Chemical control of function beyond the unit cell for new electroceramic materials

先进材料的发现和发展需要理解和控制组成、结构和功能之间的关系。在晶体材料中，有相当多的注意力集中在由布拉格衍射确定的平均晶体晶胞定义的单一宏观结构的设计过程上。这是一种强有力的方法，但越来越明显的是，即使在结晶系统中，局部化学和位置偏离结构的长期平均视图也可以产生决定性的影响。高温超导体中的电荷条纹以及跨越介观到纳米长度尺度的“全景”有序在热电性能中的作用，只是平均结构考虑因素在解释表面上微小的成分变化如何改变功能行为方面的局限性的两个例子。这进而限制了这种结构观点在设计具有增强性能的新材料中的用途。这对于许多功能材料来说尤其关键，在这些功能材料中，通过诸如外加磁场的刺激对铁性序参数(即极化或磁化)的调制或切换产生在器件(例如致动器或数据存储器)中使用的属性(例如，压电性或磁阻)。它们的性能是通过形成固溶体来优化的，例如，在PbZrO_3-PbTiO_3(PZT；)中，90%的压电器件都是由PbZrO_3-PbTiO_3(PZT)组成的，在这一点上，调整了Zr/Ti的比例，使之与三方和四方对称之间的边界相一致，在该边界处，压电电荷系数达到最大。这是由于在平均晶胞中占据相同位置，但对产生极化的位移有很大不同影响的锆阳离子和钛离子的局域构型的随机效应和有利于平均极化方向的长程偶极和弹性相互作用的效应之间的竞争。这种局部结构效应及其产生的有限尺寸相关性对功能施加了决定性的控制，这是传统设计的中心平均结构所看不到的。因此，固溶体的性质不是末端成员的平均值，并且不存在简单的设计规则。项目组最近展示了基于定量局部结构分析的设计如何使材料族具有传统平均结构设计方法无法获得的重要特性。使用来自全布拉格散射研究的纳米级信息来控制性质，他们发现了基于平均结构而被忽视的化学，但导致了新的无铅压电族(先进材料2015)，然后是第一个块状室温铁磁铁电多铁体(自然2015)：这两个长程有序在单个相中的组合一直是一个长期的科学挑战。这个项目将通过理解和操纵单位晶胞长度尺度之外的对称性和结构来发展对功能的控制。我们将建立一个工具包，将固态材料化学、材料科学和凝聚态物理结合起来，将合成、晶体化学、结晶学、局部结构分析、扫描探针显微镜、磁学、电陶瓷测量物理和材料加工集成在一起。该工具包利用了两个参与小组的技能之间的协同效应。通过设计然后制备新的压电和多铁材料，我们将展示这种方法如何指导功能的合成，以及对所研究样本领域以外的性能的控制，例如在非均相催化剂和电极(燃料电池、电池)材料中，为EPSRC物理科学纳米级功能材料设计的重大挑战做出贡献。

Discovery and development of advanced materials requires understanding and control of the relationship between composition, structure and function. In crystalline materials, there is considerable focus on a design process that is informed by a single macroscopic structure defined by the average crystallographic unit cell determined by Bragg diffraction. This is a powerful approach, but it has become increasingly apparent that local chemical and positional deviations from this long-range average view of the structure can have decisive effects even in crystalline systems. Charge stripes in the high temperature superconductors and the role of "panoscopic" order spanning meso- to nano-scopic length scales in thermoelectric performance are just two examples of the limitations of average structure considerations in explaining how an apparently small compositional change can transform functional behaviour. This in turn restricts the utility of such a view of structure in designing new materials with enhanced performance.This is particularly critical for the many functional materials in which modulation or switching of a ferroic order parameter (i.e., polarization or magnetization) by a stimulus such as an applied field produces the property (e.g., piezoelectricity or magnetoresistance) used in devices (e.g., actuators or data storage). Their properties are optimised by formation of solid solutions e.g., in PbZrO3-PbTiO3 (PZT), responsible for >90% of piezoelectric devices, the Zr/Ti ratio is adjusted to coincide with the boundary between rhombohedral and tetragonal symmetries, at which the piezoelectric charge coefficient maximizes. There is competition between the randomising effect of the local configuration of Zr and Ti cations (which occupy the same position in the average unit cell, but locally exert quite different influences on the displacements producing the polarisation) and the effect of the long-range dipolar and elastic interactions favouring the average polarisation direction. This local structure effect and the finite size correlations it produces exerts decisive control of function that is invisible from the average structure central to traditional design. The properties of the solid solutions are thus not an average of the end members, and simple design rules do not exist. The project team have recently shown how design based on quantitative local structure analysis can afford materials families with important properties that had not been accessed by classical average structure design approaches. Using nanoscale information from total Bragg scattering studies to control properties, they identified chemistry that would have been disregarded based on the average structure but led to a new lead-free piezoelectric family (Advanced Materials 2015) and then to the first bulk room temperature ferromagnetic ferroelectric multiferroic (Nature 2015): combination of these two long range orders in a single phase has been a longstanding scientific challenge.This project will develop the control of function by understanding and manipulating symmetry and structure beyond the unit cell length scale. We will build a toolkit that enables this approach by combining solid state materials chemistry, materials science and condensed matter physics to integrate synthesis, crystal chemistry, crystallography, local structure analysis, scanning probe microscopy, magnetism, electroceramic measurement physics, and materials processing. The toolkit exploits the synergies between the skills of the two participating groups.By designing then preparing new piezoelectric and multiferroic materials, we will demonstrate how this approach can guide synthesis for function, with ramifications for control of properties beyond the exemplar areas studied, for example in heterogeneous catalyst and electrode (fuel cell, battery) materials, contributing to the EPSRC Physical Sciences Grand Challenge of Nanoscale Design of Functional Materials.