Own C S, Marks L D, Sinkler W
Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA.
Acta Crystallogr A. 2006 Nov;62(Pt 6):434-43. doi: 10.1107/S0108767306032892. Epub 2006 Oct 21.
Precession electron diffraction (PED) is a method that considerably reduces dynamical effects in electron diffraction data, potentially enabling more straightforward solution of structures using the transmission electron microscope. This study focuses upon the characterization of PED data in an effort to improve the understanding of how experimental parameters affect it in order to predict favorable conditions. A method for generating simulated PED data by the multislice method is presented and tested. Data simulated for a wide range of experimental parameters are analyzed and compared to experimental data for the (Ga,In)(2)SnO(4) (GITO) and ZSM-5 zeolite (MFI) systems. Intensity deviations between normalized simulated and kinematical data sets, which are bipolar for dynamical diffraction data, become unipolar for PED data. Three-dimensional difference plots between PED and kinematical data sets show that PED data are most kinematical for small thicknesses, and as thickness increases deviations are minimized by increasing the precession cone semi-angle phi. Lorentz geometry and multibeam dynamical effects explain why the largest deviations cluster about the transmitted beam, and one-dimensional diffraction is pointed out as a strong mechanism for deviation along systematic rows. R factors for the experimental data sets are calculated, demonstrating that PED data are less sensitive to thickness variation. This error metric was also used to determine the experimental specimen thickness. R(1) (unrefined) was found to be about 12 and 15% for GITO and MFI, respectively.
进动电子衍射（PED）是一种能显著减少电子衍射数据中动力学效应的方法，这有可能使利用透射电子显微镜更直接地解析结构成为可能。本研究聚焦于对PED数据的表征，旨在加深对实验参数如何影响该数据的理解，以便预测有利条件。本文提出并测试了一种通过多切片法生成模拟PED数据的方法。对在广泛实验参数下模拟得到的数据进行了分析，并与（Ga,In）(2)SnO(4)（GITO）和ZSM - 5沸石（MFI）体系的实验数据进行了比较。归一化模拟数据集与运动学数据集之间的强度偏差，对于动力学衍射数据是双极的，而对于PED数据则变为单极。PED数据集与运动学数据集之间的三维差值图表明，对于小厚度，PED数据最接近运动学数据，并且随着厚度增加，通过增大进动锥半角φ可使偏差最小化。洛伦兹几何结构和多束动力学效应解释了为何最大偏差集中在透射束周围，并且指出一维衍射是沿系统行产生偏差的一个重要机制。计算了实验数据集的R因子，表明PED数据对厚度变化不太敏感。该误差度量还用于确定实验样品的厚度。发现GITO和MFI的R(1)（未精修）分别约为12%和15%。
