Dear Rietvelders,
after some years of being a happy user of TOPAS & Rietveld method (after
working-out a good approach, thanks to a great help from users of this
mailing list for which I am really thanful, and having it checked by
attendance in Reynolds Cup) I was recently confronted with few questions
I couldn't answer. I thus have a kind request of a help here:
- does the Absorption correction also influences microabsorption? This
question arose due to somewhat lowered expected wt.% of pyrite (19
instead of 25), FeS2, even though all the used parameters (Absorption,
CrySize, Strain - for all the sample components) were checked and proven
to give value > error, i.e., being physically meaningful for the
refinement model. I was wondering how this supposed non-precision for
pyrite could be addressed...
- Is it worthy to use the parameter known as "Sample Thickness",
associated with the Absorption Correction in TOPAS? I was taught in a
5-day course of TOPAS that it's not a good idea, especially with the
Bragg-Brentano geometry, but I'm not sure...
- Is there any difference, to be expected, in the calculated unit cell
parameters, between approximating a phase using (1) a chemically pure
STRUCTURE model (i.e., taken directly from a database / CIF file), and
(2) a STRUCTURE with Occupancy information updated with the known
content of impurities? I always thought that it shouldn't matter, as the
software simply "iteratively" compares the sample' reflections with the
structural information contained in STRUCTURE models (?). I have
compared these two situations, and option "2" actually gave me worse
fit. I thus wondered: does it really matter that much which model
(approximation) of the structure - chemically pure or with added
impurity info - is used? For most substances, the reflection positions
vary even for different standards found in databases like PDF or COD...
- Is the negative intensity in the difference curve only informing us of
underestimation of the content of a particular phase, suggesting to
replace the Structure model used with another one?
We have been working using few computers with, supposedly the very same
version of the software. However, even though we've been using exactly
the same Structure models, results were varying depending on the
computer stand used. I was wondering if this could have to do with some
errors in the JAVA script, or maybe a non-identical installation
conditions, or something about the Windows system?
I would be thankful for any tips.
Best regards,
--
Łukasz Kruszewski, Ph.D., Associate Professor
Polish Academy of Sciences
Institute of Geological Sciences
Twarda 51/55 str.
00-818 Warsaw
Poland
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