Advanced Certificate in Powder Diffraction on the Web Chapter 13 School of Crystallography, Birkbeck College, University of London

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Aims & Objectives

This is the second of two chapters on the subject of indexing. The principle aim of this chapter is to provide you with an explanation of how some of the indexing programs in use today actually work. There is no attempt to discuss specific indexing programs, though you will find references to many of them in the external links section at the end. This section introduces one more program that we will be using in this course: this is identified by the icon below:

  A zone-indexing program.

Make sure you do the exercises that use the interactive programs. The data have already been supplied in the examples, so in most cases you just submit the form without further ado as in the previous demonstrations.

Specific objectives include:

• To remind you of the principles involved in indexing powder data, together with a definition of the most commonly used figure of merit for identifing a good match between observed and calculated line positions.
• To discuss simple, trial (and error), dichotomy, and zone-indexing methods for indexing powder diffraction patterns. In addition, you will be an outline of how Monte Carlo indexing methods work.
• Finally, you will learn about some of the common pitfalls that occur and some strategies for avoiding them.

If any of the exercises give problems, please let us know. If the forms are incompletely downloaded you should get an error message. Likewise, if the form contains bad data values. A reload can sometimes solve this problem. If this fails, please contact us. The zone-indexing program is not as robust as the programs used previously: JKC was not the original author of the code, though he did write the web interface used here. Hence, duff input data may cause the interface to hang sometimes!

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