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Discussions on handling hkl dependent peak asymmetry within Rietveld software

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Subject:   hkl-dependent peak asymmetry?
Date: Tue, 17 Dec 2002 10:51:46 -0500
To: rietveld_l@ill.fr
From: "Maxim V. Lobanov" [lobanov@rutchem.rutgers.edu] 

Dear Rietveld list subscribers,
I would be interested if somebody could suggest a software (and, maybe, a
corresponding example) which can handle hkl-dependent peak asymmetry. I
mean, we are now trying to refine the structure from high-resolution
synchrotron data, and clearly see different asymmetry for different types
of reflections (specifically, "Lorentz tails" on either left or right
side). It might indicate (of course) some kind of symmetry lowering, but of
very low magnitude (it is actually high resolution data), and at present I
would prefer to treat this as "hkl-dependent asymmetry". 
Thank you in advance.
Sincerely,					Maxim.
__________________________________
Maxim V. Lobanov
Department of Chemistry
Rutgers University
610 Taylor Rd
Piscataway, NJ 08854
Phone: (732) 445-3811


Subject:   Re: hkl-dependent peak asymmetry?
Date: Tue, 17 Dec 2002 10:12:52 -0600
From: Frank May [frank.l.may@umsl.edu]
To: rietveld_l@ill.fr

Hello Maxim,

Do you also see differences in peak width which you can relate to the
various HKL classes?  I've been looking since 1985 for a way to handle
high resolution data with these effects.  Keep me posted if you find
anything.

Regards from St. Louis,

Frank May
Department of Chemistry
University of Missouri-St. Louis
(314) 516-5098


Subject:   Re: hkl-dependent peak asymmetry?
Date: Tue, 17 Dec 2002 16:39:15 +0000 (GMT)
To: rietveld_l@ill.fr
From: I.Margiolaki@sussex.ac.uk (Irene Margiolaki)

Hello,

The following reference which describes
anisotropic peak broadening in powder diffraction, might be of some
help:

P. Stephens, J. Appl. Cryst. Vol. 32, page: 281-289, 1999

In GSAS, there is a specific profile function (type 4) which involves
this effect and calculates automatically the allowed anisotropic strain
coefficients for different crystal structures.

 Of course, this could be an idea in the case that you observe difference
in the broadening of the individual peaks in the profile.

  Rena.


Subject:   Re: hkl-dependent peak asymmetry?
Date: Tue, 17 Dec 2002 18:03:17 +0000
From: Andreas Leineweber [a.leineweber@mf.mpg.de]
To: rietveld_l@ill.fr

Dear Maxim,

Sample dependent ASYMMETRIES are treated only rarely quantitatively. According
to my knowledge, no free Rietveld program can handle such a thing. There is the
program Topas by Bruker, where you can program nearly anything you want, if you
have a good phenomenological model for that.
The origin of such effects lies probably in anisotropic micro-strain
distributions, which may be, furthermore, anisotropic (size effect should be
impossible). Such effects may, e.g. be caused by composition distributions. An
example how to treat it in a relatively rude way (but it is working) is
described in Ehrenberg, H., Theissmann, R., Gassenbauer, Y., Knapp, M.,
Wltschek, Weitzel, H. Fuess, H. Herrmannsdörfer, T. & Sheptyakov, D. (2002). J.
Phys.: Condens. Matter 14, 8573-8581: You just assume the presence of several
phases of different lattice parameters. For that it is good (but not
neccessary) to know the concentration dependence of the lattice parameters. You
may try to look whether this effect occurs only for certain direction in the
reciprocal space.
Another example, which involves, however, a phenomenological treatment of the
asymmetry, is found in Langford, J. I., Louer, D. (1991), Journal of Applied
Crystallography 24, 149-155.
You can also ask me directly if you have further questions.

Yours sincerely
Andreas Leineweber

--
Dr. Andreas Leineweber
 please note the new address
Max-Planck-Institut fuer Metallforschung
Heisenbergstrasse 3
70569 Stuttgart
Germany
Tel. +49 711 689 3365
Fax. +49 711 689 3312
e-mail: a.leineweber@mf.mpg.de
home page of department:
http://finix.mpi-stuttgart.mpg.de/mittemeijer/english/index_english.htm


Subject:   Re: hkl-dependent peak asymmetry?
Date: Tue, 17 Dec 2002 18:14:24 +0000
From: Andreas Leineweber [a.leineweber@mf.mpg.de]
To: rietveld_l@ill.fr

