American Museum of Natural History, New York, Tuesday 29th March 2003 at 1pm.

"Beyond Classical Rietveld Analysis - using Le Bail fitting of X-ray
 Powder Diffraction data to help answer the questions of the world:
 can the Earth's outer core contain Oxygen?"

by L. M. D. Cranswick

   CCP14 - School of Crystallography,
   Birkbeck University of London,
   Malet Street, Bloomsbury,  London,  WC1E 7HX, U.K.
   (presently visiting the Geochemistry department at the Lamont-Doherty
   Earth Observatory of Columbia University)
   E mail: l.m.d.cranswick@dl.ac.uk
   WWW: http://www.ccp14.ac.uk/

ABSTRACT

To investigate a scientific problem may require undertaking a relatively
convoluted route. This can involuntarily involve people within the
intricacies of methods that may not seem related to the overall problem
of interest. However, if the time is not taken to ensure the best choice
and use of an analytical method; the asking of relevant scientific
questions may be possible, but not of obtaining robust analytical
results to credibly support the conclusions. This talk will 
elaborate on powder X-ray diffraction analysis with an earth sciences
(planetary core/meteoritic cores(?)) theme.

The Le Bail method [1] is a way of performing whole profile Rietveld
fitting [2] to powder diffraction data without the need for atomic
structure information (just a cell and a trial spacegroup). Examples of
the power of the Le Bail method include the ability for whole profile
unit cell refinement involving overlapping reflections, extraction of
HKLs for structure solution and spacegroup determination. Le Bail
fitting is considered as standard functionality in modern Rietveld
software[3]. Formerly the domain of single crystal diffraction, it has
been possible to solve crystal structures from powder diffraction data
for quite some time; of which the Le Bail method was the main
breakthrough which made this possible[4].

Using Le Bail fitting on a modern PC computer, in a matter of seconds a
whole X-ray diffraction profile can be reliably and accurately fitted.
This makes Le Bail fitting suitable for where a large amount of data has
been recorded and requires accurate analysis. A recent example showing
the power of the Le Bail method by its use in providing high quality
unit cell volume data necessary in the determination of the volume of
Oxygen in the Earth's outer core.[5] [6] This, and the use of
synchrotron based high pressure energy dispersive powder X-ray
diffraction (EDX) will be the focus of this talk. 

References:

[1] A. Le Bail, (2002) http://sdpd.univ-lemans.fr/iniref/lbm-story/
[2] Hugo Rietveld: http://home.wxs.nl/~rietv025/
[3] http://www.ccp14.ac.uk/solution/lebail/#software
[4] R.E. Morris, W.T.A. Harrison, J.M. Nicol, A.P. Wilkinson &
A.K. Cheetham, Nature 359 (1992) 519-522
Determination of complex structures by combined neutron and
synchrotron X-ray powder diffraction.
[5] D. Walker, G. Hughes, L. M. D. Cranswick, S. M. Clark 
and S. Buhre. "Synthesis and thermal decomposition of tetragonal 
RbClO4 and volume of fluid O2 from 2 to 9 GPa",  Geochemistry, 
Geophysics, Geosystems, Vol. 2, 2001
[6] D. Walker, S.M. Clark, L.M.D. Cranswick, M.C. Johnson and R.L.
Jones, "O2 volumes at high pressure from KClO4 decomposition: D" as a
siderophile element pump instead of a lid on the core." (2002),
Geochemistry, Geophysics, Geosystems (G^3), 23 November 2002.