>I just received the 48th Denver X-ray Conference program, and was
astonished by the title of >the D-092 conference: "Ab initio structure
solution as part of the Rietveld refinement process"
>I believed till now that the Rietveld method was the inevitable last part
of an ab initio structure >solution.
With this contribution we are going to present some evidence, that the
strict distinction between structure refinement (particularily Rietveld
refinement) and structure solution is loosing its base. Moreover we will
proove, that the following commonly stated "characteristics" of the Rietveld
method are no longer true in this rigid form:
- "...the Rietveld, being nothing else than a complex minimization
procedure, cannot be taken as an active tool, since it can only slightly
modify a preconceived model built upon external knowledge."
- "The starting parameters must be reasonably close enough to the final
values."
- "The sequence into which the different parameters are beeing refined needs
to be carefully studied..."
(Masciocchi, N., 1997, Rigaku Journal)
Concerning structure solution in our paper we are going to answer the
following question:
"How far off the position of an atom may be and the "refinement" still
brings it in?"
Lets consider the following simple example PbSO4 for Rietveld refinement.
Without any doubt the following stucture model (ICSD collection code 100625)
can be refined easily with any Rietveld program:
site Pb x 0.1667 y 0.1879 z 0.2500
site S x 0.1842 y 0.4367 z 0.7500
site O x 0.0960 y 0.5920 z 0.7500
site O x 0.0430 y 0.3060 z 0.7500
site O x 0.3090 y 0.4180 z 0.974
What about the following model:
site Pb x 0.0 y 0.0 z 0.0
site S x 0.0 y 0.0 z 0.0
site O x 0.0 y 0.0 z 0.0
site O x 0.0 y 0.0 z 0.0
site O x 0.0 y 0.0 z 0.0
Actually this is still a model. And if a "Rietveld program" sucessfully
refines this model to the true coordinates this is * structure solution *.
Isnt it? By the way: WITHOUT preceeding intensity extraction in a separate
step!
E.g. the "Zhu" example of the structure solution round robin last year (A.
Le Bail & L.M.D Cranswick) is solved within about 1.5h in the same way.
We are going to present the new Software, its basics (paper is submitted to
J. Appl. Cryst) as well as several new structures recently solved (up to 13
atoms in the asymmetric unit) at the following meetings:
- Denver Conference, D-092
- Glasgow IUCr meeting, P05.OD.001
- Glasgow Workshop "30 Years of Rietveld Analysis: The Next Generation",
http://www.bruker-axs.com/Events/glaspg1.htm
We are looking forward to meet you either at the poster or in our workshop.
Arnt
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Dr. Arnt Kern
Bruker AXS GmbH
AXS M
�stliche Rheinbr�ckenstr. 50
76187 Karlsruhe
Germany
Tel.: (+49) 721-595-5669
Fax: (+49) 721-595-6693
EMail: [EMAIL PROTECTED]
URL: http://www.bruker-axs.de
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