The Development of Precession Electron Diffraction for High Resolution Electron Crystallography

高分辨率电子晶体学进动电子衍射的发展

• 批准号: EP/E037275/1
• 负责人: Paul Midgley
• 金额: $ 35.55万
• 依托单位: University of Cambridge
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2007
• 资助国家: 英国
• 起止时间: 2007 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FE037275%2F1
• 关键词: Development; Precession; Electron; Diffraction; Resolution

By forming a parallel beam in the transmission electron microscope (TEM) and selecting a crystal with an aperture, or by forming a fine convergent beam of only a few nanometres diameter, a diffraction pattern can be recorded from crystals too small to be studied by x-ray diffraction. In addition, electron diffraction appears to offer many fundamental advantages for the study of crystalline materials: electrons have a greater interaction cross-section than x-rays or neutrons, they are scattered by light atoms relatively more strongly and electron diffraction patterns can show reflections corresponding to a resolution beyond that available with x-rays. However, in general, the reflections will be sensitive to dynamical effects and their intensity is a complex function of the crystal potential and the specimen thickness. To minimise dynamical effects, a relatively new approach, 'electron precession', is proposed in which the beam is rocked in a hollow cone both above and below the specimen plane. The resulting diffraction pattern has the same geometry as a conventional diffraction pattern but there are considerably more reflections excited. Most importantly, the intensities in precession patterns suffer less from dynamical perturbations and, at least in some cases, can be treated as kinematical.A. Technique Development: To solve structures in three dimensions, electron precession patterns must be recorded at a number of crystal orientations, ideally from mutually perpendicular zone axes. In practice, this ideal may not be possible but instead precession patterns can be recorded at many orientations about a tilt axis (or axes) to sample reciprocal space systematically and used for 3D structure determination. In the longer term, we would make such an acquisition semi-automatic.We will develop novel structure determination algorithms which are more suited to the rather sparse data sets recorded with electron diffraction. We are developing a method that applies constraints, in both reciprocal space and real space, which appear to be very good at limiting the possible solutions when data is sparse. We plan to formulate a theory that places the precession technique on a firmer footing that should allow better optimisation of the precession angle and sample thickness for diffraction experiments.B. Applications. Tin oxide is an important industrial compound for gas sensors, as an opacifier in ceramic glazes, as a heterogeneous petrochemical catalyst and, with indium oxide, a transparent conductive coating. It forms a metastable phase whose structure and precise composition is unknown. It cannot be grown as a single crystal, and in powder form it grows in combination with the equilibrium phases of tin oxide and elemental tin. We plan to solve the structure of this phase using electron precession.Ordered mesoporous silicas are solids with extremely high specific surface areas leading to catalysts of high activity and selectivity in the hydrogenation of key organic molecules. We propose to use electron precession to reveal the full three-dimensional crystallography of one of these ordered silicas, MCM-48; results from previous work disagree on the size of an important secondary pore structure.The high percentage of unknown crystal structures in the pharmaceutical industry is testament to the difficulty in determining their structures by x-ray diffraction (e.g. polymorphism). Electron precession offers valuable experimental data from minute quantities of crystal that can be used, at best, to determine the crystal structure directly, or at worst, offers experimental evidence to support powder refinements or computational analyses. We will investigate the viability of electron precession for pharmaceutical materials, studying, for example, the structure of polymorphs of 5-fluorouracil, a common anti-cancer drug, and anhydrous theophylline used as a vasodilator and a muscle relaxant whose structure is still not fully solved.

通过在透射电子显微镜中形成平行光束并选择有孔的晶体，或通过形成直径仅为几纳米的精细会聚光束，可以从太小而无法用X射线衍射研究的晶体记录衍射图。此外，电子衍射似乎为晶体材料的研究提供了许多基本优势：电子具有比X射线或中子更大的相互作用截面，它们被轻原子散射得相对更强，电子衍射图案可以显示出与X射线所能获得的分辨率相对应的反射。然而，通常情况下，反射对动力学效应很敏感，其强度是晶体电势和样品厚度的复函数。为了减小动力学效应，提出了一种相对较新的方法--“电子进动”，在这种方法中，束流在样品平面上方和下方的中空锥体中摇摆。所得到的衍射图具有与传统衍射图相同的几何形状，但有相当多的反射被激发。最重要的是，进动图案中的强度受到动力学扰动的影响较小，至少在某些情况下，可以被视为运动学。a.技术发展：为了求解三维结构，电子进动图案必须记录在许多晶体取向上，理想情况下是从相互垂直的带轴。在实践中，这一理想可能是不可能的，但可以在围绕倾斜轴(或多个轴)的许多方向上记录进动图案，以系统地对倒易空间进行采样，并用于3D结构确定。从长远来看，我们将使这种采集半自动进行。我们将开发新的结构确定算法，这些算法更适合于用电子衍射记录的相当稀疏的数据集。我们正在开发一种在倒易空间和真实空间中应用约束的方法，当数据稀疏时，这种约束似乎非常擅长限制可能的解决方案。我们计划制定一种理论，将进动技术建立在更坚实的基础上，以便为衍射实验更好地优化进动角和样品厚度。氧化锡是一种重要的气体传感器工业化合物，作为陶瓷釉的乳浊剂，作为多相石化催化剂，以及与氧化铟一起作为透明导电涂层。它形成了一种亚稳相，其结构和精确成分尚不清楚。它不能作为单晶生长，它以粉末的形式与氧化锡和元素锡的平衡相结合生长。我们计划用电子进动来解决这一相的结构。有序介孔二氧化硅是具有极高比表面积的固体，在关键有机分子的加氢反应中具有高活性和高选择性的催化剂。我们建议使用电子进动来揭示其中一种有序二氧化硅MCM-48的完整三维结晶学；以前的工作结果在一个重要的二次孔结构的大小上存在分歧。在制药行业中，未知晶体结构的高比例证明了用X射线衍射(例如多晶性)确定其结构的难度。电子进动提供了来自微量晶体的有价值的实验数据，充其量可以用来直接确定晶体结构，或者在最坏的情况下，提供支持粉末精炼或计算分析的实验证据。我们将研究电子进动在制药材料中的可行性，例如，研究常见的抗癌药物5-氟尿嘧啶的多晶型结构，以及用作血管扩张剂和肌肉松弛药的无水茶碱的结构，其结构仍未完全解决。

项目成果

Precession Electron Diffraction: Application to Organic Crystals and Hybrid Inorganic-Organic Materials

进动电子衍射：在有机晶体和无机-有机杂化材料中的应用

• 发表时间: 2008
• 作者: N/a Midgley

Structure Solution of Intermediate Tin Oxide SnO2-x by Electron Precession

电子进动法解中间体氧化锡SnO2-x的结构

• 发表时间: 2008
• 作者: N/a Moreno