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Re: [sdpd] preferred orientation
Dear Jalil,
a long time I stocked your email, finally going to answer !
>Can anybody advise me about the use of spherical harmonic for
>modelling the prefered orientation in Rietveld refinement using
>fullprof?
The only adice I could make using the SH formalism to fit a texture is:
take care !
SH are mathematically defined function that are able to fit a lot of
things. However, unless you know a bit about your texturation process,
there is no a priori analytical shape of the texture. This is sometimes
wrong. But, the main handicap of using SH from diffraction experiments is
that, dealing with texture, normal diffraction does not "see" the even
orders of the harmonic coefficients (many articles from S.M. Matthies).
This gives rise to the well known "ghost" phenomena, which means that
negative (non physical) orientation densities appear in the orientation
distribution function.
>How many spherical harmonic terms should I use for obtaining a good fit?
The number of terms you have to develop in the series depends on the
crystal and sample symmetries (see Bunge in "Quantitative Texture Analysis").
>Also: Is there any other method for modelling the prefered orientation
>besides the traditional Rietveld-Toraya and March-Dollase functions?
The traditional models you propose are, for the least, very incomplete.
They only work for very peculiar texture cases. For instance March Dollase
only works for a single fiber texture component (in some programs two). In
any case they do not provide a complete determination of the ODF.
Other models like WIMV, ADC, Maximum entropy are much better to fit a
powder diagram using Rietveld, but they require to be implemented in a
program, and a set of experimental data that generally is large (typically
600 2theta diagrams at as many sample orientations) measured on four
circles diffractometers. I only know MAUD that can practice this.
Conclusion:
- You can destroy your sample cause the texture is not of interest and does
not influence the other parameters you are looking for: then powderise it
and practice a usual Rietveld
- Your sample is expensive, too long to prepare or rare, and textured:
don't destroy it and measure more than usual and treat the full thing.
- Your measures have been operated on a textured sample and thats it: use
one or the other model to describe the texture in a non-physically
interpretable way, and cross fingers !
daniel
>Regards,
>
>bouchekara messaoud
>X-Ray Diffraction Lab,
>Division of Chemistry,
>university houari boumedien alger
>algérie
>
>
>
>
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A Quantitative Texture Analysis Internet Course:
http://qta.ensicaen.ismra.fr/
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The Crystallographic Open Database:
http://www.crystallography.net
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Daniel Chateigner
Professeur
Co-Editor Journal of Applied Crystallography
CRISMAT-ISMRA, UMR CNRS n° 6508
Bd. Maréchal Juin, 14050 Caen FRANCE
Tel prof: 33 (0) 231452611
Fax: 33 (0) 231951600
http://www.ensicaen.ismra.fr/~chateign/danielc/
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