Hello Maria,
the main reason for using ADS is avoiding the beam-overflow at low
angles by FDS and limited sample dimension, together with the increasing
of the intensity at higher angles compared to FDS data. So we get more
information at higher angles and less biased intensity at very low
angles, provided that the ADS system is working correctly. This is
advantageous for example in routine phase analysis, and practically
working well if the maximum diffraction angle is limited to about 70-80
deg 2theta, if a certain ADS divergence angle is not exceeded. In our
experience, ADS systems with diffracted beam monochromators can be also
used for structure refinement, see
http://www.bgmn.de/vardiv.html
but again only up to a certain divergence angle. As Alan stated,
deviations from the correct sin(Th) intensity relation can arise at high
angles when the high divergence of the ADS beam can not completely reach
the detector, mainly by cutting the diffracted beam by a narrow fixed
antiscatter slit or by the limited dimension of the monochromator,
starting from a certain diffraction angle.
In general, it is recommended to use untreated data in Rietveld
refinement. Thus, the recalculation from ADS to FDS data prior to a
Rietveld refinement is only the "second choice" method, necessary for
Rietveld programs what are unable to treat the ADS data directly
(inclusive the intensity function and profile shape dependency over
2theta) inside of their models. That's why (i) such a recalculation
changes the counting statistics, (ii) the intensity scaling may fail if
any non-sample dependent background contribution like air-scatter is
present, and (iii) the angle-dependent changes in peak shape can hardly
be recalculated from ADS to FDS. The better solution is surely to model
the instrumental profile shape and intensity versus 2theta by a more
fundamental approach, for example by MonteCarlo simulation. Doing so,
most of these biasing things like the cutting antiscatter slits can be
treated satisfyingly, in our experience.
Reinhard Kleeberg
AlanCoelho schrieb:
Hi Maria
Automatic Divergence Slits (ADS) illuminate different parts of the post
monochromator as a function of 2Th. I am guessing but I think that the
crystals are good enough not to change the intensity too much; this is my
experience with Y2O23 in any case.
At around 70 to 80 degrees 2Th however the beam (typically around 4 degrees
in the equatorial plane) spills out of the post monochromator. This
situation should be avoided; instead above that angle the slits should be
fixed. Thus you have the situation where part of your pattern is analysed
using ADS corrections and part using FDS corrections.
ADS corrections comprise:
- A Sin(Th) scaling of intensities
- A change in peak shape
Note, that anti scatter slits could also influence intensities at large
divergences.
Cheers
Alan
-----Original Message-----
From: Fabra-Puchol, Maria [mailto:[EMAIL PROTECTED]
Sent: Tuesday, 27 March 2007 12:03 AM
To: rietveld_l@ill.fr
Subject: Use fix or programmable slits for Rietveld analysis
Hi all,
I have a question using fix (FDS) and programmable slits (ADS) for
Rietveld analysis.
Actually, I work with a X'PERT equipment with fast detection and with
programmable divergence slits in the incident beam and a programmable
antiscattering slits in the diffracted beam.
I have also the possibility to use them in a fix mode.
I would like to know the opinion of the community about the best
configuration of this slits (fix or programmable) if a Rietveld analysis
is required.
In fact, using programmable slits, the software corrects data and
changes from ADS to FDS. What is the interest to use ADS in this case?
Thank you all
Maria Fabra Puchol
Microanalysis Engineer
Saint-Gobain CREE
---------------------------------------
550, Avenue Alphonse Jauffret
84306 Cavaillon Cedex-France
[EMAIL PROTECTED]
telf: +33 (0)4 32 50 09 36
fax: +33 (0)4 32 50 08 51
begin:vcard
fn:Reinhard Kleeberg
n:Kleeberg;Reinhard
org;quoted-printable:TU Bergakademie Freiberg;Institut f=C3=BCr Mineralogie
adr:;;Brennhausgasse 14;Freiberg;Sachsen;D-09596;Germany
email;internet:[EMAIL PROTECTED]
title:Dr.
tel;work:(+49) (0)3731 393244
tel;fax:(+49)(0)3731 393129
url:http://www.mineral.tu-freiberg.de/mineralogie/roelabor/
version:2.1
end:vcard
