[Ifeffit] Coordination of ball-milled alloys
newville at cars.uchicago.edu
Fri May 4 21:56:56 CDT 2007
I would not expect lambda to depend on alloy structure -- it's
generally expected to be fairly insensitive to structural details.
Your assertion that the EXAFS sigma2 should be larger for Cr in a
disordered alloy than for Cr in Cr-Cr foil, and coordination numbers
should be on the order of 10 for metal alloys is certainly reasonable.
Then again, there are a couple of questions and points to consider:
1. If you're expecting a bcc structure, you should definitely include
the first two shells (~14 neighbors), not trying to resolve the
coordination shells with 8 and 6 neighbors separately.
2. For highly disordered systems, diffraction and EXAFS can give
quite wildly different results. Most diffraction measurements (as
opposed to scattering measurements) implicitly select the crystalline
portion of a sample -- amorphous portions may not contribute at all.
EXAFS averages equally over all atoms. In some cases, this can be an
important distinction. In addition, for liquid metals it does often
appear that the sigma2 doesn't change much, but that N drops
dramatically. I wouldn't expect that to be a huge effect for Cr-Fe
alloys, but it's possible for very high defect concentrations.
3. Related to that, a doubling of a diffraction DWF for statically
disorder really does not imply a doubling of the EXAFS sigma2. In
fact, the evidence is generally that the EXAFS sigma2 (ie, the range
of bond lengths) doesn't really change much, while the increase in
diffraction DWF simply implies a loss of long-range (which is, after
all, what diffraction DWFs measure).
4. Finally, it's probably impossible to tell Cr-Cr scattering from
Cr-Fe. How are you taking into account the ~30% of neighbors that are
Fe instead of Cr?
So I would suggest that there's probably not a good physical basis
for floating lambda, and there really is not compelling argument for
forcing sigma2 to be twice that for Cr metal.
What happens if you try to fit the whole first peak (bcc 1st and 2nd
neighbors) without making assumptions about sigma2?
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