Advanced Certificate in Powder Diffraction on the Web Chapter 12 School of Crystallography, Birkbeck College, University of London

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Aims & Objectives

This is the first of two chapters on the subject of indexing and I am starting first with the concepts involved in unit-cell refinement. This may seem to be "putting the cart before the horse", but good data is essential if time is not to be wasted at the indexing stage of data analysis. Unit-cell refinements of standard materials are one of the best checks on instrument and sample alignment. This section introduces two web-interfaced programs that we will be making more use of later in the course. These are identified by the two icons below:

a program to generate hkl, d, and 2q values, and

a program to refine unit-cell parameters.

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

Specific objectives include:

• A discussion of the relationship between d spacing, unit-cell parameters, and reflections. Simplifications are then provided for different crystal symmetries. The principles underlying the generation of hkl indices for use in the analysis of powder diffraction data are then explained. The use of standard samples for instrument calibration is then reviewed.
• The refinement of unit-cell parameters using radiation of a known value and the refinement of wavelength using standard samples is then discussed, with examples drawn from simple data sets obtained on silicon and nickel powders.
• Finally, using we discuss the accuracy of the refined parameters using 3 different data sets of yttria to illustrate a few points.

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 all else fails, please contact us.

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