Hi,
as William Bisson stated, the main purpose of automatic divergence slits
is to enhance the intensities at higher angles. But, the changes in
intensity and profile shape compared to fixed slit systems are no
mysterium. They can calulated and introduced into the refinement.
However, some (?) Rietveld programs can not deal with this problem,
consequently artifacts like negative temperature factors can occur
during refinement of data measured with automatic slits. If you are
using such a program, recalculation of the intensities to fixed slit
data is necessary, but this procedure may cause other problems.
I disagree that TOPAS is the only program that is able to calculate peak
profiles and intensities of variable slits, see:
http://www.bgmn.de/vardiv.html
BGMN simulates the instrumental profile shapes by a MonteCarlo
technique, at certain steps over 2theta. So, the profile function can be
very well modeled.
In my personal experience, the automatic slit data can also be used for
structure refinement, provided that they are programmed correctly. For
phase analysis work, we always prefer ADS data.
Regards
Reinhard Kleeberg
[EMAIL PROTECTED] schrieb:
Dear Silvina,
The purpose of divergence slits is to increase the intensities of
reflections at
high angle, the flip side is an increase in background and peak
broadening (that
will extenuate peak overlap) at high angle.
For structural work it is best to stay with fixed slits, no
modification needed
for the profile function - though TOPAS (academic) is the only program
to date (I know of) that can model divergence slits.
Old school techniques of counting for longer at high angle is the best
way to
improve stats and will not unduly effect your profile function.
Divergence slits are useful for phase identification especially if you
are
processing a bulk number of samples. Stick with fixed slits for Rietveld
analysis.
Regards
William Bisson
CCP14 administrator
http://www.ccp14.ac.uk
Quoting Silvina Pagola <[EMAIL PROTECTED]>:
Hi,
I have a question regarding the use of fixed radiated length data in
a laboratory powder diffractometer, with Bragg-Brentano geometry and
programmable divergence and antiscatter/receiving slits, which can
also be used in the "fixed slit" mode (in which the irradiated length
on the sample varies with the theta angle).
This is, for what cases the fixed radiated lenght is recommended
instead of fixed slits data?.
I have only these two choices for structure solution. For one
compound I solved, it seems to work better the fixed radiated length
data, although the background increases at high angles and I have to
refine absorption (surface roughness), to get positive thermal
factors. Could someone explain why to use one dataset or the other
for structure solution and refinement?
Thanks,
Silvina.
