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Chapter 5

School of Crystallography, Birkbeck College, University of London


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Chapter 5

Aims & Objectives

Why do we need to know about symmetry? Molecules often interact symmetrically, producing crystals which intrinsically possess symmetry. The technique of powder diffraction exploits diffraction of the tiny crystallites in a powder in a variety of ways, some of which require a detailed knowledge of symmetry to refine or determine a crystal structure. Even a subject such as indexing requires some knowledge of symmetry. So there is no escaping from it.

Text books often deal with this subject in a way far removed from practical use. It is also a difficult subject. There is no escaping that. This section describes the 3-D symmetry elements with a minimum amount of mathematics, and it does not strive to be exhaustive. Concepts are important here, and it may take a while for the concepts to sink in. This is quite normal, so do not worry if you cannot pick up all the concepts on the first read through of the material. Please do not hesitate to ask questions, either via the the e-mail list, or privately.

Specific objectives are to provide an understanding of the following 3-D symmetry elements:

We hope that this approach will be a major aid in your understanding of the subject.

Assignments

We shall shortly be setting some assignments on the material covered so far. Further instructions concerning these will follow. The assignments are quite separate from questions posed in the course material itself. The latter, indicated by the light bulb icon, are designed to stimulate your thoughts on the subject.


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