Transparency

Transparency

For the Bragg-Brentano geometry, poor data may be obtained when the sample is too transparent for X-rays. This situation is common for organic materials containing only light elements such as C, H, N, and O. For transparent samples, the X-rays penetrate deeper into the sample than normal as shown in the figure below:

The green lines represent the path of the X-rays when the diffraction is from the surface layer, while the red lines show the path of the X-rays for diffraction well below the surface. The Bragg angles of the scattered X-rays are the same in each case, but the path of the X-rays shown by the red lines will be seen by the detector at a slightly smaller 2θ angle than those represented by the green lines.

The effect of sample transparency on the powder diffraction data is illustrated below using the saccharide glucose (which has the chemical formula C₆H₁₂O₆):

You will see that the data set has very broad peaks which appear shifted to lower scattering angle in comparison to an alternative powder diffraction pattern of glucose as shown below:

The latter was collected also in Bragg-Brentano geometry, but as a fine powder coating on a silicon single-crystal sample holder. The second pattern has much sharper peaks and provides better data for, say, indexing when the unit cell is unknown, but it has much less reliable peak intensities due to preferred orientation as demonstrated by the abnormally intense peak at approximately 28.5°. In addition, the intensities of the diffracted peaks are much weaker for the silicon mounted sample due to the fact the volume of diffracting material is much smaller in this case.

Under these circumstances, maybe the best way to collect data for multi-purpose use is to exploit transmission geometry, which is ideal for organic materials with low absorption.

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Author(s): Jeremy Karl Cockcroft
Martin Vickers