I guess that I should try to clarify a couple of points about structure
determination from neutron powder diffraction and about the resolution of
neutron powder diffractometers.
(i) Structure determination from neutron powder diffraction
Armel is quite right when he says that you see all the atoms with neutrons
whereas the heavy atoms dominate with X-rays. The effective number of atoms
is thus lower with X-rays and this makes the initial structure solution
easier. As a corollary, neutrons are usually better for refinement as more
of the atoms are visible. One area in particular where X-rays wipe the floor
with neutrons is in the organic molecular structural work - hydrogens can be
taken to be invisible in the initial stages of structure solution with X-ray
data whereas they are a proverbial nuisance with neutrons because they
produce such a large background. Thus anyone in their right mind will use
X-rays for structure determination of organic structures - we certainly do!
The situation with non-hydrogen structures is not quite so clear cut but
again principally rests on the fact that X-rays again have the advantage
that the heavy atoms (e.g. cations in an oxide) often dominate the
scattering and thus simplify the structure solution. The other hurdle that
needs to be overcome before a successful structure solution from neutron
powder diffraction is an interesting one - that is the peer review panel.
Almost all neutron sources in the world peer review the proposals for
experiments - ISIS and the ILL certainly do. I have sat on both ILL and ISIS
peer review committees and have found on almost every occasion when a
structure solution is proposed that it is knocked back because the reviewer
states (usually correctly) that it would be better done with X-rays. Thus
the drought!
(ii) the resolution of neutron powder diffractometers
Armel is again correct is his assessment of conventional constant wavelength
diffractometers
He states ...
Let us examine the lowest FWHM measured on powder patterns
at constant wavelength :
neutron : minimal FWHM ~0.10° (2-theta), pushing the machine
conventional X-ray : minimal FWHM ~0.04°
synchrotron radiation : minimal FWHM < 0.01°
and concludes ...
There is an order of magnitude between the neutron high
resolution and the synchrotron high resolution.
This is just a fact, not a joke.
Well ...
I am sure that the likes of Alan Hewat would have something to say about the
implicit presumption that constant wavelength neutron powder diffractometers
are an order of magnitude poorer - the position of the minimum is important
and the information in a D2b diffraction pattern is very high because of the
lack of a form-factor fall-off for neutrons. Kenneth Shankland was
discussing TOF diffractometers and HRPD in particular - now before you say
there's only one, it must also be said that there are few synchrotron
machines in the world that have minimal FWHM < 0.01°. Be that as it may, the
bottom line for neutron TOF machine resolution is that the (delta d)/d of
the machine is determined by the flight path uncertainty which in turn is
essentially the width of the neutron moderator. On HRPD, the moderator width
is 5cm. and the flight path 100m. This makes
(delta d)/d = (delta L)/L = 5 x 10-4 = measured FWHM of HRPD
Now (delta d)/d = (delta theta)* cot(theta). Putting 2 theta = 90 and
cot(theta)=1 then
(delta 2 theta) = 10-3 radians = 0.06 degrees ( at 90
degrees equivalent)
and !! ...
(delta 2 theta) = 10-3 radians = 0.006 degrees ( at 11
degrees equivalent)
and essentially linear in between.
Now before you say this cannot be true and effective 2 thetas means nothing
- if you plot HRPD and synchrotron data on a common scale ( say d* = 1/d)
then HRPD matches even the best synchrotrons at d-spacings above ca. 1.5 -
2A. I hope that this helps clarify the point that Kenneth was trying to
make. For those who remain to be convinced why not apply for some HRPD
experiments.
Call for Proposals Round 2000/1 at the ISIS Facility.
Scheduling period April 2000 to September 2000.
DEADLINE 16 OCTOBER 1999.
For further information contact : ISIS User Liaison Office;
Tel +44(0)1235 44 5592
Fax +44(0)1235 44 5103
e-mail [EMAIL PROTECTED]
The best proposals always get through!!
All the best,
Bill David
