[Ifeffit] how to consider Jahn-Teller distortion?

[Matthew Marcus]

Moving the Fe atom off-center as if it were the Ti atom in a perovskite isn't a good way to simulate JT distortion.
In Fe3+ compounds such as goethite (I assume Fe3+, not the rare Fe4+), the distortion consists of the two axial
distances being shorter than the 4 equatorial.  You could simulate the 1st shell by moving two of the O atoms
artificially.  Alternatively, in Artemis, you could clone the Fe-O 1NN path and set a parameter for the JT distortion,
but that wouldn't get the MS or higher neighbors correct.  Does anyone know of a program which would be like ATOMS,
except with parameters, thus generating parameterized distances?

The beat you get from the JT distortion will make a big difference in the fit quality once you get to something
like 14A^-1.
	mam

On 8/8/2011 6:21 PM, Matt Newville wrote:
> 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
>
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