Armel Le Bail wrote:
> I note that this is mainly the neutron community
> that is not preoccupated by improving the positions of the
> heavy atoms by using X-ray ;-).
With a few exceptions, neutrons are sensitive to most heavy atoms. For
many materials the improvement obtained using both x-rays and neutrons
vs neutrons alone are minor, but for complex materials, or materials
such as zeolites, that do not scatter well, use of only one radiation
can result in a severely flawed result (examples on request).
> However, I tend to think
> that a simultaneous refinement could eventually degrade
> the heavy atom position accuracy AND degrade the light
> atoms accuracy.
This is a common misconception. As a gedanken experiment, suppose you
have a structure containing D atoms and V atoms. The neutron fit will be
completely insensitive to the V positions and the x-rays insensitive to
the D positions. (This is easily verified.) The only case where the
positions are degraded is where there is a systematic experimental
anomaly with one (or both) datasets.
> Giving the results of 2 independent refinements seems better
> to me.
As supplementary material to demonstrate the combined refinement is
valid? Sounds like a good idea to me! But, the combined refinement
result will be the most accurate and thus the one to stick in the paper.
> The confirmation of what I suspected : some peoples are
> more equal than others !-).
Armel you may indeed be more equal than some but, I would like to
welcome you to visit us in the suburbs of Washington, along with some of
your favorite samples. DC is no match for Paris or even Grenoble, and
our cafeteria does not come near the quality of the ILL, but there are
some other attractions.
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Brian H. Toby, Ph.D. Leader, Crystallography Team
[EMAIL PROTECTED] NIST Center for Neutron Research, Stop 8562
voice: 301-975-4297 National Institute of Standards & Technology
FAX: 301-921-9847 Gaithersburg, MD 20899-8562
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