Sorry, Maxim, once more,

Another important case of anisotropic line broadening occurs for stacking
faults, e.g. in fcc materials. Even for the simplest cases this is relatively
complicated (e.g. L. Velterop, R. Delhez, Th.H. de Keijser, E.J. Mittemeijer,
D. Reefman: X-ray diffraction analysis of stacking and twin faults in f.c.c.
metals: a revision and allowance for texture and non-uniform fault
probabilities, J. Appl. Crystallogr. 33 (2000) 296-306.). In a Rietveld similar
approach this was applied (Scardi, P. & Leoni, M. (2002). Acta Cryst. A58,
190-200.). But the simple, freely available programs (see last mail) cannot do
that according to my knowledge.

Best regards
Andreas Leineweber

"--
Dr. Andreas Leineweber
 please note the new address
Max-Planck-Institut fuer Metallforschung
Heisenbergstrasse 3
70569 Stuttgart
Germany
Tel. +49 711 689 3365
Fax. +49 711 689 3312
e-mail: a.leineweber@mf.mpg.de
home page of department:
http://finix.mpi-stuttgart.mpg.de/mittemeijer/english/index_english.htm


Subject:   Re: hkl-dependent peak asymmetry?
Date: Tue, 17 Dec 2002 09:32:44 -0800
To: rietveld_l@ill.fr
From: Luca Lutterotti [Luca.Lutterotti@ing.unitn.it]

Dear Maxim,

what compound are you analysing and what kind of symmetry it has (space 
group)?
I am asking this because, the more common cause of hkl dependent 
symmetry are faults, either deformation, stacking, growth or twin 
faults. The problem for a phenomenological approach on it is that there 
is no actually a general model or treatment for them.
Only for close packed cubics (like FCC, BCC) and hexagonal there is the 
well known treatment of Warren. Some people have tried to model (not 
refine) such faults in clay materials.

So we have implemented the Warren (+recent modifications) treatment in 
Maud (Lutterotti and Gialanella, Acta Materialia,  46[1], 101-110, 
1998); but as I said before it is only for close packed materials. 
Maud, I recall, is just a free Rietveld program; it's problem is that 
it is in Java, so it is not Microsoft certified ;-))

	Best regards,

			Luca Lutterotti


Subject:   Re: hkl-dependent peak asymmetry?
From: "Joerg Bergmann" [bergmann@rcs1.urz.tu-dresden.de]
To: "rietveld_l@ill.fr" [rietveld_l@ill.fr]
Date: Tue, 17 Dec 2002 21:24:12 +0100 (CET)

On Tue, 17 Dec 2002 10:51:46 -0500, Maxim V. Lobanov wrote:

>Dear Rietveld list subscribers,
>I would be interested if somebody could suggest a software (and, maybe, a
>corresponding example) which can handle hkl-dependent peak asymmetry. I
>mean, we are now trying to refine the structure from high-resolution
>synchrotron data, and clearly see different asymmetry for different types
>of reflections (specifically, "Lorentz tails" on either left or right
>side). It might indicate (of course) some kind of symmetry lowering, but of
>very low magnitude (it is actually high resolution data), and at present I
>would prefer to treat this as "hkl-dependent asymmetry".
 
You may handle this in BGMN by defining "multiplet" lines
(RefMult=2 or 3...). In following, you may define some hkl-dependent
expression for the shift/intensitiy/with parameters of the 2/3...
particular lines. Giving different shifts/intensities/width parameters
for iref=1 resp. iref=2... gives some anisotropy for a hkl line as
representated by a non-resolved multiplet. We have done such for example
for the description of hkl-dependent anisotropies of clays.

Regards

J. Bergmann, Dresden
email@jbergmann.de


Date: Wed, 18 Dec 2002 08:23:21 +0100
From: "Kern, Arnt A." [Arnt.Kern@bruker-axs.de]
To: [rietveld_l@ill.fr]

Dear Maxim,

I already posted this about one month ago. Check it out - it works extremely well! 

Cheers,

Arnt

-----------------------

For the upcoming Topas V2.1 I am working on two tutorial examples (Pawley or
LeBail fitting) for anisotropic line broadening, which are available on
request (data and INP files):

- Norbornane, taken from A. LeBail's PowBase
(http://pcb4122.univ-lemans.fr/)
- Pboxas, taken from the KoalaRiet distribution (http://www.ccp14.ac.uk/)

For Pboxas the use of spherical harmonics is shown for "gauss_fhwhm" and /
or "lor_fwhm" and / or asymmetry using a "circles convolution" (-> axial
divergence). Its easy to use an "exponential" or "1/x convolution" instead
of the circles convolution.

