Electron Nano-Crystallography: Precession Electron Diffraction in an Aberration-Free Environment

绝大多数重要的材料本质上都是结晶的。因此，能够确定新材料晶体结构的技术是很重要的。x射线和中子方法现在非常复杂和精确，但往往不能解决结构，因为材料是多相的，感兴趣的晶体体积太小，或者在x射线/中子束的长度尺度上有太多的无序。电子显微镜提供了一种方法来记录纳米级区域的衍射数据，克服了x射线和中子技术的局限性。然而，电子与晶体强烈相互作用，由此产生的衍射强度通常不能直接用于确定晶体结构。进动电子衍射是一种方法，通过该方法，电子束在样品上方的一个空心锥体中扫描，然后在下面去扫描，从而产生通过布拉格条件积分的衍射强度；这种几何结构相当于沿固定光束对晶体进行加工。衍射进动强度不容易受到动力效应的影响，可以用来求解晶体结构。从广义上讲，扫描角或进动角越大，动态散射对衍射强度的影响就越小。在这个提议中，我们计划在像差校正电子显微镜上实现进动，这样可以在保持超小纳米级光束的同时使用大进动角。在传统的仪器上，探针形成透镜的球差导致除了最小的进动角外，所有的探针都非常大。进动将被优化，以产生最小的探头，最大的进动角。我们将评估实现这一目标的两种方式。第一种方法需要动态控制像差，随着光束的进动改变校正光学元件。理论上，这样会产生很小的光束，但在实践中很难实现。第二种可能是首选的选择是选择透镜离焦，使通带能够匹配进动角，并限制整个光束在扫描进动锥时的畸变。在这里，修正是固定的，而且在实践中实施起来要容易得多。然后，我们计划使用来自许多不同结构类型的大量模拟来确定特定材料类型的最佳进动角的通用曲线，以建立趋势和相似性。同时进行的将是利用电子通道和原子弦散射强度概念的进动理论的发展。规范化数据是结构解决方案成功的关键，我们将对此进行深入研究。如何最好地整合包含高空间频率信息的高阶（HOLZ）数据也将被研究。PI最近在Acta crystal a上发表了一篇论文，详细介绍了一种利用电子数据结合“电荷翻转”和相对称来解决晶体结构的新方法。我们计划将其扩展到与更传统的“直接方法”元素相结合，这是一种更统计的结构分析方法。通过结合在倾斜轴上记录的衍射图案，应该可以直接在3D中解决晶体结构。层析成像采集和进动衍射的结合应该被证明是一个强大的工具。这些方法可以应用于许多晶体体系。在提案中，我们重点关注三个方面：第一种是铋锰矿，一种多铁性（铁电和铁磁性）材料，其行为取决于晶体结构和氧亚化学计量。第二种是氧化铼，与晶体学有关，其热膨胀为负。最后，我们计划研究金属有机框架，以及如何优化进动来研究这类令人着迷的新材料，这些新材料在催化、燃料电池技术和气体储存方面具有前景。

The vast majority of important materials are crystalline in nature. As such it is important that techniques are available which enable the crystal structure to be determined for new materials. X-ray and neutron methods are now very sophisticated and accurate but often fail to solve structures because the material is multi-phased, the crystal of interest has too small a volume or there is too much disorder at the lengthscale of the x-ray/neutron beam. Electron microscopy offers a way to record diffraction data from nm-sized regions overcoming the limitations of x-ray and neutron techniques. However electrons interact strongly with the crystal and the resultant diffraction intensities cannot in general be used directly to determine the crystal structure. Precession electron diffraction is a method by which the electron beam in scanned in a hollow cone above the specimen and then de-scanned below to give rise to diffracted intensities which are integrated through the Bragg condition; this geometry is equivalent to precessing the crystal about a fixed beam. The diffracted precession intensities are less prone to dynamical effects and may be used to solve crystal structures. In broad terms, the larger is the scan, or precession, angle, the smaller the effects of dynamical scattering on the diffracted intensities. In this proposal we plan to implement precession on an aberration-corrected electron microscope so that large precession angles can be used whilst retaining ultra-small nanoscale beams. On conventional instruments the spherical aberration of the probe-forming lens gives rise to very large probes for all but the smallest of precession angles.Precession will be optimised to generate the smallest probe for the largest precession angle. We will evaluate two ways of achieving this. The first requires dynamical control of the aberrations, changing the correction optics as the beam precesses. In theory this gives a very small beam but will be difficult to implement in practice. The second, probably preferred, option is to choose a lens defocus that enables a pass-band to match the precession angle and limit the distortion in the overall beam as it scans around the precession cone. Here the correction is fixed and far easier to implement in practice. We then plan to determine universal curves for the optimal precession angle for particular material types using a large number of simulations from many different structure types to establish trends and similarities. Running concurrently will be a development of precession theory using electron channelling and the concept of scattering strength from atomic strings. Normalizing data is crucial to successful structure solution and this will be investigated thoroughly. How best to incorporate high order (HOLZ) data, which contain high spatial frequency information will also be studied. The PI recently published a paper in Acta Cryst A detailing a new method to solve crystal structures using electron data combining 'charge-flipping' and phase symmetry. We plan to extend this to combine with elements of more conventional 'direct methods', a more statistical approach to structure analysis. By combining diffraction patterns recorded about a tilt axis it should be possible to solve crystal structures directly in 3D. This combination of tomographic acquisition and precession diffraction should prove to be a powerful tool. These methods can be applied to many crystal systems. In the proposal, we focus on three: The first in bismuth manganite, a multiferroic (ferroelectric and ferromagnetic) material, whose behaviour depends upon the crystal structure and oxygen sub-stoichiometry. The second is rhenium oxide with a negative thermal expansion dependent on the crystallography. Lastly, we plan to study metal-organic frameworks and how precession can be optimised to study this fascinating class of new materials, which have promise for catalysis, fuel cell technology and gas storage.