Pawel,
For these two structures, the location of the anions (Se for ZnCr2Se4
and O for MgAl2O4) are not in in exactly the same location in the unit
cell. The nearly identical double scattering paths:
Al->O->O->Al
and
Cr->Se->Se->Cr
each have small estimated scattering amplitudes ("Zabinsky Curved Wave
Importance Factors"). The Cr->Se->Se->Cr path is filtered out with
the default settings, and so not included in the path list, while the
Al->O->O->Al path is not filtered out.
You can guarantee to see all the paths if you put
CRITERIA 0 0
in the feff.inp file.
For a more detailed reason of why Al->O->O->Al has an estimated
amplitude of 10.7 (and so not filtered out) while the Cr->Se->Se->Cr
has an estimated amplitude of 3.9 (and so is filtered out), there are
a few points to consider:
1. The amplitude reported is relative to the strongest scattering
path, which always has amplitude 100. In both cases, this is the
first shell Al->O->Al and Cr->Se->Cr. Se is a stronger back-scatterer
than O.
2. Both absorber and scatterer are heavier in the Zn-Cr-Se case
than the Mg-Al-O case, and the the heavier atoms are more point-like
in their electron density.
3. The Se atoms are closer to 0.25, 0.25, 0.25 in the unit cell
than the O atoms. This makes the Cr->Se->Se->Cr path have a
scattering angle beta a little closer to 90 degrees, which gives less
scattering amplitude.
I think all three of these conspire to give the Cr->Se->Se->Cr path
less amplitude than the Al->O->O->Al path.
Hope that helps,
--Matt
2011/7/28 Paweł Zajdel
Dear All,
I am facing a strange issue while playing with EXAFS for a spinel type structure. I tried two typical examples: MgAl2O4 and ZnCr2Se4 with octahedral site K edge of Al or Cr. The MgAl2O4 is fine but for ZnCr2Se4, FEFF seems to omit one scattering path. It is not reported as being rejected by <2.6% amp criterion, so I wonder, while it is not shown. This is path 3 (as seen in MgAl2O4), which goes from Cr/Al to anion, then over the "octahedron" edge to another anion and back to Cr/Al. For oxide Reff=3.2251 <cut from feff.run> path cw ratio deg nleg reff 1 100.000 6.000 2 1.9371 2 37.913 6.000 2 2.8853 3 10.687 12.000 3 3.2251 <-- It's here 4 9.664 2.000 2 3.3444 </cut> For selenide Reff=4.2354 <cut from feff.run> path cw ratio deg nleg reff 1 100.000 6.000 2 2.5248 2 47.173 6.000 2 3.7084 <-- should be after this one. 3 25.382 6.000 2 4.3485 </cut> I am attaching two atoms.inp files, which may help. It is space grup 227, standard setting (2), atom list is correct. I am using Ifeffit 1.2.11c with Feff 6L.02. It s present under WinVista and Linux on two different machines. Am I missing something? I would appreciate any suggestions.
Best Pawel
-- dr Pawel Zajdel Division of Physics of Crystals Institute of Physics University of Silesia Uniwersytecka 4 40-007 Katowice Poland [48]323591978
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