partial and differential PDFs
Seyma Ortatatli
Hi everyone!
I have one question related to PDFgui software. Actually I want to do some partial and differential PDF analysis. But I am really confused about how to do that. Because honestly, when I checked the questions/answers, it sounds a bit complicated for inexperienced users. For example I saw the following answer,
“It is not possible to define calculations of different partials under the same fit. Instead one needs to copy the whole fit and adjust the included pairs in the new phase.” Then, in which order one should refine the parameters of a multiphase-structure in PDFgui? How to sum up these phases afterwards to obtain the total PDF?
It would be nice if some of you give an example (in my case I have three phases!) about how to make a partial and differential PDF analysis by the use of PDFgui, I would be grateful.
Many thanks in advance for any help.
Regards,
Seyma.
-------------------------------------------------------
Seyma Ortatatli
Department of Heterogeneous Catalysis
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1
D-45470 Mülheim an der Ruhr (Germany)
phone: +49.208.306-2173
eMail: orta...@mpi-muelheim.mpg.de
Pavol Juhas
> Hi everyone!
>
>
>
> I have one question related to PDFgui software. Actually I want to do some
> partial and differential PDF analysis. But I am really confused about how to
> do that. Because honestly, when I checked the questions/answers, it sounds a
> bit complicated for inexperienced users. For example I saw the following
> answer,
>
>
>
> "It is not possible to define calculations of different partials under the
> same fit. Instead one needs to copy the whole fit and adjust the included
> pairs in the new phase." Then, in which order one should refine the
> parameters of a multiphase-structure in PDFgui? How to sum up these phases
> afterwards to obtain the total PDF?
The quoted sentence means that it is not possible to evaluate
several partials of some structure (phase) by defining multiple
Calculations within the same Fit node, because the pair selection
happens in the phase node. In other words, to simulate
all possible partials for NaCl within a single run, one needs to
define 3 fits, say "NaNa", "ClCl", "NaCl" (and a Calculation within
each of these fits) that are identical with the exception of the
"Included Pairs" setting in the NaCl phase. Note that this is just
a recipe for simulating all partial PDFs from a known structure
without doing any refinement.
Partial PDFs can be refined provided that the "Included Pairs"
setting is consistent with the pair contributions that are
present in the measured signal.
It is hard to answer your question without knowing more details about
the analyzed multi-phase material and the measured differential PDF.
As a general rule, the first step, before setting up any complicated
refinements, would be to try to simulate the measured PDF and check if
there is any appreciable response of the simulated curves to the
physical quantities(s) (atom positions, phase content?) that you are
trying to extract from the data.
Hope this helps,
Pavol
--
Dr. Pavol Juhas
Condensed Matter Physics and Materials Science Department
Brookhaven National Laboratory
P.O. Box 5000
Upton, NY 11973-5000
Seyma Ortatatli
First of all, thanks very much for the reply. Regarding to the material, you
are right I could not provide the required info about it, since it was not
synthesized by me. But as I know, it is an alloy: Au@Pt on Carbon. But when
doing Rietveld refinement of the data set, I did not see any peaks coming
from Pt. They were just Au peaks and amorphous Carbon as a contribution to
the bkgr. However, in PDFgetX2 software I defined 3 elements (Au, Pt, C) to
obtain the ".gr file". For this reason, in PDFgui I added 3 phases together
in the same fit and refined it step by step (Rw=0.16). I sent the pic. after
the refinement. But you can fit an elephant by doing refinement, this is the
problem... So, I should not rely on this result, right?
Regards,
Seyma.
-------------------------------------------------------
Seyma Ortatatli
Department of Heterogeneous Catalysis
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1
D-45470 Mülheim an der Ruhr (Germany)
phone: +49.208.306-2173
eMail: orta...@mpi-muelheim.mpg.de
http://www.kofo.mpg.de
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