在同形相界压电晶体、铁磁形状记忆合金和前马氏体状态中的花呢状结构中找到。在这些材料中，孪生边界的特性代表了给定系统功能的主要贡献者。事实上，对域边界主导现象的进一步研究可能会产生为所需结果定制功能特性的新潜力。还应该注意的是，可以将新属性设计为不是域属性的孪生边界。本文回顾了自适应结构和功能孪生边界，并提供了各种观察到的功能（例如超导性、极性和铁电性）和相应孪生边界结构的示例。此外，这篇综述证实，各种理论上预测的、结构上桥接的低对称相确实存在。此外，自适应状态的晶格常数值是由立方、菱形/四方相和几何不变条件的组合确定的可调参数。最后，我们表明，在这种情况下，宏观特性由双壁的独特功能控制。展望未来，域边界主导的现象提供了一种重要的方法来增强体的特性，以及“双胞胎是设备”的独特局部特性。