[Ifeffit] How to select important MS paths in wurtzite?

[Anatoly Frenkel]

By some reason, when I googled "wurtzite structure", the first link with an
image was to the following page:

http://britneyspears.ac/physics/lipgloss/gloss.htm

and they even posted her picture in the bottom, next to the wurtzite
structure. Weird.

I would add to Bruce's and Scott's comments:

If it is an alloy, not a pure material, the importance criteria for
multiple-scattering contributions change. For example, due to the size
mismatch of randomized atoms, they are displaced from their regular lattice
sites, causing lattice buckling which is not taken into account by FEFF
which generates the table of "importance factors" - calculated for the
perfert and periodic model compound. Thus, what may be important for a model
compound with wurtzite structure, may not be important for an actual
compound - an alloy, or a nanoparticle, e.g., CdSe. The reason is that
buckling changes the bond angles - and the ms contributions are extremely
sensitive to that. The change of bond angle that was 0 degrees for a
collinear path to 20 degrees reduces may reduce its f(k) by 20-40%,
depending on material. That makes such paths comparable with nominally
non-collinear triangular ms paths which are, of course, a majority.

Thus, they all compete, and if one were able to neglect non-collinear paths
(except short triangles, as Bruce pointed out) in favor of collinear paths
in pure compounds, or standards, it is often not possible to do it in
alloys, or when some other type of disorder is present, for example - in
nanoparticles where proximity to the surface or support distorts atomic
order. In this case, the short length triangles (equilateral or like) are
the most rigid structural units, and they survive buckling - they can be
seen even in nanoparticles.

Anatoly





-----Original Message-----
From: ifeffit-bounces at millenia.cars.aps.anl.gov
[mailto:ifeffit-bounces at millenia.cars.aps.anl.gov]On Behalf Of Bruce
Ravel
Sent: Tuesday, May 15, 2007 11:35 AM
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] How to select important MS paths in wurtzite?


On Tuesday 15 May 2007, Mauro Rovezzi wrote:
> Dear all,
>
> Do you know if in literature there are studies on the important multiple
> scattering (MS) paths in fitting wurtzite XAFS spectra? I mean which are
> the important paths to properly fit a wurtzite structure up to 6 A in a
> elegant way (<20 paths)?
>
> I'm asking this question thinking at old works by Filipponi et al. on
> the fcc and bcc structures explaining very well the importance of some
> specific triangular and collinear paths to fit the EXAFS spectrum...
>
> Well, this was my coffee-break question :)

OK, let me make a cup of coffee for myself ....



Mmmmmm ... espresso!  Now I'm ready to offer an answer.



I would say that one of the best features of Artemis is that it makes
it easy to examine the MS paths.  You don't have to place faith in the
literature -- you can go look at the paths themselves.

It is very easy to import data and import a large number of paths.
Then you can make useful, interesting plots with relatively little
effort.  You don't even have to do a fit to get a sense of which paths
are big and which are small.

Once you start actually doing some fits, it fairly simple to throw
paths in the mix, take paths out, and tweak the fitting model.  Again,
you don't have to take on faith which paths are important.  You can
evaluate fits with and without certain paths and decide for yourself
which ones are important.

There is a rule of thumb that you need the SS, the (nearly) collinear
MS, and the short triangles.  In my experience, that rule of thumb is
surprisingly effective.

Shelly uses an interesting approach, which is described on page 25 of
this PDF file:


http://xafs.org/Tutorials?action=AttachFile&do=get&target=Basics_of_XAFS_ana
lysis.pdf

It involves filtering paths on the basis of Feff's approximation of
the amplitude, another technique that is surprisingly effective.

HTH,
B

--
 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

 Molecular Environmental Science Group, Building 203, Room E-165
 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007

 Argonne National Laboratory         phone and voice mail: (1) 630 252 5033
 Argonne IL 60439, USA                                fax: (1) 630 252 9793

 My homepage:    http://cars9.uchicago.edu/~ravel
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/

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