"Just for fun" it is also shown how to accommodate hkl-dependent peak shifts
by linking spherical harmonics to e.g. the zero point error. 

Please keep in mind: With spherical harmonics it is always extremely
important to check parameter correlations and therefore also to avoid to use
higher orders than necessary. When fitting second-rank tensors the situation
is similar.

--

By the way: By convoluting the sum of two exponentials with a Voigt or a
pseudo-Voigt function you not only can get a nice description of e.g. ISIS
TOF neutron data with downto  5 refineable profile parameters only; by using
spherical harmonics to make the exponentials hkl-dependent you also get a
wonderful fit of extreme profile anisotropy e.g. due to disorder as in
clays. 

The physical meaning of the refined (profile) parameters is another story
though..., at least the fit looks good ;-). This is meant to clearly
illustrate my opinion, that in general such refinement models should be only
used with great care! They should never be used to just simply get a better
fit... This is why I like the example of Lachlan very much - he thoroughly
checked the results.

Cheers,

Arnt


Subject:   Re: hkl-dependent peak asymmetry?
Date: Wed, 18 Dec 2002 12:01:04 +0100
To: rietveld_l@ill.fr
From: Armel Le Bail [alb@cristal.org]

Hi,

I remember Davor Balzar giving account of the Size/Strain
Round Robin at IUCr XIX Geneva : the Rietveld approach of
these problems was clearly not recommended...

I disagree when structures are complex enough to give
more than a few Bragg peaks typical for a cubic sample
with small cell parameter (CeO2 for instance). In complex
cases, with huge overlapping, nothing else than a global
pattern modelling is applicable, and this includes the
Rietveld approach.

However, if experts who will be the manuscript reviewers,
think that Rietveld is not recommended in size/strain studies,
then the publication of your results will be difficult, maybe.

Can we expect the next size/strain round robin step with
some samples more complex than a cubic size-only one ?

With all these computer programs claiming to cope with
hkl-dependent peak broadening and asymmetry, there is a
gulf between expert recommendations and software
possibilities (though these programs often produce only
phenomenological fits that do not give any size/strain parameters).

The cases of Norbornane and Pb oxalate are quite nice examples
with data available on the Internet. There are even more bizarre data
in Powbase than PbC2O4 and Norbornane. A query with broadening
as a keyword returns 25 entries :
                   http://sdpd.univ-lemans.fr/powbase/
Enough non-cubic cases to frighten experts...

So, even if we know that no high accuracy in size/strain
parameters can be expected by using the Rietveld approach
in complex cases, should we do nothing instead ?

Best,

Armel


Subject:  RE: hkl-dependent peak asymmetry? 
Date: Fri, 20 Dec 2002 10:20:33 +0100
To: Rietveld_L@ill.fr
From: Alan Hewat [hewat@ill.fr]

Forwarded from Davor Balzar:

>>I remember Davor Balzar giving account of the Size/Strain
>>Round Robin at IUCr XIX Geneva : the Rietveld approach of
>>these problems was clearly not recommended...

>Well, to set the record straight about my Geneva lecture and before the full
>paper with some recommendations and recipes in regard to Rietveld-based
>determination of size/strain, based on the evaluations of ceria round-robin
>sample, is out of review (Armel is one of the paper co-authors and I thought
>we all agreed on this:-):
>
>- Results obtained through Rietveld refinement agreed extremely well with
>other, more traditional techniques. This, of course, for cynics would
>implicate that for simple cases such as this one, any method would do.
>However, the round-robin indicated that the results could very up to 100 or so
>%, if the refinement is not carried out properly. The most important mistake
>is not to refine all profile-related parameters that are pertinent to the
>size/strain effects -- typical example is overlooking the Gaussian size term
>(P).
>
>- Rietveld programs become more sophisticated and some of them can handle a
>variety of line-broadening sources. However, this implies there are more
>parameters to refine and more possibility of correlation with other refined
>parameters especially if a limited theta (Q) space was covered in an
>experiment.
>
>- As a consequence, size/strain related broadening is compensated by another
>parameter with similar effect on line widths -- pattern fit looks excellent,
>Rwp is low, but physical interpretation is wrong (as it was pointed out by
>Armel and Arnt).
>
>- As this is directed toward newcomers in this field, one should always be
>reminded that Rietveld and similar approaches are based on solving a
>linearized system of nonlinear equations -- therefore, unless a guessed
>parameter value is not close to the "true" one, the refinement may never reach
>the "true" value and there is no way of knowing when or whether that value was
>reached.
>
>- Therefore, one should try different methods to check the validity of results
>obtained. However, most of the traditional line-broadening methods cannot be
>applied or also carry large systematic errors if applied to low-symmetry
>materials. Rietveld of similar full-powder pattern approach might be your only
>method available.
>
>Thus, my full summary in Geneva (and our paper) was/is something like:
>- Rietveld is good and getting better for size/strain IF you are aware of
>pitfalls.
>- In case of complicated structures with overlapping lines, multiphase
>mixtures, etc.., the results should be taken cautiously, but it maybe your
>only method available.
>- Always check obtained results for physical soundness and compare to other
>methods if possible.
>
>The bottom line: I think we all pretty much agree that the Rietveld route
>should be used, and will be used more often in future. However, very carefully
>because Rietveld programs are getting more user-friendly but far from being
>laymen user-proof.
>
>Davor Balzar
>
>P.S: In this field, there are no experts, only different opinions.


