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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