[Ifeffit] Clarification needed on the structured 2nn peak

Kaustubh Priolkar krpriolkar at yahoo.co.in
Tue Jul 6 22:36:55 CDT 2010


Dear Varadarajan,

Infact if you look at the magnitude of FT's of all compounds together in Athena, 

you can see the evolution of "double peak" with increasing Ca in CdO. 
Therefore it should be most likely due to reasons pointed out by Matt in his 
mail.


cheers
Kaustubh




----- Original Message ----
> From: Matt Newville <newville at cars.uchicago.edu>
> To: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
> Sent: Tue, 6 July, 2010 6:22:16 PM
> Subject: Re: [Ifeffit] Clarification needed on the structured 2nn peak
> 
> 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.
> 
> Cheers,
> 
> --Matt
> 
> 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
> >
> >  _______________________________________________
> > Ifeffit mailing  list
> > Ifeffit at millenia.cars.aps.anl.gov
> > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
> >
> >
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