[Ifeffit] FEFF6L.02 - missing path?

Matt Newville newville at cars.uchicago.edu
Thu Jul 28 21:27:57 CDT 2011


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:
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

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,


2011/7/28 Paweł Zajdel <pawel.zajdel at us.edu.pl>:
> 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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