Advanced Certificate in Powder Diffraction on the Web Chapter 14 School of Crystallography, Birkbeck College, University of London

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Aims & Objectives

This is the fourth and final chapter on the subject of symmetry. By now you should be familiar with the basic concepts regarding symmetry: it is now time to put theory into practice. The principle aim of this chapter is to provide you with an explanation of how space-group information is determined from diffraction data, with special attention being paid to the case of powder samples. The reason that so much emphasis is given to this topic is that the procedures for space group determination are much more difficult to automate so as to give reliable results than in the equivalent singe-crystal diffraction experiment. Specific objectives are listed below:

• We will start with a quick recap of the symmetry information that may be deduced from the geometrical interpretation of the powder diffraction data, i.e. the results obtained from cell indexing and refinement.
• You will then learn about the concept of reflections conditions & systematic absences and how these provide information on centred lattices, glide planes, and screw axes.
• The discussion on reflection conditions is based around the single-crystal scenario so we then discuss the differences from the point of view of symmetry determination between the single-crystal and powder diffraction data.
• You will then learn how the knowledge learnt in the previous pages is then applied in practice to the problem of space-group determination from powder diffraction data. On this page the tables are provided in order to demonstrate certain concepts: you are not expected to memorize the detail in them!
• Finally, a case study is provided to demonstrate on a step-by-step basis how a space group is determined from a real experimental data set.

Your tutor for this session is: Jeremy Karl Cockcroft

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