Dear Andreas,
I didn't said "it cannot be done". Only that "was not made for" and so it is not easy as using other tools. In principle every diffraction fitting program can be used for size-strain.
Few questions: have you ever tried to do such analysis with GSAS the right way using the instrumental profile correction?
Did you use only GSAS for that or you had to use other external tools/computations? (this to get a feeling about your statement: "Thus, on this level of line broadening analysis, everything necessary is contained in GSAS"; may be level should be clarified).
I would like to know what there is inside GSAS for crystallite size and microstrain analysis in particular.
Best regards,
Luca LutterottiOn Mar 25, 2005, at 12:55, Andreas Leineweber wrote:
Dear all,
I think the statement that one cannot do line-profile analysis using GSAS is
too strong. In principle it is possible to do some
size strain analysis using GSAS, if the instrumental profile is e.g. sufficiently described previously
by the Thompson-Cox-Hastings (TCH) profile function (includes measuring
corresponding data on a suitable standard). I think, even involvement of the
Finger asymmetry correction does not introduce systematic errors. Then the
increase in the tantheta
and 1/costheta related Gaussian and Lorentzian line width components of the TCH
description upon Rietveld refinement on the basis of diffraction data
exhibiting physical line broadening can
in principle be associated with microstrain- and size-related quantities.
This can also be extended by involving anisotropic size and microstrain models.
Thus, on this level of line broadening analysis, everything neccessary is
contained in GSAS. Of course, something like microstrain and size distributions
cannot be obtained using GSAS.
Of course there are problems:
0. Microstrain broadening must be proportional to tan(theta). This is not
neccessarily the case. There mustn't be further line broadening contributions
like stacking faults, complicating the situation.
1. If both size and strain contributions are present, one most be aware of the
correlation between the tantheta and 1/costheta dependent compontents.
2. One has to be aware to which average values of the size and microstrain
distributions the increases of the line width parameters can be associated
with.
This requires for GSAS a close analysis of the GSAS manual (how are the
line-width paramerters defined!) and line broadening literature, how such
pseudo-Voigt line width parameters can be related with averages microstructure
parameters.
Thus it is not made easy for the user to extract something like that from the
line width parameters. But perhaps it is better that way, because consequently
the user is forced to to deal himself with the required theory, rather than
just refining a parameter called size and one called microstrain, believing in
the results and publish the values....
Definitely there are much better procedures to analyse size and microstrain
than by GSAS.
So, going to the problem of Apu: If the instrumental profile is well described
before using TCH, refinement of (only) LX (and perhaps P) gives you quantities
whiuch you should be able to relate to size related quantities upon reading the
GSAS manual and some line broadening literature.
Best regards
Andreas Leineweber
Dear Prof. Lutterotti,
I was also aware of the fact that GSAS is not made for Size Strain analysis.I got interested to use the Size strain refinement feature of GSAS only after
going through the article :
"Size-strain line broadening analysis of the ceria round-robin sample" byProf. D. Balzar et. al. Journal of Applied Crys. 37(2004)911-924.
In that round robin results they have reported the size strain obtained fromGSAS.
I my case also when I am trying with GSAS, the diffraction pattern is fittingwell except the peak braodening. I think this brodening is due to small domain
size effect. I that case how will I obatin a good fit with GSAS.
Thanking you.
Best Regards,
Apu
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Apu Sarkar
Research Fellow
Variable Energy Cyclotron Centre
Kolkata 700 064
phone: 91-33-2337-1230 (extn. 3190)
Fax: 91-33-2334-6871
INDIA
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----- Original Message -----
From: Luca Lutterotti <[EMAIL PROTECTED]>
Date: Friday, March 25, 2005 3:31 pm
Dear Apu,
I know I will start up a "good" debate here, but size-strain
analysis
with GSAS is a non-sense. The program was not written with that
purpose
in mind and in fact it does not contains the instrumental
aberration
part of the broadening that is necessary for such computation.
Indeed it is possible to get at end some size-strain data, but
quite
hard as you have to do all correction later and out of the
program. So
it is like using GSAS for peak fitting, so better to use a peak
fitting
dedicated program.
Best wishes,
Luca Lutterotti
On Mar 25, 2005, at 7:15, [EMAIL PROTECTED] wrote:
Dear All,
I am trying to perform Rietveld refinement on a very simple
system
using GSAS. I have obtained a reasonable fit except the peak widths.
I want to use the size and strain refinement option in GSAS to
make
the fit well.
Please tell me how to use the SIZE STRAIN refinement option in GSAS.
P.S. I am using the EXPGUI.
Thanks in advace.
Regards,
Apu
/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
Apu Sarkar
Research Fellow
Variable Energy Cyclotron Centre
Kolkata 700 064
phone: 91-33-2337-1230 (extn. 3190)
Fax: 91-33-2334-6871
INDIA
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