Anomalous X-ray Diffraction (AXD)

 

The variation of the energy dependent part of the x-ray form factor of an element close to an absorption edge can be used in order to obtain a contrast in the scattering of that element compared to other elements in the sample. Therefore if a sample contains more than one element two contrasting diffraction pattern can be collected. For disordered materials such as liquids and glasses this allows a more detailed measurement of the structure. The following figure shows absorption measurements at ID1 with the Si(111) monochromator pair and demonstrates the wide range of absorption edges that are accessible. As stated a Si(311) pair is also available to allow higher energies and has been used for AXD measurements at the Cs K-edge at 36 keV.

For instance in PSe glass we can change the incident energy close to the Se K-edge (12.66 keV) and change the scattering power of the Se atoms. These measurements were carried out using a flat Si(400) crystal analyser. A new focusing analyser gives a 50 fold increase in countrate so AXD measurements in which the concentration of the anomalous element is much weaker can now be considered.

The resulting diffraction patterns show a clear difference. Each diffraction pattern is made up of a weighted combination of the three pair correlations in the system: P-P, P-Se and Se-Se. By subtracting  these patterns (a first-order difference) we can eliminate completely the contribution of P-P because the weighting has not changed. Immediately by using anomalous scattering it can be deduced that the first sharp diffraction peak at about 1 A-1 is due to P-P correlations. This is an excellent example of how varying the energy near the absorption  edge allows us to obtain more detailed information about the structure of disordered materials.  Furthermore in this case we can multiply one of the structure factors to eliminate any one of the three correlations. Hence we obtain three differences and the Fourier transform of these functions gives us a pair distribution  function g(r) from which we get the coordination distances and numbers for this material. This work appears in the ESRF highlights 1997/1998 and more information can be obtained here.