Dear Rietvelders,
I agree with Leopold answer but the experimental procedure need to
take care of f', f'' experimental value if you to ensure good data. On
BM2-CRG beam line at ESRF we record often anomalous data using the
following procedure, the same for powder or amorphous samples.
Each edge to be used is recorded, this allow to calculate using
Kramers-Kroening relation the effective value of f' and f'' at the
experimental wavelength then a data scan is performed near -10, -20eV
below the edge ( the f' value is not at this point the highest but it
remains stable enough to allow small experimental shifts ) and another
scan is performed far away from the edge, typically -200, -500eV below.
In powder, we are not concerned by hkl, -h-k-l difference, then we
take more information from f' variation than from f'' one, that explain
that we do not use scan above the edge in which the signal to noise
ratio is decreased by fluorescence.
You can find references in Hodeau review paper (Chem. Rev. (2001) 98)
or practical example in Pallancher paper (Angew. Chem. Int. Ed. (2005)
43, 44).
Regards,
Jfrancois
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
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MCMF, Institut Neel - CNRS, BP 166, 38042 Grenoble Cedex 09, France
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