[Ifeffit] how to consider Jahn-Teller distortion?

Matt Newville newville at cars.uchicago.edu
Mon Aug 8 20:21:39 CDT 2011


Hi Enyuan,

On Mon, Aug 8, 2011 at 7:47 PM, Enyuan Hu <bearcharge at gmail.com> wrote:
> Dear all,
> I'm trying to fit the Fe K-edge EXAFS data and I appreciate that Fe in my
> sample mainly exists as Fe4+, which is associated with Jahn-Teller
> distortion. The one shell model that considers Fe residing in the standard
> octahedral apparently didn't give satisfactory result. I'm sorry that even
> though I found people do fitting considering Jahn-Teller distortion, I am
> still not aware of how to carry that out in Artemis. Can someone helps me to
> look at my fitting and give me some guidance on how to implement fitting
> that considers Jahn-Teller distortion in Artemis? Thank you.

I noticed a few things that might lead you to thinking about the problem better:

You started with the crystal structure for LiCoO3, which has
octahedral symmetry, and calculated for the Co edge, even though your
data is for Fe edge (and one can see the Co K edge showing in the
chi(k) data at ~12.5 Ang^-1).

What you want to do is *edit* the feff.inp that results from running
Atoms on the crystallographic data.  When doing, you will definitely
want to make the central atom Fe, by changing

     POTENTIALS
     *    ipot   Z  element
            0   27   Co
            1    3   Li
            2   27   Co
            3    8   O
to

     POTENTIALS
     *    ipot   Z  element
            0   26   Fe
            1    3   Li
            2   27   Co
            3    8   O

While you're in there messing with feff.inp, you can introduce your
own hypothetical Jahn-Teller distortion, by moving the absorbing atom
away from 0, 0, 0, from

     ATOMS                          * this list contains 103 atoms
     *   x          y          z      ipot  tag              distance
        0.00000    0.00000    0.00000  0 Co              0.00000

to perhaps

     ATOMS                          * this list contains 103 atoms
     *   x          y          z      ipot  tag              distance
         0.00       0.04       0.00     0 Fe              0.00000

(though I'm not at all sure I have the direction for the distortion
correct in the R -3 m structure).  That will lead to a splitting of
the Fe-O distances, for the distortion you're expecting.

I also guessed at the size of that distortion.  Since the resolving
power of EXAFS (especially with data only out to 12.5Ang^-1) is
limited, so you probably will have a difficult time distinguishing the
split in distance from an increased sigma2.  That is, you may hae
trouble proving that there is a Jahn-Teller distortion from the EXAFS
data alone.

My final pieces of advice would be: don't use dk=1 with the
Kaiser-Bessel window (it makes an otherwise ideal window function
horrible) -- use dk=4 or dk=5.

Hope that helps,

--Matt




More information about the Ifeffit mailing list