I can't resist adding one more to Mike's excellent list:
5. When peaks overlap strongly it becomes difficult to determine the
background level. Negative Uiso is a consequence of the background
refining to a value which is too low, especially where the peaks are
most dense in the pattern (shorter d-spacings or higher angles).
All the best,
Jon
Michael Glazer wrote:
Negative U's in Rietveld can arise from several causes, so that there is
not one single answer. Some of the reasons are
1. The structural model is simply incorrect.
2. High absorption means that the low-angle data are weaker than they
should be, or conversely that the high-angle data appear stronger than
they should be. Abnormally strong high-angle data give rise to a
decrease in U's, even making them appear negative
3. Correlation between the refinement parameters. For instance U's will
tend to be highly correlated with site occupation parameters, often
making it difficult to separate them.
4. In general, many of the errors that one encounters tend to end up in
the refined U's, and this is why their precise values have to be treated
with caution.
Rietveld refinement (as opposed to single-crystal refinement) is in fact
refinement of degraded data (it is one-dimensional instead of
three-dimensional) and so the errors will be more significant.
Mike Glazer
-----Original Message-----
From: carolina.zip...@fi.isc.cnr.it
[mailto:carolina.zip...@fi.isc.cnr.it]
Sent: 03 March 2010 16:08
To: rietveld_l@ill.fr
Subject: Negative Uiso in GSAS
Dear all,
could someone explain to me the meaning of obtaining a negative Uiso in
GSAS?
I thought it was always positive...(p. 123 manual)
thanks
Carolina
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Dr. Carolina Ziparo
Istituto dei Sistemi Complessi - sezione di Firenze,
C.N.R. - Consiglio Nazionale delle Ricerche
via Madonna del Piano, 10
I-50019 Sesto Fiorentino Italy
tel.: +39 055 5226693
fax: +39 055 5226683
e-mail: carolina.zip...@fi.isc.cnr.it
