[Ifeffit] Inversion Degree

[Juan de la figuera]

Hi, Carlos

I do not agree in the general case with Bokky about using either a Rietveld
analysis or XRD is good to get the inversion factor. In fact, I think EXAFS
is perfect for that. But that is for the general case of AB2O4 with A and B
being different atoms. The EXAFS spectra are really different if A is in
tetrahedral positions or in octahedral ones. But in your case both A and B
are Co. I think you cannot get the inversion in that case, because the
differences arise just from the difference between Co+3 or Co2+ scattering
factors. Not enough.

In any case, in order to do the fit you just select the paths from the two
sites to fit in Artemis, check the link that Bokky sent. And use a factor
multiplying the contribution from one site and (1-x) for the other site.

Juan

----------------------------------------------------------------------------
Juan de la Figuera
Instituto de Quimica Fisica "Rocasolano"
c/ serrano, 119
Madrid, Spain 28006
Phone: +34 91 745 9517
http://surfmoss.iqfr.csic.es/
<http://surfmoss.iqfr.csic.es/surflab/people/juan-de-la-figuera>
http://scholar.google.com/citations?user=GOiP624AAAAJ
-----------------------------------------------------------------------------


On Sat, Jul 6, 2019 at 7:10 AM dcnguyen <dcnguyen at gate.sinica.edu.tw> wrote:

> Dear Carlos,
>
> My understanding is that you would like to explore the ratio of Co (or Co
> ions) either in octahedral phase or tetrahedral phase.
> Please be referred to a similar issue at
> https://millenia.cars.aps.anl.gov/pipermail/ifeffit/2013-July/006759.html
> If your samples are in forms of crystalline, you may want to use X-ray
> diffraction technique. Performing a Rietveld refinement for the obtained
> XRD patterns can give you the required results.
> If your samples are amorphous, you may want to measure XANES spectra, then
> perform fitting for those using the Athena package, to get the percentage
> of Co.
> EXAFS fitting for 2 phases might give you more information than the
> above-mentioned stoichiometry. That can be done using the Artemis package.
> The EXAFS equation for a mixture of materials contributing to the measured
> edge is given in the following reference:
> "Scott Calvin, XAFS for Everyone, CRC Press, Boca Raton, 2013 p. 342."
> The following paper is highly recommended for your reference
> S. Kelly, R. Ingalls, F. Wang, B. Ravel, and D. Haskel, Phys. Rev. B 57,
> 7543-7550 (1998).
>
> Furthermore, from the Co *K*-edge spectrum, that is proportional to the
> absorption per atom of Co, you can also calculate the volume fraction in a
> cubic centimeter of material.
> To do so, you need to firstly obtain the (short-ranged) lattice parameter
> (in angstrom) of each phase from an EXAFS fitting with an R-factor less
> than 0.02.
> Then the volume (in angstrom cube) of the unit cell of each phase is
> known. Given that the number of Co atoms /unit cell is known for each
> phase, the ratio of moles of Co in octahedral phase to total moles of Co
> for 1 cm3 of sample can be conveniently calculated. This value is
> equivalent to the volume fraction (%) in 1 cm3 of the sample.
> Good luck with your analysis.
>
> Sincerely,
> Bokky, D.C. Nguyen
> Postdoctoral Fellow at IoP, Academia Sinica, Taiwan
>
>
>
> -----Original message-----
> *From:*Carlos Triana Estupinan<ctrianae at unal.edu.co>
> *To:*XAFS Analysis using Ifeffit<ifeffit at millenia.cars.aps.anl.gov>
> *Date: * Fri, 05 Jul 2019 20:37:15
> *Subject:* [Ifeffit] Inversion Degree
> Dear
> All
>
> Dear
>
> I have EXAFS spectra for spinel Co3O4, and I have to determine the
> inversion degree X, or the cation distribution of Co at the octaedral and
> tetraedrical sites.
>
> The EXAFS fitting can be performed employing two clusters of atoms, one
> having the absorbing atom in tetrahedral site and one having the absorbing
> atom in octahedral site. However, I do not know how to exactly determine
> the fraction of cations in octahedral or tetraedral sites from fitting the
> spectra with two sites independently.
>
> Could you please tell me how exactly should I proced, can I define a EXAFS
> fitting parameter giving me the cation distribution of Co at the octaedral
> and tetraedrical sites. If so, how can I do it.
>
> Thanks and best regards,
>
> --
>
> *Carlos Augusto Triana Estupiñan*
> * M.Sc. Sciences - **Physics*
> *Physicist*
> * Tel: +57 322 256-2309*
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