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成分、结构和功能之间的关系。在晶体材料中，有相当多的焦点放在设计过程上，该设计过程是由布拉格衍射确定的平均晶体学单位胞定义的单个宏观结构所决定的。这是一种强有力的方法，但越来越明显的是，局部化学和位置偏离这种长期平均结构视图，即使在晶体系统中也会产生决定性的影响。高温超导体中的电荷条纹和跨越中观到纳米尺度长度的“全景”顺序在热电性能中的作用只是平均结构考虑在解释明显的小成分变化如何改变功能行为时的局限性的两个例子。这反过来又限制了这种结构观点在设计具有增强性能的新材料时的实用性。这对于许多功能材料来说尤其重要，在这些功能材料中，通过施加的场等刺激调制或切换铁序参数（即极化或磁化）会产生用于器件（例如致动器或数据存储）的特性（例如压电或磁阻）。通过形成固溶体来优化它们的性能，例如，在占压电器件bbb90 %的PbZrO3-PbTiO3 （PZT）中，调整Zr/Ti比以符合菱形对称和四边形对称之间的边界，此时压电电荷系数最大。Zr和Ti阳离子（它们在平均晶胞中占据相同的位置，但在局部对产生极化的位移施加完全不同的影响）的局部构型的随机效应与有利于平均极化方向的远程偶极和弹性相互作用之间存在竞争。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，这是传统设计中心的平均结构所看不到的。因此，固溶体的性质不是末端构件的平均值，简单的设计规则不存在。项目团队最近展示了基于定量局部结构分析的设计如何能够提供具有经典平均结构设计方法无法获得的重要特性的材料家族。利用来自总Bragg散射研究的纳米级信息来控制性能，他们确定了基于平均结构而被忽视的化学反应，但导致了新的无铅压电家族（Advanced Materials 2015），然后是第一个室温铁磁铁电多铁质（Nature 2015）：将这两个长范围顺序结合在一起一直是一个长期的科学挑战。该项目将通过理解和操纵超越单位细胞长度尺度的对称和结构来发展功能控制。我们将建立一个工具包，通过结合固态材料化学、材料科学和凝聚态物理来整合合成、晶体化学、晶体学、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，从而实现这种方法。该工具包利用了两个参与小组的技能之间的协同作用。通过设计和制备新的压电和多铁性材料，我们将展示这种方法如何指导功能合成，并对所研究的范例领域之外的性能控制产生影响，例如异相催化剂和电极（燃料电池、电池）材料，为EPSRC纳米级功能材料设计物理科学大挑战做出贡献。

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.