Hi all,
This may be a non-issue given the high accuracy of diffraction measurements,
and the internal checks on sample position, such as dependent peaks that lead
to the sample shift, but there is something I've always wondered about when an
internal standard is used in combination with refinement.
You can fix the lattice parameter of silicon, as Patrick says below, but the
extent to which this fixed lattice parameter enters the refinement depends on
how much silicon is in the sample. If you keep adding more silicon, in effect
you are weighting the data to have an increasing dependence on the silicon.
So, what is the correct ratio of silicon to unknown, to be sure that you are
making the best measurement?
I suppose this is only an issue if the sample shift or zero error is quite
large, in which case you might want to work on that problem first. But still,
it's something I've wondered about.
- Kurt
*********
Kurt Leinenweber
Dept. of Chemistry and Biochemistry
Arizona State University
Tempe, AZ 85287-1604
phone (480)-965-8853
fax (480)-965-2747
***********
________________________________
From: Patrick Weisbecker [mailto:weis...@yahoo.fr]
Sent: Thursday, May 14, 2009 10:16 AM
To: leopo...@fq.edu.uy
Cc: rietveld_l@ill.fr
Subject: Re : Introduction and Unit cell parameter determination
It is indeed advisable to start with an internal standard to check that there
is no instrumental error or sample position issue (sample displacement is the
more common cause of peak shift).
Silicon powder is often used as a standard, a standard reference material (SRM)
is sold by the NIST (SRM640) but you can also use good quality silicon powder
(99.99%) if you don't need an accuracy better than 0.0001A (a=5.4309A).
Rietveld programs are indeed not necessary if you only the lattice parameters,
standard peak fitting and lattice parameter refinement using several peaks at
high angle should give you good results.
Fullprof can handle internal standard: you have to fix the lattice parameter of
Si to its known value and refine zero error or sample displacement parameter
(use Full profile fit prerentially as indicated by Leopoldo).
Patrick
________________________________
De : Leopoldo Suescun <leopo...@fq.edu.uy>
À : Gumelar Pritosiwi <pritos...@gmail.com>; rietveld_l@ill.fr
Envoyé le : Jeudi, 14 Mai 2009, 15h45mn 16s
Objet : RE: Introduction and Unit cell parameter determination
Dear Gumelar Pritosiwi,
There are two independent topics in your question. If you are using a well
calibrated conventional diffractometer to determine your cell parameters then
you don't need an internal standard. That would be useful if you, additionally,
need to do a quantitative phase analysis.
In a conventional machine with sealed tube source the cell parameters
determination can be done easily in the case of cubic magnetite, because you
already know the incident wavelength and d-spacings for the observed maxima are
linearly related to "a" cell parameter by d(hkl)=a/sqrt(h^2+k^2+l^2).
Programs such as FULLPROF, GSAS, TOPAS, etc. are meant for Rietveld refinement,
that is something a little bit more complex than cell parameters determination,
although most of these programs have additional routines (peak search and
indexing in Winplotr) that can help determine cell parameters of an unknown
phase, or in your case, of the magnetite you are studying. You can also try
full profile fit (LeBail or Pawley methods) to extract best possible cell
parameters provided your data is of enough quality and you are willing to cope
with profile shapes, background modeling, etc. (too complicated to start with
in my opinion).
If you use data with internal standard for lattice parameters determination you
may choose to ignore the internal standard peaks or just collect new data on
the raw material where you just see your magnetite peaks (be careful, if your
sample contains other crystalline materials you will need to identify the extra
peaks and avoid using them for the cell parameters determination).
Free programs like checkcell will allow you to determine your cell parameters
by refinement of previously determined peak positions, other programs like
Winplotr, PowderX, etc will allow you to perform a peak search and extract a
peak list, other programs like CMPR can fit individual peaks and output a list
of fit results. Any of these (and many more) you can then fit to obtain "a"
using the abovementioned equation. There are probably many more free programs
to do either thing more or less automatically so you should check the www or
ccp14 site for information.
If your magnetite show any kind of structural distortion this may be a bit more
complex, but in that case you'll need to check some crystallography book to get
a better idea of what can you do with your powder diffraction data and how can
you do it. A good book for beginners that gets all the way up to complex stuff
is "Fundamentals of Powder Diffraction and Structural Characterization of
Materials" by V.K. Pecharsky and P.Y. Zavalij edited by Springer.
Hope this helps.
Leo
Dr. Leopoldo Suescun
Prof. Agr (Assoc. Prof.) de Física Tel: (+598 2) 9290648/9249859
Cryssmat-Lab./DETEMA Fax: (+598 2) 9241906
Facultad de Quimica, Universidad de la Republica
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\__Montevideo, Uruguay
From: Gumelar Pritosiwi [mailto:pritos...@gmail.com]
Sent: Thursday, May 14, 2009 8:30 AM
To: rietveld_l@ill.fr
Subject: Introduction and Unit cell parameter determination
Dear all,
I am a new member of this mailing list. My background is environmental
engineer. At the moment I am conducting a research related to the crystalline
iron oxide magnetite. One of the analyses that I have to do is the
determination the unit cell dimension (lattice parameter) of the crystal from
X-ray powder diffraction data. So far, for the XRD analysis I have mixed my
sample with an internal standard. Because I am a beginner in the
crystallography, I would like to hear opinions on two things:
- Is using an internal standard a good way of sample preparation for the
determination of unit cell?. Can anybody give me an example of a good internal
standard that can be used for XRD analysis?.
- Can anybody give me a suggestion which free software can be used for the
determination of unit cell parameter from XRD powder diffraction data (with
internal standard)?. I have learned a little about FullProf but I don?t know
whether this program can handle XRD data with internal standard or not. My
knowledge about FullProf is very limited, therefore if some hints about this
program are more than welcome. Is perhaps FullProf too complicated for me as a
beginner?.
?
Thanks a lot and best regards
?
Gumelar Pritosiwi
--
Gumelar Pritosiwi
Institute of Environmental Technology and Energy Economics
Hamburg University of Technology
Eissendorfer Str. 40
21073 Hamburg
Tel ?+49 40 42878 3319
Fax ?+49 40 42878 2315
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