[Ifeffit] Clarification needed on the structured 2nn peak

Matt Newville newville at cars.uchicago.edu
Tue Jul 6 07:52:16 CDT 2010

Dear Varadarian,

I didn't look at your attachments, but have you considered that there
might be some Cd-Ca in the second shell?   I didn't see what value of
x you had in Cd(1-x)CaxO, but if the oxides form a solid solution one
might naively expect the second shell to have  (1-x) Cd and x Ca
coordination.   Ca and Cd scatter photoelectrons  differently enough
to be distinguishable in the EXAFS.  It might be that there is extra
disorder too, say if the typical Cd-Ca distance was different from the
typical Cd-Cd distance.



On Tue, Jul 6, 2010 at 7:29 AM, Varadarajan Sridharan
<varadu.sridh at gmail.com> wrote:
> Greetings to all
>    I am a beginner to EXAFS.
>   Recently I had carried out Cd K-edge EXAFS  measurements on Cd1-xCaxO
> solid solution. The end members CdO and CaO belong to same crystal structure
> Fm-3m.
>    I had analysed the spectrum with Atthena and fitted with Arthemis. I am
> attaching the Athena project file (***.prj) and that of Arthemis project
> file (***.apr) for your reference.
> Athena:
>    As per the crystal structure of CdO, the first nearest neighbour is at
> 2.345 A corresponding to Cd-O1 inter atomic distance and 2nn distance is
> 3.3.16 A corresponding to Cd-Cd1 inter atomic distance. Though I am able get
> the first peak in chi(R) plot at expected position, I am having some
> problems for the second peak. The second peak has features; main one and a
> shoulder. The main peak itself appears at lower R compared to expected one.
> Intensity of the shoulder increases with Ca substitution at the expense of
> main peak itself.
>             Can any one comment on this?
>      My predicament: If I associate, based on the structure of second peak,
> bimodal distribution for the Cd-Cd1 interatomic distance, the difference of
> 0.4 A is 11%. This value gives me an uneasy feeling. Is it possible to
> accommodate this difference within the setting of F m-3m?
>                                   OR
>                                  It is due the multi scattering is the
> problem?
> Arthemis:
> For CdO:  The k^3 weighted chi fitting, I agree is far from satisfaction.
> Std of delR is a bothering point. I had included 20 paths. Even for the CdO
> with F m-3m, the calculated second peak has a shoulder at lower R. I could
> see a small contribution from path 4. But I am not sure if that is all.
> How to float the individual GDS parameter for each and every path? What can
> be done to improve the fitting?
> Thanking you all in advance
> Varadarajan Sridharan
> ----------------------------------
> Condensed matter Physics Division
> Materials Science division
> Indira Gandhi Centre for Atomic Research
> Kalpakkam 603 102
> India
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