Fyzikální ústav Akademie věd ČR

Lukáš Palatinus, Mariana Klementová, Markéta Jarošová, Michal Dušek

Precession electron diffraction (PED) is a newly emerged diffraction method that can be used for determination of atomic structure of very small crystalline samples. Its potential is highlighted in combination with other common diffraction techniques, namely with x-ray diffraction on single crystals and on powders. It is the goal of the Department of structure analysis to have these three methods available within the next few years.

Introduction to PED. Electron diffraction is an easily accessible technique, because it is available in all transmission electron microscopes. Electrons with energy 100-300 keV interact very strongly with matter, and thus electron diffraction can be observed already on very small crystals starting from a few nanometers. However, when trying to use electron diffraction for structure analysis, we face a serious problem. As a result of the strong interaction the electrons scatter multiply inside the crystal and the diffracted intensity cannot be described within the relatively simple kinematical theory. However, kinematical theory of diffraction is the basis of most methods for solution of the phase problem and for determination of the crystal structure. For this reason the electron diffraction was not, until recently, considered useful for routine structure analysis.

Dynamical effects increase with increasing number of simultaneously diffracting reflections. Such case occurs mainly when the electron beam is oriented parallel to some zone axis in the crystal. On the other hand it is just this orientation that allows collection of most intensities at once, and allows also determination of symmetry properties of the crystal. The dilemma was solved by Vincent and Midgley (Vincent & Midgley, 1994), who developed the so called precession electron diffraction technique. In this technique the crystal stays oriented, but the electron beam exhibits a fast precessing motion along a surface of a cone with vertex on the sample, and with axis along the zone axis of the crystal. In this geometry the diffraction never occurs in the perfectly oriented position, and thus the dynamical effects are suppressed, but thanks to the rotation of the beam the diffraction image contains all reflections from the zone. Diffracted intensities acquired with this technique are much closer to the kinematical intensities, and can be used for solution of the crystal structure.

Electron diffractometer. In the Laboratory of structure analysis we have rich experience with structure solution by experiments on single-crystal x-ray diffractometer. We would like to use this experience in collection and analysis of electron diffraction data. Our goal is to develop techniques of automated data collection and analysis in a way that is standard in x-ray diffraction, but that only starts to be used in electron diffraction. The result should be an experimental and computational system that will allow for a relatively routine determination of structures of micro- and nanocrystals by electron diffraction similarly to the procedures common in contemporary x-ray single-crystal diffraction.

Literature
Vincent, P.A. Midgley, Ultramicroscopy 53 (1994) 271