[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [sdpd] Indexing problem
Dear Cristian
Consider the possibility that you have a small triclinic distortion of
this monoclinic cell, which becomes significant only at higher 2Theta.
A good way to test this would be to use the monoclinic cell as basis set
in Mmap, and allow alpha* and gamma* to be varied in small steps from
their initial monoclinic values of 90 deg. One of these reciprocal
angles would need to be allowed to take both acute and obtuse values, the
other need only take acute values.
If a triclinic distortion is present, you should find that the figure of
merit maximum has moved out into one or other of these quadrants of
alpha*/gamma*.
The effect should become more clearly determined if you calculate against
only the apparently anomalous higher angle reflections, since the low
angle lines will not be contributing much signal.
With best wishes
Robin Shirley
On 18 Nov 2004 at 10:43, Cristian Perca wrote:
Dear all,
I encounter a (serious) problem when trying to index an X-ray powder
pattern. The data was measured on a good diffractometer (Philips X-
celerator, Cu Kalpha1;-Brag-Brentano geometry) (lowest FWHM~0.06 °) with
a step of 0.016° and a lot of counting time. The sample has a perovskite
like structure and undergoes a number of structural transitions when
heating. Triclinic at room temperature (I have refined this one), unknown
intermediary temperature phase, and a tetragonal high temperature phase
(I have refined this one too). When indexing with the majority of the
available software (Dicvol, Treor, Ito?) alone or in the Crysfire suite,
in the front of the list I get a monoclinic cell a=7.86 b=7.68 c= 7.72
A and Beta~90.3° with rather good agreement factors (M20 about 180).
Refining the cell in FullProf, gives a perfect fit for the low 2Theta
pattern (2Theta<80°), but at high 2Theta values there are some shifts in
the positions or the reflections. I have thought that this could came
from some instrumental error, but the tetragonal high temperature phase
give a perfect fit even in this are, so I have excluded this possibility.
Is someone has any idea or maybe has encountered the same kind of
problem, please help me. Thank you in advance
=====
Cristian PERCA
Laboratoire de Physico-Chimie de l'Etat Solide
Bât. 414 - Université Paris XI
91405 ORSAY Cedex - France
http://www.u-psud.fr/chimsol
Tél : 01 69 15 48 04
cristian.perca...@lpces.u-psud.fr
cristianperca...@yahoo.com
------------------------ Yahoo! Groups Sponsor --------------------~-->
Make a clean sweep of pop-up ads. Yahoo! Companion Toolbar.
Now with Pop-Up Blocker. Get it for free!
http://us.click.yahoo.com/L5YrjA/eSIIAA/yQLSAA/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/