Dear Ross,
I presume on the flat sample you measured regular theta-2theta diagrams,
then only probed those planes parallel to the sample plane. In such a
situation, you do not probe correctly the texture, and all the models
you could envisage are just giving back parameters that have been
refined on non-physic grounds. Harmonics will render a very nice fit,
just adding on the number of parameters in the development. Measuring
the full spectra in order to resolve structural parameters of a textured
sample requires that you acquire data in more than few sample
orientations, then the quantitative textural information brings
orientational physics in the Rietveld fits
(http://www.ecole.ensicaen.fr/~chateign/texture/combined.pdf). For this
you need to put the sample in a 4-circle diffractometer.
cheers
daniel
Ross H Colman a écrit :
Dear All,
Could anybody help me with a question I have related to preferred
orientation?
I have collected some spectra of a sample, on a D4 using flat plate
geometry, that I suspect is showing preferred orientation effects. I was
under the impression that a way of testing for this is to re-pack the
sample, re-run and compare the spectra, as the level of preferred
orientation should change between packings and so the spectra should
differ slightly. Despite re-packing and comparing spectra from up to 8
scans the spectra overlay perfectly (within noise). This suggests to me
that the refinement residuals are probably not due to preferred
orientation.
I am using Topas to refine the data and despite the comment above, the
residuals seem best fitted using a spherical harmonic-prefered orientation
model. I have also tried using other parameters to fit the residuals such
as the occupancy of some of the sites, using anisotropic displacement
parameters or anisotropic broadening effects, but the SH-PO fits best and
seems the most physically reasonable treatment.
As a test, I have also run the sample in capillary transmission geometry
on another instrument, and the refined value of the SH-PO parameter is
reduced. This seems to confirm that PO is the problem, but the question I
have is how effective the re-packing comparison method is for confirming
PO? If I see no difference in the spectra does it confirm that PO is NOT
the problem or can I still have PO effects that are very repeatable upon
repeat packing?
If PO is not the problem in this case, are there any other structural or
instrument effects that could lead to refinement residuals that would be
well treated with a SH-PO model?
Thank you all very much for any information or light you can shed on the
subject.
Ross Colman
--
--
Daniel Chateigner
Professeur, Université de Caen Basse-Normandie
Co-editor Journal of Applied Crystallography, www.iucr.org
Editor-in-Chief Texture, Stress and Microstructure, www.hindawi.com
--
address: CRISMAT-ENSICAEN and IUT-Caen,
Université de Caen Basse-Normandie, campus 2
6, Bd. M. Juin 14050 Caen, France
tel: 33 (0)2 31 45 26 11
fax: 33 (0)2 31 95 16 00
daniel.chateig...@ensicaen.fr
http://www.ecole.ensicaen.fr/~chateign/danielc/
--
A Quantitative Texture Analysis Course:
http://qta.ecole.ensicaen.fr/
An Open Source for Crystallographic Data: the COD
http://sdpd.univ-lemans.fr/cod/
--
