[Ifeffit] EXAFS Fitting with 2 CIF files

[Zihao Yan]

Dear Scott, Matthew and Peng:

Thank you so much for your answers!
I have a couple of follow up questions:
1) for the CIF(Pt3Fe), there are 2 paths (Pt-Pt and Pt-Fe). Do the 2 paths
share the same amplitude as: S02 * (1-fracoxide) ?
I attached screenshots of my settings. Could you please take a look?
2) Is there any way to set up constraints on Artemis? For example, I want
to set 0<fracoxide<1. Sometimes, I got weird values of  fracoxide,  even
with abs()

Thank you again for your help!

Sincerely,
Zihao Yan

On Sun, Sep 5, 2021 at 4:08 AM Peng Liu <liupeng5182 at gmail.com> wrote:

> Just a quick question. For Zihao' sample, the 1st peak for Pt-O from PtO2
> is at 2.07 A and the 2nd peak for Pt-Pt or Pt-Fe from Pt3-Fe is at 2.736 A.
> If these peaks are at different positions, could we use the molar ratio
> method to distinguish the contribution from each reference?
> My understanding is that if two paths contribute to the same peak, then
> the molar ratio method can be applied.
> I am not sure if my understanding is right or not.
>
> Best Regards,
>
> Peng Liu
>
> China University of Geosciences
>
> On Sun, Sep 5, 2021 at 5:00 AM Matthew Marcus <mamarcus at lbl.gov> wrote:
>
>> Extending what Scott said, I find it useful to make the coefficients
>> abs(fracoxide) and abs(1-abs(fracoxide)) to avoid false minima with
>> negative S0^2.
>>         mam
>>
>> On 9/4/2021 1:21 PM, Scott Calvin wrote:
>> > Hi Zihao,
>> >
>> > Do it by folding the molar ratio in to the amp parameter for each CIF.
>> Of course S02 is part of that parameter, too.
>> >
>> > So the amp for PtO2 might be parameterized as S02 * fracoxide and the
>> amp for Pt3Fe as S02 * (1-fracoxide)
>> >
>> > You could either assume the S02 is the same for the two phases (as I
>> implicitly did by writing it with only one S02 variable), or you could use
>> a standard for each (from a sample prepared and measured in a similar way)
>> to fix the S02 for each phase.
>> >
>> > Best,
>> > Scott Calvin
>> > Lehman College of the City University of New York
>> >
>> >> On Sep 4, 2021, at 4:02 PM, Zihao Yan <zihao.yan.sci at gmail.com> wrote:
>> >>
>> >> Dear Ifeffit members:
>> >>
>> >> I have a sample which is a mixture of PtFe and PtO2. In order to do
>> the EXAFS fitting, I imported 2 CIF files on Artemis and generated the
>> following 3 paths:
>> >>  From PtO2: Pt-O @ 2.07 nm
>> >>  From Pt3Fe: Pt-Pt @ 2.736 nm, Pt-Fe @ 2.736 nm
>> >> Then, I used these 3 bonds to do the fitting as in the online tutorial
>> (which used 1 CIF file). The fitting result is good in terms of R^2 but it
>> is not scientifically correct.
>> >>
>> >> I think I didn't set up a parameter which determines the ratio between
>> CIF(PtO2) and CIF(Pt3Fe). I mean there should be a parameter that regulates
>> how much each CIF contributes to the fitting or the molar ratio between
>> PtO2 and Pt3Fe in my system.
>> >>
>> >> Could someone please give me some suggestions on how to do EXAFS
>> fitting with 2 CIF files? Is there anywhere I can set up the molar ratio
>> between PtO2 and Pt3Fe?
>> >>
>> >> Yours sincerely,
>> >> Zihao Yan
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>
>
> --
> Best Regards,
>
> Peng Liu
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