[Ifeffit] amplitude parameter S02 larger than 1

Fri Mar 20 12:57:39 CDT 2015

Hi Bruce,

Thank you for your insight. The current materials I am talking about  
is something a little different. The first-shell distance for one site  
is about 2.5 angstrom, and for the other site is 3.3 angstrom.

I feel happy that you still remember that bizarre system which I am  
still working on.  Your brought up a very interesting test to do. I  
will definitely do this test experiment to see what would happen to  
the amplitude S02 if the first-shell distance is larger than 3  
angstrom. This might take me for a while. I will come back to you  
after I do it.

Best,
Yanyun
Quoting Bruce Ravel <bravel at bnl.gov>:

> On 03/20/2015 12:48 PM, huyanyun at physics.utoronto.ca wrote:
>> Thank you. Our group has one copy of your book, I'll read it again after
>> my colleague return it to shelf. I still want to continue our discussion
>> here:
>>
>> If we treat S02 as an empirically observed parameter, can I just set
>> S02=0.9 or 1.45 and let other parameters to explain the k- and R-
>> dependence? Because S02 is not a simplistic parameter which may include
>> both theory and experimental effects, I feel that S02 is not necessarily
>> to be smaller than 1, although I admit S02 smaller than 1 is more
>> defensible as it represents some limitations both in theory model and
>> experiment, but I have a series of similar sample and all their S02 will
>> be automatically be fitted to 1.45~1.55, not smaller than 1. Could this
>> indicate something?
>>
>> I actually found in my system, when I set S02=0.9 (instead of letting it
>> fit to 1.45), other parameter will definitely change but the fitting is
>> not terrible, it is still a close fit but important site occupancy
>> percentage P% changed a lot.  So how should I compare/select from the
>> two fits, one with S02=0.9 and one with S02=1.45 with two scenarios
>> showing different results?
>
>
> Yanyun,
>
> As I recall, you are looking at those bizarre skuttuderite materials
> which consist of a metal framework with an enormous gap.  Sitting in
> the gap is your absorber atom.  The center point of the gap is, as I
> recall, over 3 angstroms away from the nearest vertex of the
> framework.  The point I am about to make hinges upon all that being
> more or less correct.
>
> Feff drops neutral atoms into the specified lattice positions then
> does a rather simple-minded algorithm to overlap the charges and come
> up with the radii that are used to compute the muffin tin potentials.
> In the case of one of those atoms rattling about inside the cage, I am
> skeptical that Feff's model produces a highly reliable set of
> scattering potentials.  Probably ain't bad -- as you said in your
> first email, your fits look good.  But it probably ain't quite right
> either.  As Scott hinted, mistakes in the theory can show up in
> surprising with surprising k- or R-dependence, and surprising
> amplitude and phase dependence.
>
> I have absolutely no intuition for how Feff might introduce systematic
> error into a fit for the physical situation of a nearest neighbor at a
> distance of 3 or more angstrom, so I don't know how to "explain away"
> an oddly large S02.
>
> That said, I can think of some experiments that /might/ give some
> insight.  Pick something simple, like a metal oxide or a metal
> sulfide -- something with a cubic structure.  You don't want this
> experiment to get to complicated.
>
>   1. Before generating the feff.inp file, make the lattice constant
>      nonphysically large such that the near neighbor distance is about
>      3 angstroms.
>
>   2. Run Feff and add up all the paths to make a theoretical chi(k)
>      spectrum for your nonphysically large crystal.  For a later
>      iteration of this, you might add some synthetic noise to the
>      spectrum.
>
>   3. Treat the chi(k) you just made as your "data".  Import it and the
>      normal crystal data into Artemis.  Run Feff on the normal
>      crystal.
>
>   4. Use Artemis's single scattering path tool to make a path for the
>      first shell scatterer at the distance you used to make your
>      theoretical data.
>
>   5. Make a simple first shell, four-parameter fit using that SS path.
>
> Can you make a reasonable looking fit?  With sensible error bars?
> What happens with the amplitude?  Is it very large or very small?
>
> Perhaps try the experiment the other way around.  Fit the "normal"
> theoretical data with the unphysical Feff calculation.
>
> The point I am driving at that I wonder if you can figure out what
> happens to the amplitude in a decent fit when you contrive a situation
> with an unusually large first neighbor distance.  If you see a trend
> in these "Feff experiments", perhaps that can help you understand the
> amplitude in your skuttuderite fits.
>
> Again, I have no intuition about this.  I have no idea if my suggestion
> will be fruitful or not.  For that matter, I have no idea if my memory
> of your problem is correct.
>
> But maybe this is a brilliant suggestion.  Unlikely, but stranger
> things have happened :)
>
> B
>
>
>
> -- 
>  Bruce Ravel  ------------------------------------ bravel at bnl.gov
>
>  National Institute of Standards and Technology
>  Synchrotron Science Group at NSLS-II
>  Building 535A
>  Upton NY, 11973
>
>  Homepage:    http://bruceravel.github.io/home/
>  Software:    https://github.com/bruceravel
>  Demeter:     http://bruceravel.github.io/demeter/
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