Dear Franz, The best thing to do is collect 2 patterns, one just below the Fe absorption edge and one well above both absorption edges to get normal scattering from both atoms.
If you manage to collect data a few eV below the actual Fe edge (remember that the edge energy changes a bit with charge and coordination environment so an energy scan to determine the edge position is recommended to select the energy to use) that will increase the contrast from 4 electrons (assuming CuII and FeIII) to ~8 or 9 electrons (~30% difference in scattering power, enough to refine occupancies). Collecting data at the Cu edge will just reduce the contrast between Fe and Cu possibly making Cu weaker scatterer than Fe in some conditions. The tricky part is to determine exactly the f' and f'' corrections (and therefore the real change in scattering power) due to the energy change of the absorption edge and the steep change in f' and f'' close to the edge that makes small errors in energy (1eV error of selected energy radiation) have a big effect on the correction . The closer you get to the edge the larger the contrast but the higher the uncertainty in the f' and f'' correction values. Hope this helps. Leo Dr. Leopoldo Suescun Argonne National Laboratory Materials Science Division Bldg 223 9700 S. Cass Ave. Argonne IL 60439 Phone: 1 (630) 252-9760 Fax: 1 (630) 252-7777 ----- Original Message ----- From: Franz Werner <[EMAIL PROTECTED]> Date: Friday, November 16, 2007 10:44 pm Subject: X-ray scattering contrast Fe/Cu > Dear Rietvelders > > Does anyone know whether the scattering contrast of Fe and Cu is > large enough to refine their occupancies from a single X-ray powder > pattern or would multiple wavelength patterns help? > > Thanks for your advice. > > Regards > Franz Werner > -- > Psssst! Schon vom neuen GMX MultiMessenger gehört? > Der kann`s mit allen: http://www.gmx.net/de/go/multimessenger >
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