Re: Fe3O4

[Bruce Ravel <ravel@phys.washington.edu>]

On Wednesday 02 July 2003 06:59 am, Sidorenko wrote:

> I try to calculate XAFS spectra for Fe3O4.
> 
> Should I use ION card for more reliable result?
> 
> How I can use it in the case of inverse spinel structure, where Fe3+
> and Fe2+ ions mixed in the octahedral sites?
> 
> What distribution of Fe and O atoms in the elementary sell
> (atoms.inp) will be in the case of the rocksalt structure?

Hi folks,

I find this to be an interesting question because there are some
subtle issues hidden beneath the surface.  For those of you who were
at the Wednesday evening session of last week's XAFS conference, this
is the sort of problem for which it is especially hard to build a
"black box" because the best solutions to the problem require
thoughtful action by the user.

Fe3O4 is an interesting problem from the Feff perspective because it
involves 3 kinds of absorbers -- the tetrahedral Fe, the octahedral
Fe3+, and the octahedral Fe2+.  In the measured spectrum, these three
are averaged.  Thus, in the computed spectrum, we have to do a similar
average.  At the very least, the user must sum multiple calculations
in a way that is consistent with stoichiometry and with the relative
energy shifts of the different absorber types.

The simplest thing to do is to add calculations on the tetrahedral and
octahedral sites in their proper ratio.  That doesn't address
Dr. Sidorenko's question, though.

One way of allowing Feff to consider the presence of two different
kinds of octahedral Fe sites is to create a new potential.  Initially,
the feff8 potentials list might look like this:

 POTENTIALS
 *    ipot   Z  element            l_scmt  l_fms   stoichiometry
        0   26   Fe_oct             -1       -1       0.01
        1   26   Fe_t               -1       -1       1
        2   26   Fe_o               -1       -1       2
        3    8   O                  -1       -1       4

You could introduce another type of iron atom by editing this to read

 POTENTIALS
 *    ipot   Z  element            l_scmt  l_fms   stoichiometry
        0   26   Fe_oct             -1       -1       0.01
        1   26   Fe_t               -1       -1       1
        2   26   Fe_o1              -1       -1       1
        3    8   O                  -1       -1       4
        4   26   Fe_o2              -1       -1       1

Then, in the atoms list, change some of the atoms of potential type 2
to type 4.  That will give Feff some freedom to allow the two
octohedral irons to have different potentials.

How best to distribute the new iron type throughout the atoms list is
unclear to me.  One possibility is that this new degree of freedom in
the calculation will make no difference at all.  Another possibility
is that it will depend on how the new potential type is distributed
throughout the list.

Determining the answer to those issues will require experimentation.
You will have to test several different distributions and compare the
resulting calculations both to one another and to the data.

Finally, my (admitedly limited) experience using the ION card is
unencouraging.  I find that it has strong and uninuitive (at least to
me) results on the calculation.  In any case, integer values of the
IONization are almost certainly not what you would want to use.  If
you look at the charge transfer numbers form a typical feff8
calculation, the amount of transfered charge is non-integer and
usually much smaller than the formal valence.

Hope that helps,
B

P.S.  I think it is not a well-known feature of Atoms (I am speaking
of the desktop version now, not the web-based version) that it is very
simple to change how the potentials list is determined.  There is a
place on your computer were the so-called Atoms Template files are
kept.  On unix this is something like
/usr/lib/perl5/site_perl/5.8.0/Xray/lib/atp/ and on Windows it is
C:\Program Files\Ifeffit\perl\Xray\atp\.  In that place you will find
a file called feff8.atp.  On unix, copy file to ~/.atoms/.  On
windows, you can just edit it in place.

If you edit this file, you will see a line that looks something like this

<potentials :ipots species :gcd 0>
        <ipot>   <znum>   <elem>         <l>       <l>       <stoi>

This line determines how the potentials list is created.  The possible
values of the ":ipots" argument are "species", "sites", and "tags".
Here is what these mean:
  "species"
     a unique potential will be given to each atomic species in the
     unit cell.  This is the default.
  "sites"
     a unique potential is given to each crystallographic site in the
     unit cell.  (This is a bit dangerous in that feff only allows 7
     unique potentials and unit cells often have more than 7 unique
     sites.)
  "tags"
     a unique potential is given to each unique tag in the atoms
     input.  Suppose that a unit cell has 5 crystallographically
     distinct oxygen atoms.  If you give them all the same tag, say
     "O", then they will all get the same unique potential when
     ":ipots tags" is used.

"tags" or "sites" seems like a more appropriate choice for Fe3O4 than
"species".
     
You may want to keep all three options lying around.  You could have
files called, for example, "feff8_species.atp", "feff8_sites.atp", and
"feff8_tags.atp" where the only difference is the ":ipots" argument.
If all three of these are found by TkAtoms in the ~/.atoms/ or in
C:\Program Files\Ifeffit\perl\Xray\atp\, then all three will show up
in the drop-down menu of output types in TkAtoms.

If you use the command line version of atoms, the -t command line
switch allows you explicitly specify the atp file.



-- 
 Bruce Ravel  ----------------------------------- ravel@phys.washington.edu
 Code 6134, Building 3, Room 222
 Naval Research Laboratory                          phone: (1) 202 767 5947
 Washington DC 20375, USA                             fax: (1) 202 767 1697

 NRL Synchrotron Radiation Consortium (NRL-SRC)
 Beamlines X11a, X11b, X23b, X24c, U4b
 National Synchrotron Light Source
 Brookhaven National Laboratory, Upton, NY 11973

 My homepage:    http://feff.phys.washington.edu/~ravel 
 EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/