[Ifeffit] Clarification needed on the structured 2nn peak

[Varadarajan Sridharan]

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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