Dear All,
here is my (biased) opinion on the whole matter.
1) Intensity data: neutron powder diffraction *always* yield better
intensity data than x-ray powder diffraction, including synchrotron.
Contrary to popular belief, this is true not only for mixtures of heavy and
light atoms, but also for all light or all heavy atoms. An interesting
example of this was given by Bill David with his spherical harmonic
refinement of C60 (a very fair test for x-rays). HRPD data yielded correct
harmonic components up to l=18 (this was subsequently confirmed by a
sigle-crystal XRD measurement). For synchrotron data, even the first
component had the wrong sign (!). The exact reasons of this superiority
have never been precisely quantified (to my knowledge), but it is generally
believed to be due to the smaller amount of systematic errors. Preferred
orientation has already been mentioned, but another big source of systematic
errors in synchrotron XRD is powder statistics, because the beam is highly
collimated on a small sample. You just need to take a look at the
oscilloscope while the sample is spinning to understand what I am talking
about. The advent of hard x-ray beamlines is not easing the problem: true,
the beams are more penetrating, but you are selecting even fewer grains
because the reciprocal space is compressed.
2) Atomic contribution. In general, x-ray data are dominated by the heavy
atoms. This is not necessarily always a disadvantage, and, in fact, it can
be a big advantage for complex structures. Here, the classic example is
drug structure solution, where neutrons are hopeless just because they see
too much.
3) Signal-to-noise ratio: at the moment, x-rays give far better STN ratios
than neutrons. This means, that they are better at identifying weak
features (e.g., weak superlattice peaks), even when have large light atom
contributions.
4) Resolution: many neutron diffractometers have sufficient resolution so
that this is not an issue in structural refinements, because peak widths at
high q are usually sample-limited. The exceptions are large low-symmetry
unit cells, where you can start telling the difference, say, between HRPD
and D2B in terms of refinement stability. However, this assessment changes
dramatically when you start talking about indexing and strain analysis.
Here, only HRPD can compete with synchrotrons.
5) Complementary vs. simultaneous use: I personally think that there are
many more cases for complementary use than for simultaneous use. This is
certainly the case of my own work on perovskite superlattices, where I have
published synchrotron and neutron data in the same paper many times, but I
have never published a single simultaneous refinement. Not that I have
never tried: in fact, I have several published examples of simultaneous
refinements of two neutron wavelengths. However, when I tried to add the
x-rays, I almost invariably spoiled the neutron results. I used the x-ray
data to determine the strain model and to extract weak superlattice peaks.
At the other extreme (small organic molecules) there is also a good case for
complementary use, where x-rays are used to refine the non-hydrogen
positions, which are then fixed in the neutron refinements to get the
hydrogen positions.
In general, I believe that simultaneous refinements are not worth the effort
unless there is a clear reward in sight, like in the case of contrast
variations to get occupancies in multiple solid solutions.
6) Accessibility. Armel's argument sounds a lot like the Aesop's fox (nolo
acerbam sumere=these grapes are too sour), with the important difference
that Armel could jump a little higher if he really wanted to get to the
neutrons. Accessibility is an issue, but it can only be addressed if there
is a consensus on the need to use the technique. I am absolutely sure that
if enough chemists make enough noise that they want to measure their hot
sample one week after they synthesized, they are going to get a rapid NPD
service. And there need not be a human sample changer, you know. We've got
machines for that.
Best
Paolo
