Hello again. Just want your opinion on a thought that I have. Due to the absence of Mg-ferrite (MgFe2O4) K-edge XANES spectrum, I have decided to do my Fe(II)/Fe(III) quantification with only maghemite and magnetite as end-member standards.

Because Fe in MgFe2O4 exists entirely as Fe(III) and its structure is the same as both magnetite and maghemite (inverse-spinel structure with Fe (III) occupying both tetrahedral and octahedral sites), I am inferring that its K-edge XANES spectrum would be similar to that of maghemite.

I am planning to take the quantified fraction of maghemite as a sum of the contributions of maghemite and Mg-ferrite.

For instance, from Linear Combination Fitting:

Fraction of Magnetite=0.05, Fraction of Maghemite = 0.95;
Due to absence of Mg-ferrite spectrum, i will be taking (Fraction of Maghemite) + (Fraction of Mg-ferrite)= 0.95

What are your thoughts on this?

Kind regards.

Teck Kwang


On 14 August 2013 18:46, Matthew Marcus <mamarcus@lbl.gov> wrote:
Yup.  That's consistent.  That sort of thing is why every XANES paper should state clearly
the basis of the energy calibration.  With Newville's database, you can often figure that
out yourself because many of the spectra were done with a reference foil.
    mam
----- Original Message -----
Sent: Wednesday, August 14, 2013 1:07 AM
Subject: Re: [Ifeffit] Linear Combination Fitting using ATHENA

Yes Matthew, I used 7112 eV for my Fe K-edge calibration. I am guessing that is the reason a ~+1 eV shift in your maghemite spectrum is required is required to fit mine. Both pre-edge features match in the sense that they are roughly bound by the same energy values.


On 14 August 2013 17:30, Matthew Marcus <mamarcus@lbl.gov> wrote:
That depends on the way you did your calibration.  If it really matches, including the
pre-edge features, with a 1eV shift, then that's what you probably have.  Did you take
into account my calibration, which puts the Fe K-edge at 7110.75eV?  Some people use other
values like 7112eV.
    mam
----- Original Message -----
Sent: Tuesday, August 13, 2013 9:20 PM
Subject: Re: [Ifeffit] Linear Combination Fitting using ATHENA

Also Matthew, I have found that my sample spectrum is an almost exact replica of the maghemite spectrum, the only differences being the absorbance height of the edge-peak (highest peak), which is ok but more importantly
I have found that there is a +1eV shift of my spectrum relative to the maghemite one, probably due to slight differences in beamline. Do you think it is alright to make a +1 eV shift to the maghemite spectrum (or -1eV shift to my sample spectrum)?

Thanks.

Kind regards.

Teck Kwang


On 14 August 2013 10:07, Teck Kwang Choo <teck.kwang.choo@monash.edu> wrote:
Thanks Matthew for the spectra.

Is it correct to use the spectra that was taken in transmission mode (the one you did) to fit those taken in fluorescence mode (my case)?

Teck Kwang


On 14 August 2013 01:37, Matthew Marcus <mamarcus@lbl.gov> wrote:
Here are reference spectra.  The calibration is defined with the Fe metal edge at 7110.75eV.  The maghemite spectrum has been corrected for mild
overabsorption by reference to magnetite, which is very similar.  The magnetite is synthetic Fe3O4 from Aldrich, and done in TEY, so no overabsorption.
The maghemite is from oxidation of Fe oxide nanoparticles and has been verified by XRD and comparison with the ETH group's maghemite spectrum.
        mam


On 8/13/2013 1:33 AM, Teck Kwang Choo wrote:
Hi all,

@Drew Latta: I was wondering why the end members should be magnetite and maghemite. Is it because they are both of inverse-spinel structure, the difference only being that the latter has all Fe completely oxidized into Fe(III)? Would you be able to provide the standard spectra to me if that is the case? Thanks very much!

Previously I have only thought of using hematite (alpha-Fe2O3) and FeO as end-members, both representing Fe(III) and Fe(II) respectively without giving much thought to the structure Fe takes. I also had my doubts using magnetite (Fe3O4) as a standard as it is a mixture of both Fe (II) and Fe (III). But thanks to the insights Drew has given, I now think I should take the Fe-coordination environment into account with the use of magnetite and maghemite as standards.

@Alexandre and Matthew: I have obtained Mossbauer spectra of some of my samples but like Drew said, it is not straightforward at this point in time. Will see what I can do with the data I have!

Thanks for all your responses! It is greatly appreciated! This forum has been more helpful than I thought!

Teck Kwang


On 13 August 2013 01:34, Matthew Marcus <mamarcus@lbl.gov <mailto:mamarcus@lbl.gov>> wrote:

    While it's true that Mossbauer is the gold standard for Fe valence determination, access to the technique isn't all that common, and I don't know
    of any facility that can do it on a micro scale.  Is there one?
             mam


    On 8/12/2013 7:01 AM, Alexandre dos Santos Anastacio wrote:

        Hello,

        maybe that fingerprinting technique would be Mossbauer spectroscopy. The Fe(II)/Fe(III) ratio comes easily and also there are lots of papers about Mg-ferrite, an example:

        10.1109/TMAG.2009.2018880 <http://dx.doi.org/10.1109/__TMAG.2009.2018880 <http://dx.doi.org/10.1109/TMAG.2009.2018880>>

        Alexandre.





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--
Teck Kwang Choo
PhD Student
Department of Chemical Engineering
Room 225, Building 36
Monash University
Mobile No.: 04-11489904


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--
Teck Kwang Choo
PhD Student
Department of Chemical Engineering
Room 225, Building 36
Monash University
Mobile No.: 04-11489904



--
Teck Kwang Choo
PhD Student
Department of Chemical Engineering
Room 225, Building 36
Monash University
Mobile No.: 04-11489904

_______________________________________________
Ifeffit mailing list
Ifeffit@millenia.cars.aps.anl.gov
http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit


_______________________________________________
Ifeffit mailing list
Ifeffit@millenia.cars.aps.anl.gov
http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit




--
Teck Kwang Choo
PhD Student
Department of Chemical Engineering
Room 225, Building 36
Monash University
Mobile No.: 04-11489904

_______________________________________________
Ifeffit mailing list
Ifeffit@millenia.cars.aps.anl.gov
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--
Teck Kwang Choo
PhD Student
Department of Chemical Engineering
Room 225, Building 36
Monash University
Mobile No.: 04-11489904