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Re: [sdpd] background soustraction and sample standard
>Does anyone know the best standard sample to make deconvolution,
Visit the Rietveld Mailing List archive :
http://www.mail-archive.com/rietveld_l...@ill.fr/
See this email for instance :
http://www.mail-archive.com/rietveld_l...@ill.fr/msg02635.html
About the "super NAC", some 2003 emails are lacking in the
archive :
==============================================
Subject: LaB6 versus NAC + synchrotron // beam
Date: Fri, 18 Apr 2003 09:30:32 +0200
To: rietveld_l...@ill.fr
From: Armel Le Bail <alb...@cristal.org>
Hi,
If I understand well the J. Appl. Cryst. 36 (2003) 286-294
paper :"Instrument line-profile synthesis in high- resolution
synchrotron powder diffraction,"
The NAC compound (Na2Ca3Al2F14) is better (with less size/
strain broadening) than LaB6 NIST SRM 660 ! The revenge
of the Fluoride Lab !-).
Hum, shame on NIST, that means that if the classical
line broadening approach is used (rather than the fundamental
parameters -FP- approach), then LaB6 NIST SRM 660 should be
studied by using NAC as a standard (in fact, a size effect
as large as 3µm is detected for the NAC ;-).
Adding more confusion now (sorry ;-), this is not verified by using
conventional X-ray and a Bragg Brentano reflexion geometry -
and the conclusion is (again) that the universal reference compound
does not exist - but this is only my own conclusion, there is not a word
in the JAC paper on that point. Then in your lab equipped with a Bragg-
Brentano system, without the splendid synchrotron parallel
beam, do not use NAC as a reference for LaB6, use NAC for
NAC and similar absorbing compounds, or use the FP
approach if you trust it in all circumstances.
Armel
=================================================
Subject: Re: LaB6 versus NAC + synchrotron // beam
From: "Joerg Bergmann" <bergmann...@rcs1.urz.tu-dresden.de>
To: "rietveld_l...@ill.fr" <rietveld_l...@ill.fr>
Date: Fri, 18 Apr 2003 09:52:04 +0200 (CEST)
On Fri, 18 Apr 2003 09:30:32 +0200, Armel Le Bail wrote:
>Hi,
>
>If I understand well the J. Appl. Cryst. 36 (2003) 286-294
>paper :"Instrument line-profile synthesis in high- resolution
>synchrotron powder diffraction,"
>The NAC compound (Na2Ca3Al2F14) is better (with less size/
>strain broadening) than LaB6 NIST SRM 660 ! The revenge
>of the Fluoride Lab !-).
>
Assuming both SRM660a and NAC having the same size value
(above 2...5 micron), the SRM660a will have broader lines
due to the higher dynamic scattering broadening, see
http://www.bgmn.de/srm660a.pdf
SRM660a contains strong scattering La elements, this will give
large F and therefore broader lines. NAC consists of
elements from the uppper (light weight) part of the periodic
table. In my opinion, this will hold for Si, too.
But SRM660a was designed for conventional diffractiometers
in reflection geometry. Therefore, it _must_ contain
some heavy elements for small penetration depth. NAC
as well as Si will give broader lines due to penetration
depth.
Greetings
J. Bergmann, Dresden, Germany
email...@jbergmann.de
================================================
Subject: Re: LaB6 versus NAC + synchrotron // beam
Date: Fri, 18 Apr 2003 10:32:53 +0200
To: rietveld_l...@ill.fr
From: Andy Fitch <fitch...@esrf.fr>
At 09:30 18 04 03 +0200, Armel wrote:
>Hi,
>
>If I understand well the J. Appl. Cryst. 36 (2003) 286-294
>paper :"Instrument line-profile synthesis in high- resolution
>synchrotron powder diffraction,"
>The NAC compound (Na2Ca3Al2F14) is better (with less size/
>strain broadening) than LaB6 NIST SRM 660 ! The revenge
>of the Fluoride Lab !-)
>................etc
Before anyone gets the wrong end of the stick, the paper is about
calculating the instrumental contribution to the peak shape of a very high
resolution synchrotron-based powder diffractometer. It's not about standard
samples for peak shapes. With a very high resolution instrument you need a
very well crystalline sample, and NAC is the one we find most useful for
our purposes. We measure in capillary geometry so, as well as excellent
crystallinity, low X-ray absorption is required. Once you have accurately
modelled what the instrument is giving, it's possible to see the
broadening coming from the sample, and deduce that it has a particle size
of around 3 um, which is pretty big from the point of view of line-profile
analysis. But then, this is a high resolution setup.
The abstract of the paper is as follows
c 2003 International Union of Crystallography
#Printed in Great Britain ± all rights reserved
An accurate method for synthesizing the instrumental line profile of high
resolution
synchrotron powder diffraction instruments is presented. It is shown
that the instrumental profile can be modelled by the convolution of four
physical
aberration functions: the equatorial intensity distribution, the monochromator
and analyser transfer functions, and the axial divergence aberration function.
Moreover, each equatorial aberration is related to an angle-independent
function by a scale transform factor. The principles of the instrument
line-profile
calculation are general. They are applied in the case of the angle-dispersive
powder X-ray diffraction beamline BM16 at the ESRF. The effects of each
optical element on the overall instrument profile are discussed and the
importance of the quality of the different optical elements of the
instrument is
emphasized. Finally, it is shown that the high resolution combined with the
precise modelling of the instrument profile shape give access to a particle
size as
large as 3 um.
Best regards
Andy
==============================================
Subject: Re: LaB6 versus NAC + synchrotron // beam
Date: Fri, 18 Apr 2003 11:00:13 +0200
To: rietveld_l...@ill.fr
From: Armel Le Bail <alb...@cristal.org>
>Before anyone gets the wrong end of the stick,
What wrong end ? Champagne is not boycotted here.
You cannot beat NIST SRMs every day ;-). the
excellent crystallization state of NAC is obtained
in a simple way : platinum tube sealed under argon,
~650°C a few days.
Tchin-tchin,
Armel
Fluorides Lab
==================================================
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