Subject:   RE: hkl-dependent peak asymmetry?
From: "Joerg Bergmann" 
To: "rietveld_l@ill.fr" 
Date: Fri, 20 Dec 2002 14:03:09 +0100 (CET)

On Fri, 20 Dec 2002 10:20:33 +0100, Alan Hewat wrote:

>Forwarded from Davor Balzar:

>>The bottom line: I think we all pretty much agree that the Rietveld route
>>should be used, and will be used more often in future. However, very carefully
>>because Rietveld programs are getting more user-friendly but far from being
>>laymen user-proof.

This is indeed a very good statement. In future, Rietveld programs
(as well as other crystallographic programs) will need much less
_computer_knowledge_. That ist the way they will become user-friendly.
On the other hand, nobody can solve a crystallographic task (like
Rietveld refinement) without having learned the crystallographic
fundamentals of his task. Using a Rietveld program without
_crystallographic_knowledge_ (and/or related, like mineralogy/materials
science/chemistry/solid state physics) is _nonsense_.

Regards

Joerg Bergmann, Dresden
email@jbergmann.de


Subject:   RE: hkl-dependent peak asymmetry?
Date: Fri, 20 Dec 2002 14:55:40 +0100
To: rietveld_l@ill.fr
From: Armel Le Bail [alb@cristal.org]

Dear Davor,

Thanks for more opening the Rietveld route to (anisotropic)
line broadening studies.

>P.S: In this field, there are no experts, only different opinions.

Anyway, the advice is to not give your manuscript a title like :
"Opinions about size/strain effects in PbC2O4, as revealed by
the Rietveld method", though that title would reflect closely some
reality. Most éditors dislike opinions unless in editorials. They
prefer facts and exact interpretation. However, expert opinions may
be heavier than simple fellow opinions.

Best,

Armel  


Subject:   Re: Other experts opinion
From: "Joerg Bergmann" [bergmann@rcs1.urz.tu-dresden.de]
To: "rietveld_l@ill.fr" [rietveld_l@ill.fr]
Date: Fri, 20 Dec 2002 16:04:59 +0100 (CET)

Hi,

I agree. For some years, Reinhard Kleeberg (and I) know:
Even an exact phase analysis is impossible without bothering
with an as much as possible precise real structure
description. Which, in normal, includes gaussian like 
as well as lorentzian additional broadening parts.
Maybe anisotropic. Maybe some more complicated
real structure models, as for clay minerals.

Of course, BGMN may assume a gaussian like size contribution,
see the k2 variable. I use k2 routinely.

Joerg Bergmann, Dresden
email@jbergmann.de


Subject:   Re: Other experts opinion
From: "Joerg Bergmann" [bergmann@rcs1.urz.tu-dresden.de]
To: "Rietveld mailing list" [rietveld_l@ill.fr]
Date: Fri, 20 Dec 2002 16:10:57 +0100 (CET)

On Fri, 20 Dec 2002 16:04:59 +0100 (CET), Joerg Bergmann wrote:

>Of course, BGMN may assume a gaussian like size contribution,
>see the k2 variable. I use k2 routinely.

Oops, sorry. This means the k1 variable instead. k2 is for
describing the micro strain gaussian like part.

Regards

Joerg Bergmann, Dresden
email@jbergmann.de


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