I also use combined CW neutron and synchrotron refinements. A simple
minded justification goes as follows. Most of the problems I work on are
badly underdetermined -- at least by the crystallographic rule-of-ten
(10 crystallographic observations for each structural variable). By
changing scattering lengths, I get a second set of observations which
gives me more observables. Thus, I agree strongly with all of Dr. Jaap
Vente's points:
> 1) in general the refinement is more stable.
> 2) their is the possibility to study much more complicated structures
> than with only one of the techniques.
> 3) because you now have two really different sets of data your structural
> model is more reliable.
> 4) you can study compounds which contain elements that are difficult to
> locate precisely with one technique, think of vanadium oxides or
> manganese/iron oxides.
Andrew Wills is correct that X-rays see the electronic distribution and
neutrons see nuclei positions, but electrons distributions are pretty
close to spherical (our form factors assume this) for high-Z elements
and are usually well centered around the nucleus. One can make a good
argument that displacement parameters (aka temperature factors) can be
completely different for x-rays vs neutrons, but experimentally this is
seldom true. In any case, for all but the simplest systems, with powder
work we don't have the precision to tell. Besides, x-ray displacement
parameters are pretty meaningless anyway :-).
I do not know of any codes other than GSAS that do combined
x-ray/neutron fits, but in GSAS all the experimental effects
(orientation, absorption, etc) are segregated by dataset so one only
needs to apply these corrections to the x-ray data. (Neutron data seldom
have either problem). In any case, if you can't model them well, you
can't use the data.
The "weighting" problem is overstated. The data are weighted by how well
you know them. Usually the x-rays do contribute more to the Chi2 than
the neutron, but the algorithm will minimize the deviations in both
appropriately. One could downweight the x-ray data artificially, since
you will probably have worse precision on the more structurally accurate
neutron data, but this will screw up the Chi2 value.
The biggest problem for combined refinements is that you need to have
exactly the same sample and the same conditions for both the x-ray and
neutron work. Since single crystals are frequently grown under different
conditions than bulk samples, the utility of combined x-ray single
crystal - powder neutron refinements is limited. Alas, it is fairly
common that someone makes a material, measures the x-ray diffraction and
then scales up the synthesis for neutrons, but ends up with something
different. Attempts to simultaneously fit one model to x-ray data from
the first batch and neutron data from the second batch are a waste.
Other issues can also arise. We recently had a case where a material
seemed nearly pure by x-rays, but the neutron work showed that the
centers of the large particles were still composed of unreacted starting
material. The x-rays did not penetrate far enough to see the purity was
only ~70%.
It would probably be a good idea to check that the model obtained from
the combined refinement agrees well with (possibly constrained) models
using the individual datasets. Perhaps we could entice John Parise to
write a message about how to do this.
Finally, I should mention in response to Armel that at least here at
NIST, most requests for time are scheduled within 2-8 weeks of when we
get them (see http://www.ncnr.nist.gov/~toby/bt1.html).
********************************************************************
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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