Hi Alan,
In the limiting case of orienting all crystals in 3D you get a mosaic
single crystal (e.g. [1], or using an XFEL to measure 1 at a time). Then
the refined the crystal structure has somewhat better accuracy than with
a 1D Rietveld fit. Perhaps not the most popular idea for this mailing
list :-)
The intermediate case of multiple patterns with properly modelled
texture should also be better even for refinement. Simplistically - you
can add information about overlaps like 511/333 cubic overlap if you
have a series of patterns.
With only a single pattern you introduce extra texture parameters
without adding the extra information to pay for this (which is why
people tell us to get rid of it). With multiple patterns the story is
different, but the price is figuring out the texture properly.
Cheers,
Jon
===
[1] Kimura et al, Langmuir, 2006, 22 (8), pp 3464–3466
DOI: 10.1021/la053479n
http://pubs.acs.org/doi/abs/10.1021/la053479n
On 24/06/2015 19:46, Alan Hewat wrote:
Both Peter and Luca are correct :-) Preferred orientation can indeed be
used to help SOLVE (unknown) structures, but when you want to REFINE
structures you should try to eliminate systematic errors such as
preferred orientation, if possible by better sample preparation. That
may not be possible with highly oriented crystallites such as nanotubes,
and as Naveed found, modelling peak broadening will not help. Search the
program manual for "texture" or "preferred orientation" modelling.
Pax. Alan.
On 24 June 2015 at 17:44, Khalifah, Peter <kp...@bnl.gov
<mailto:kp...@bnl.gov>> wrote:
Yes, that is of course a more precise and correct answer. I did not
mean to imply that diffraction physics work differently, just that
the data processing resulted in some badly wrong assumptions about
the data being used for Rietveld refinement. ____
__ __
I tried to give a quick answer to a basic question from a basic
point of view. If it was an expert question, I would have left it
to the experts like you :-)____
__ __
-Peter____
____
__ __
*From:*Luca Lutterotti [mailto:luca.luttero...@ing.unitn.it
<mailto:luca.luttero...@ing.unitn.it>]
*Sent:* Wednesday, June 24, 2015 3:52 AM
*To:* Khalifah, Peter
*Cc:* rietveld_l@ill.fr <mailto:rietveld_l@ill.fr>
*Subject:* Re: TOPAS Macro Language (peak shape broadening macros)____
__ __
__ __
On 23 Jun 2015, at 22:22, Khalifah, Peter <kp...@bnl.gov
<mailto:kp...@bnl.gov>> wrote:____
__ __
Preferred orientation results in the central assumption that the
measured intensity is proportional to the reflection structure
being violated, ____
__ __
This is actually quite new to me as definition of preferred
orientation or better texture. Until now I thought that in the case
of preferred orientations the measured intensity is proportional to
the volume fraction of grains oriented in a certain direction…….____
__ __
As many people will tell you, it is much more effective to
eliminate preferred orientation (though improved sample
preparation) than to try to model it. ____
__ __
We actually use preferred orientation (texture) to help structure
solution, and I found more easy to model them than to eliminate
them. But may be is just me.____
__ __
Best regards,____
__ __
Luca Lutterotti____
__ __
-----------------------Luca
Lutterotti--------------------------------------------____
Dipartimento di Ingegneria Industriale, Universita' di Trento,____
via Sommarive 9, 38123 Trento, Italy____
Temporary address:____
Laboratoire CRISMAT-ENSICAEN, Université de Caen
Basse-Normandie, Campus 2____
6, Bd. M. Juin 14050 Caen, France____
e-mail address : luca.luttero...@unitn.it
<mailto:luca.luttero...@unitn.it>
Home page : http://www5.unitn.it/People/en/Web/Persona/PER0004735
New Maud page : http://maud.radiographema.com
Phone number :+39-0461-28-2414
Fax :
+39-0461-28-1977------------------------------------------------------____
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