[Ifeffit] Trouble with fitting with Arthemis

[Carlo Segre]

I think you need to start with the shortest paths first and then work your 
way to the longer pt-Pt paths.  It is likely that there is some Pt-Pt 
there but it is swamped by Pt-O and others as well.  The first thing you 
need to figure out is if the peak at 1.1A is real or not.  You might want 
to look inthe literature for Pt-H distances.  I know that Pt-O distances 
are generally close to 2.0A so that is likely the shoulder below 2.0A It 
is possible that the peak just above 2.0A is Pt-Al but you might want to 
look this up in the literature.

Once you have worked out the short distances, you can start to put the 
Pt-Pt distances in.  It is certainly likely that there is more than one 
but the Pt clusters are probably very small.

Cheers,

Carlo

On Tue, 13 Jan 2009, Kleper Oliveira Rocha wrote:

> Hi Carlo,
>
> You seem to be right. I try to do the fit in my notebook, which has the same
> Windows and Services Packs, and worked. I want to know why? Is it possible
> to resolve this?
> You right too assuming that there is an H2 adsorbed. In reality, this is a
> sample catalyst of 1%Pt supported in Al2O3 that was treated in H2. The data
> of EXAFS was colleted at 500ºC in an flux of H2. It is a catalysts prepared
> by impregnation method. In the fit model I assuming that is possible to has
> a oxigen neighbor to Pt, therefore, there is a PtO2 model atom to do the
> fit. They work well with one path unless by the peak at 1A. I already
> thought to assuming a H atom neighbor to Pt but, I not find a good model to
> this. Do you have some idea if is possible to do that? Do you belives that I
> should take more than one path for Pt-Pt distance?
>
> 2009/1/13 Carlo Segre <segre at iit.edu>
>
>>
>> Hi Kleper:
>>
>> There is a single Pt-Pt path in your fit.  This is effectively a fcc Pt
>> distance of 2.77.  I see several problems:
>>
>> This spectrum has distances which are much shorter than 2.77 and so I
>> would expect that this single path would not be a very good fit.  I am not
>> sure if the peak around 1.1A is real, but it could be given the hydrogen
>> that you indicate is present in the file name.  What is clear is that the
>> bigger peaks are not Pt-Pt peaks at all.  The Pt-Pt peak will usually
>> appear around 2.5A in Chi(R).
>>
>> The second thing that I noticed, is that you must have a problem with your
>> installation of the software because I just took your file and executed
>> the fit and did not get the strange values for Chi^2 and reduced Chi^2
>> that you have posted.  The value for amp is negative, indicating that the
>> path is not a good model, but the Chi^2 I got was around 1000.
>>
>> I can't tell much form the name of your file but I suspect that you have
>> Pt-Al and possibly Pt-H distances in your sample.  Do you have any other
>> information about this sample?
>>
>> Carlo
>>
>>
>> --
>> Carlo U. Segre -- Professor of Physics
>> Associate Dean for Special Projects, Graduate College
>> Illinois Institute of Technology
>> Voice: 312.567.3498            Fax: 312.567.3494
>> segre at iit.edu   http://www.iit.edu/~segre   segre at debian.org
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>
>
>
>

-- 
Carlo U. Segre -- Professor of Physics
Associate Dean for Special Projects, Graduate College
Illinois Institute of Technology
Voice: 312.567.3498            Fax: 312.567.3494
segre at iit.edu   http://www.iit.edu/~segre   segre at debian.org