Re: Negative Uiso in GSAS

[Reinhard Kleeberg]

To add some more simple reasons for apparently overmuch high intensity 
at high angles causing meaningless negative temperature factors:
- the low angle peak intensities are suppressed by beam overflow 
(typically fixed slit Bragg Brentano geometry)
- one is using automatic slit data instead of fixed slit ones, and the 
program used is unable to account for that
- if one has used converted ADS data to fixed slit data: The conversion 
failed because of any wrong background correction, or the primary ADS 
data are wrong because of misprogramming or misalignement of the ADS system.

And one addition to Jon's important point 5 below: This is often related 
to physically meaningless peak shape models, running out of meaningful 
peak width values, just by becoming broader and broader and "taking 
intensity" from the background into the high angle peak intensities. One 
should use a physically based and more rigid peak shape model.

Regards

Reinhard


Jon Wright schrieb:
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,
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