Using aberration corrected STEM to study the atomic structure of incommensurate antiferroelectrics

一些原子结构不适用于使用X射线或中子的传统衍射基解决方案。其中一个例子是不规则或无序的层状结构，其中纳米尺度上的无序使得对衍射数据的任何解释都非常困难。这就是掺镧富锆铅钛酸铅的情况，其中单元电池通常在一个方向上具有周期，而不是组成它们的简单电池的积木数量的简单倍数(即，整个电池与原始电池不相称)。这似乎是因为可以形成不同的层状结构，并且它们可以不规则地堆叠在一起。在这种情况下，确定局部原子结构的唯一方法是通过具有原子分辨率的基于成像的技术。该项目将在世界领先的EPSRC支持的SuperSTEM设施中使用高分辨率扫描电子显微镜，该设施可以成像到1+以下的特征。特别是，样品将使用高角度环形暗场技术进行成像，该技术对重元素特别敏感，因此对于定位铅和锆离子将是非常宝贵的；这将为单独进行的HRTEM研究(也由EPSRC根据赠款参考EP/H028218/1资助)提供强大的补充，后者对氧离子位置更敏感。然后，将对这两种技术的图像进行定量分析，以提取原子位置，并使用来自多个方向的图像，然后确定每个堆叠序列中的三维原子位置。这将使我们能够理解极化是如何在不同的堆积中排序的，以便产生反铁电结构。这将有力地展示像差校正的STEM结合定量数据分析以原子分辨率分析纳米结构的能力，也将为未来电场诱导相变的工作和反铁电锆钛酸铅化合物的可能应用奠定基础。

Some atomic structures are just not amenable to traditional diffraction-based solution using X-rays or neutrons. One example of this is irregular or disordered layered structures, where the disorder on the nanoscale makes any interpretation of diffraction data extremely difficult. This is exactly the situation in lanthanum-doped zirconium-rich lead zirconate titanate where the unit cells often appear to have a period in one direction which is not a simple multiple of the number of building blocks of simple cells that they are made of (i.e. the full cell is incommensurate with the primitive cell). It appears that this happens because different layered structures can be formed and they can be stacked together irregularly. In this situation, the only way to determine the local atomic structure is via an imaging based technique with atomic resolution.This project will use high resolution scanning transmission electron microscopy at the world-leading EPSRC-supported SuperSTEM facility, which can image features down to below 1 +. In particular, the samples will be imaged using the High Angle Annular Dark Field technique, which is especially sensitive to heavy elements and which will consequently be invaluable for locating Pb and Zr ions; this will provide a powerful complement to HRTEM investigations being performed separately (also funded by the EPSRC under Grant Reference EP/H028218/1) which is more sensitive to oxygen ion locations. Images from both techniques will then be analysed quantitatively to extract atom positions and using images from more than one direction, we will then determine the three dimensional atom positions in each stacking sequence. This will then enable us to understand how the polarisation is ordered within the different stacking in order to produce antiferroelectric structures. This will provide a powerful demonstration of the capabilities of aberration corrected STEM in conjunction with quantitative data analysis to analyse nanoscale structures with atomic resolution, and will also lay the groundwork for future work on electric field induced phase transformations and possible applications of antiferroelectric lead zirconate titanate compositions.