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Preferred Orientation

The biggest disadvantage of mounting samples on a flat-plate is that many of them (and probably even a majority of them?) end up with surface preferred orientation due to the flattening process. This is spectacularly illustrated for the hydrated cement phase Ca4Al2(SO4)O6.16H2O for which powder diffraction data was collected in a "routine" manner on a Bragg-Brentano diffractometer as shown below:

At first sight only a few peaks are seen due to very strong preferred orientation of the crystallites within the powder. (The four most intense peaks may be indexed by eye as 001, 002, 003, and 004 layer lines.) Closer inspection of the data shows the presence of other hkl reflections as shown below:
This sample was also measured in transmission geometry in a 0.5 mm capillary and the results below were obtained:
This is an extreme example which can occur when the Bragg-Brentano geometry is used. You should be aware that other less extreme cases usually pass unnoticed (unless a comparison is made using the same sample measured in transmission geometry). The effect of preferred orientation as shown here must not be confused with the effect of texture which is discussed next.

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© Copyright 1997-2006.  Birkbeck College, University of London.
Author(s): Jeremy Karl Cockcroft
Andrew Charles Jupe