Hi All,
Data collected using conventional, fixed divergence slits (FDS) result
in diffraction occurring from a fixed volume of sample (albeit varying
in shape). The use of a variable divergence slit (VDS) results in
diffraction from a fixed sample area but varying volume and hence a
variation in relative peak intensities as a function of 2theta. There
are valid arguments for use of both FDS and VDS but, in the 'standard'
mode most Rietveld analysis programs calculate the peak intensities
based on fixed sample volume (=FDS). For data analysis where intensities
matter (i.e. refinement of thermal parameters) an intensity correction
must be applied during analysis if a VDS has been used to collect the
data.
The relationship between FDS and VDS intensities should, in principle,
be well established but there are a number of traps which can affect
this. Bowen and Ryan (Powder Diffraction, 1991, V6, No2, 78-81) and
another paper I think by Toraya (I don't have the reference to hand)
discuss the intensity correction function needed. One of the traps is
the setting of anti-scatter slits (ASS) when a VDS is used. The
acceptance angle of the ASS must greater than the widest opening of the
VDS at all angles (this becomes most important at higher 2theta). If the
ASS is not wide enough, then intensity will be cut off by the ASS
resulting in a deviation from the FDS/VDS ratio - hence an incorrect
intensity distribution vs 2theta - hence thermal parameters which do not
make sense. Other traps include misalignment of the ASS.
How to make sure that you are applying the correct FDS/VDS function
during your analysis??
Collect some data from a 'trusted' standard material (under exactly the
same instrumental conditions as the real samples - collect data to the
upper limit of the diffractometer say 140deg 2theta) - the material I
use is cubic Y2O3 'a'=10.6040 A. This has two Y sites and one O site.
During analysis of the data, conduct full structure refinement including
isotropic thermal parameters (Beq). If the FDS/VDS function is working
correctly, then you should be able to obtain a reasonable value for the
Oxygen Beq (not a bad trick for X-ray data for a light atom in the
presence of two moderately heavy atom sites) - i.e. you should be able
to reproduce the published (neutron diffraction) values for Y2O3 (see
e.g. O'Connor & Valentine, ~1969).
Values of Beq which deviate significantly from the published values are
an indicator that the FDS/VDS function is not correct for your
instrumental configuration - this needs to be resolved before you
proceed! If you are using a Rietveld program that lets you refine
parameters in the FDS/VDS function (such as e.g. Topas), refine them
using the 'standard' data, then fix them at the refined values for the
analysis of your real sample data.
Personally - I try to avoid most of these problems by using an FDS
configuration. Also, there are some considerable benefits that accrue
from the use of a variable counting time (VCT) regime especially in the
refinement of thermal parameters. The VCT regime can be made to
compensate for the real changes in observed intensity resulting in
effectively constant peak counting statistics across the entire pattern
- the VDS only partially achieves this.
Cheers
o----------------------oo0oo---------------------------o
Ian Madsen
CSIRO Minerals
Box 312
Clayton South 3169
Victoria
AUSTRALIA
Phone +61 3 9545 8785 direct
+61 3 9545 8500 switch
+61 (0) 417 554 935 mobile
FAX +61 3 9562 8919
Email [EMAIL PROTECTED]
o----------------------oo0oo---------------------------o
-----Original Message-----
From: Reinhard Kleeberg [mailto:[EMAIL PROTECTED]
Sent: Wednesday, 8 August 2007 2:01 AM
To: [email protected]
Subject: Re: fixed slits or fixed radiated length Bragg-Brentano XRPD
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.
>>
>
>
