Michael Glazer said:
> 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.
While I agree with Mike's list, I can't let him get away with such a
swee
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 fix
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 (short
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 stronge
Dear Carolina,
I have the answer. It's about the electron cloud of your elements.
I'll tell you further tomorrow because I'm at home right now. I speak
about this feature in one of my publications. You will have the answer
tomorrow, if nobody replies.
Best regards.
--
Dr. Jean-Ga
As mentioned recently,
Their is no physical meaning to have negative thermal agitation factor.
It means that at high angle the observed data at upon the calculated.
It means that you should put more electron (if you are doing X-Ray
diffraction) or more electron or nuclear density in your
cryst
