Dear All:
Indeed, despite some more advanced approaches to modeling diffraction line
shapes, the good, old Cagliotti function is still in use probably for
historical reasons (as it is the case with many other things in sciences).
However, to be fair to (practically all major) Rietveld programs,
As Maxim says, strain-related parameters are U and Y. By the same token,
size-related parameters are P and X (both go with 1/cos(Theta). The reason is
that this Rietveld model (the so-called TCH) assumes a Voigt function for both
size and strain profile and therefore has to have both Lorentzian
It was shown in paragraph 6 of JAC 35 (2002) 338-346 that size-broadened
profiles given by both lognormal and gamma distributions can be approximated
by a weighted sum of Lorentz and Gauss functions for a broad range of
distribution dispersions. Besides, round robins can sometimes be long
I guess, this discussion has already died down but I couldn't find a moment for
reply soon enough:-)
As Prague was already mentioned, let me try to summarize what I think about
this subject and have said there (let's hope I actually remember it:-):
1. A careful line broadening analysis (at this
size and
strain values calculated. One can even obtain size distribution by following
the procedure that was posted to this mailing list several months ago; see
below.
Best wishes,
Davor
Davor Balzar
Department of Physics Astronomy
University of Denver
2112 E
ze and strain values.
Davor Balzar
tifies why we tried to have
measurements collected on different instruments in the round robin, although
it might have been a problem for some participants.
Best regards,
Davor Balzar
**
National Institute of Standards and Technology
Materials Science
Hi Armel:
I am impressed -- nice report:-)
One comment regarding ARIT. I didn't go carefully through all the results
but my impression is your values are a bit underestimated. If you added
additional Gaussian term to your An, you should get results much closer to
WA. That should be relatively
Dear Armel:
Although I sent the message unintentionally to the whole list, why not have
a public discussion.
including Gaussian. But the size model is currently only
Lorentzian in ARIT. There is no way to have a pure Gaussian
shape for the size effect since it corresponds to unphysical
size
Very often, a simple assumption that
size-broadened profile has both Lorentzian and Gaussian terms yields
satisfactory results.
Well, this assumption is easily entered in a pseudo-Voigtian or Voigtian
profile shape model. But the corresponding size Fourier coefficients
AnS are never
alzar/ or from the European CCP14 mirror
at the http://www.ccp14.ac.uk/ccp/web-mirrors/balzar/div853/balzar/
The deadline for returning the results to the Round-Robin organizer is set
for October 20, 2000!
Your participation is greatly appreciated
alzar/ or from the European CCP14 mirror
at the http://www.ccp14.ac.uk/ccp/web-mirrors/balzar/div853/balzar/
The deadline for returning the results to the Round-Robin organizer is set
for October 20, 2000!
Your participation is greatly appreciated
reprint can be found at
http://www.boulder.nist.gov/div853/balzar/PRB03414.pdf
As for the Eui-Suk's question, a good starting point to understand the
BaTiO3 structure and other properties is the book of Jona and Shirane,
Ferroelectric Crystals, Pergamon Press.
Regards,
Davor Balzar
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