[Ifeffit] Artemis fitting using FEFF paths plus experimentalreferences

[Wayne Lukens]

Hi Matthew,

It sounds like you have a problem caused by the finite size of the
nanoparticles, which results in lower than expected amplitude of
the shells in comparison to the amplitude of the shells generated
from the xtal structures of maghemite and magnetite.  If this is
the case and you have a good idea how large the nanoparticles are,
you can use Scott Calvin's formula for the amplitude of a scattering
shell in a nanoparticle:

APPLIED PHYSICS LETTERS 87 (23): Art. No. 233102 DEC 5 2005.

This works very well for metallic nanoparticles, but I have never
tried it on oxide nanoparticles.

Maybe you could try using Scott's formula for the amplitudes and the
distances from the crystal structures.  If I recall correctly, Scott
uses 3 Debye-Waller parameters as variables: one each for the first
two shells and one for all other shells.

The other possibility is to fit the XANES and low k portion of the
EXAFS, but I assume that you have already tried that.

Sincerely,

Wayne



Matthew wrote:
> Of course I tried PCA!  Over a certain range of temperatures, one 
> component looks tolerably like magnetite.  The other is sort of,
> but not exactly, maghemite.  However, if I go below that restricted 
> range, it's not pseudobinary anymore.  Also, I still don't know
> what the "maghemite-like" end-member actually is.  Still, for the 
> purposes of the ALS user-meeting poster I have to hang in a week or
> so, I'll probably take the PCA analysis plus a FEFF fit to the one 
> end-member, and try to do a more thorough job when I have time.
> No, it doesn't fit well to a combo of the references I have, which does 
> include maghemite.
>  
> To be fair, what I have is not an isothermal series but an isochronal 
> series at different temperatures, so there's less reason for
> expecting simple kinetics than in an isothermal series.
>     mam
> 
>     ----- Original Message -----
>     *From:* Anatoly Frenkel <mailto:frenkel at bnl.gov>
>     *To:* MAMarcus at lbl.gov <mailto:MAMarcus at lbl.gov>
>     *Sent:* Monday, September 24, 2007 6:59 PM
>     *Subject:* RE: [Ifeffit] Artemis fitting using FEFF paths plus
>     experimentalreferences
> 
>     Matthew - please forward my reply to the list if you consider it
>     appropriate. I am spammed out. Anatoly
>      
>     *************************
>      
>     Matthew,
>      
>     May be it is not relevant, since I do not know if you have a series
>     of EXAFS data at different stages of the oxidation, or just the
>     initial and the final stages. If you do have several stages,
>     including the starting (unoxidized) and final (oxidized, or a sum of
>     oxidized and unoxidized, depending where you stopped at) ones, the
>     principal component analysis seems to match your needs well.  I
>     assume you also have access to EXAFS data in bulk magnetite and
>     maghemite references.
>      
>     PCA analysis may tell you (if you are lucky and your system is
>     similar to those where such information was previously obtained):
>      
>     1) The volume fractions of intermediate species at each stage of the
>     oxidation:
>     2) Identities of each intermediate species
>      
>     The first part is entirely model-independent. The second part is
>     not, but a reasonable selection of standard compounds (e.g.,
>     magnetite or maghemite) may help you isolating the intermediates.
>     The good thing is that you do not need to fit a Fe-Fe and Fe-O bonds
>     to the spectrum of the entire heterogeneous sample, where these
>     contributions may be present in different species, and you have a
>     large number of fitting parameters that describe such a model.
>     Instead, you may end up obtaining that your species are identical
>     with their bulk reference compounds (and thus no fit is required
>     since the only thing you need to know is the volume fraction of this
>     species) or, you may have to fit a couple of FEFF paths to that
>     unknown species, after you deconvolute it from the mixture knowing
>     what the rest of the phases are (it is doable in the case of one
>     intermediate, but can be done in the case of a larger number of
>     intermediates as well).
>      
>     Anatoly
> 
>         -----Original Message-----
>         *From:* ifeffit-bounces at millenia.cars.aps.anl.gov
>         [mailto:ifeffit-bounces at millenia.cars.aps.anl.gov]*On Behalf Of
>         *Matthew
>         *Sent:* Monday, September 24, 2007 7:05 PM
>         *To:* XAFS Analysis using Ifeffit
>         *Subject:* [Ifeffit] Artemis fitting using FEFF paths plus
>         experimentalreferences
> 
>         I have a system in which Fe:C alloy nanoparticles are being
>         oxidized.  I have the EXAFS of the unoxidized nanoparticles, and I'd
>         like to use that as a reference
>         for fitting as in:
> 
>         Fe_EXAFS(oxidation_stage) = x*(unox_particles)+(1-x)*(Fe->O
>         paths + Fe->Fe paths +...)
> 
>         where the Fe->O and Fe->Fe are based on magnetite and maghemite,
>         which seem to be products of the oxidation.  It's not just a linear
>         fit to
>         unoxidized+magnetite+maghemite because the oxide is somewhat
>         disordered. If I do Artemis fits to the shells from the two oxides
>         alone, I find I lose
>         some Fe atoms (sum of fractions < 1), a result which is echoed
>         in linear-combo fits to the oxides alone.  Thus, I really want to do
>         a hybrid fit.  The unoxidized
>         particles are highly disordered to the point where they don't
>         fit well with reasonable shells, so I'd rather not have to represent
>         them as FEFF shells.
> 
>         How can I do this hybrid fit?
>             mam
> 
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