[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [sdpd] Inappropriate
I fully agree with Armel.
In the presence of too many reflections, how does one
determine the background level in the Intensity extraction (but
even in the Rietveld refinement) steps?
Too complex for 'conventional' powder structural analysis,
Riello's approach (which computes AB-INITIO the correct
physical background - in the absence of fluorescence, etc.)
is the answer?
Norberto
J. Appl. Cryst. (1995). 28, 115-120 [doi:10.1107/S002188989401037X]
X-ray Rietveld Analysis with a Physically Based Background
P. Riello, G. Fagherazzi, D. Clemente and P. Canton
Abstract: On the basis of known equations for calculating X-ray diffraction
intensities from a given number of unit cells of a crystal phase in
polycrystalline material, as due to: (i) Bragg reflections; (ii) average
diffuse scattering caused by thermal plus first-kind disorder; and (iii)
incoherent scattering, a relationship has been found that ties, in the
Rietveld analysis, the Bragg scale factor to a scale factor for `disorder'
as well as incoherent scattering. Instead of fitting the background with a
polynomial function, it becomes possible to describe the background by
physically based equations. Air scattering is included in the background
simulation. By this means, the refinement can be carried out with fewer
parameters (six fewer than when a fifth-order polynomial is used). The
DBWS-9006PC computer program written by Sakthivel & Young [(1990), Georgia
Institute of Technology, Atlanta, GA, USA] has been modified to follow this
approach and it has been used to refine the crystal structures of the cubic
form of Y2O3 and of a-Al2O3. Peak asymmetry has been described by a
function based on an exponential approximation. The results from
refinements using polynomial physically based background function are, in
terms of final structural parameters and reliability indices, very close to
each other and in agreement with results reported in the literature. The
reconstruction and optimization of the background scattering by means of
physically based equations helps the implementation in the Rietveld code of
other possible specific diffuse scattering contributions, such as that due
to an amorphous phase.
At 11.09 11/04/05 +0200, you wrote:
>Hi,
>
>Comments on a recent negative paper about the Le Bail
>method (iterative application of the Rietveld decomposition
>formula with cell restraints, but without structural
>model, in order to extract intensities):
>
>================================================
>"Lattice parameter measurement using Le Bail versus structural
>(Rietveld) refinement: A caution for complex, low symmetry
>systems", V. K. Peterson, Powder Diffraction, 20 (2005) 14-17.
>
>Abstract : The measurement of lattice parameters using the Le Bail
>method was shown to be inappropriate for a complex, low
>symmetry, structure, even with high resolution synchrotron diffraction
>data. The method failed as a result of ambiguous indexing in
>the absence of constraints on diffraction intensities, that
>arise when a structural model is used, combined with the large
>number of reflections. A caution for the use of the Le Bail
>and other whole-powder pattern decomposition methods is
>presented, particularly for high reflection density data.
>=============================================
>
>Is that true ?
>
>Yes, absolutely and obviously. The door was wide open
>to such a conclusion. I certainly have written something
>similar somewhere.
>
>Other inappropriate conditions already described alsewhere:
>the use of a negative background leading to negative intensities.
>
>Best wishes,
>
>Armel
>
>
>
>
>
>
>Yahoo! Groups Links
>
>
>
>
------------------------------------------------------------------------------
Norberto Masciocchi, Prof.
Dipartimento di Scienze Chimiche e Ambientali,
Università dell'Insubria, via Valleggio 11, 22100 Como (Italy)
Phone: +39-031-326227; FAX: +39-031-2386119
norberto.masciocchi...@uninsubria.it
http://scienze-como.uninsubria.it/masciocchi/
------------------------------------------------------------------------------
Hago cosas que me quitan un poco el mal gusto del vacìo.
Y ésa es en el fondo la mejor definiciòn del homo sapiens. (J.Cortàzar)
------------------------------------------------------------------------------
------------------------ Yahoo! Groups Sponsor --------------------~-->
What would our lives be like without music, dance, and theater?
Donate or volunteer in the arts today at Network for Good!
http://us.click.yahoo.com/TzSHvD/SOnJAA/79vVAA/UIYolB/TM
--------------------------------------------------------------------~->
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/sdpd/
<*> To unsubscribe from this group, send an email to:
sdpd-unsubscribe...@yahoogroups.com
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/