RE: Rietveld website cancelled ?

[May, Frank]

To all:

Initially I sent this to Alan who suggested I open discussion to the entire 
community.  I've posted similar in past years and have been FLAMED.  Please 
keep it civil.  ;-)
=
As a long time practitioner of powder XRD (since 1972) I was there when the 
superconductor craze hit in the mid-1980s.  At that time it was recognized the 
Rietveld Method was useful only for REFINING structures which were generally 
accepted from single crystal x-ray analysis.

The http://www.cristal.org/rietv025/ link says the method "[allows] an accurate 
determination of the structure."  I take that statement to mean no model is 
required; Rietveld Analysis can determine structure without preconceived model.

Thanks for reading.

Blessings from toasty and warm St. Louis, MO.

Frank May
Research Investigator
University of Missouri-St. Louis

formerly at Monsanto Company (which recently became part of BAYER)++
Please do NOT attach files to the whole list 
Send commands to  eg: HELP as the subject with no body text
The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/
++

What Year Is It?

[May, Frank]

Greetings from HOT St. Louis, MO.

I find it silly to discuss inclusion of a 64K image in email.  In the early 
days (1970s) of useful scientific computing, we said, "core [memory/computing 
space] is gold;" we wrote our own programs; large programs were 50K.  We have 
made progress.  Our computers use OS which measure in the gigabits; 
memory/computing space is in the tens of gigabits; storage is measured in 
multiple terabytes.

How is it a server in 2015 cannot handle 64 KILObytes in an email attachment?

Flame away!

Frank May
Research Investigator
University of Missouri - St. Louis
XRD since 1972

PS:  I removed all the preceding emails except the most recent - in the concern 
to save bandwidth !!!


From: rietveld_l-requ...@ill.fr [rietveld_l-requ...@ill.fr] on behalf of Cline, 
James Dr. [james.cl...@nist.gov]
Sent: Tuesday, July 14, 2015 10:08 AM
To: Alan Hewat; David Elbert
Cc: Fabrizio Guzzetta; Young, Lindsay Kay; rietveld_l@ill.fr
Subject: RE: Obtaining a silicon standard wafer

Hi all,

The plot was generated for inclusion in a paper I wrote originally for the 
yet-to-be-published Volume H of the International Tables for Crystallography.  
Due yet another delay in the publication of Vol H, it is due to be published in 
the Journal of Research at NIST very shortly (it may go into Vol H as well 
sometime next year).  Once this is done it will be available online for 
consumption.  It is in the final stages of review; if you would like a 
preprint, send me an email.

++
Please do NOT attach files to the whole list 
Send commands to  eg: HELP as the subject with no body text
The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/
++

Line Calibration

[May, Frank]

To all:

I have electronic grade silicon wafers (6-9s pure) which I'm willing to share.  
They are not certified by NIST.  They are from silicon single crystal 
production and are many years old.  You can grind and mix with your material of 
interest and use the line positions to ascertain peak shifts.

Contact me directly if you want to try.

Frank May
Research Investigator
Department of Chemistry & Biochemistry
University of Missouri-St. Louis

(314)623-4524
frank.l@umsl.edu



From: rietveld_l-requ...@ill.fr [rietveld_l-requ...@ill.fr] on behalf of led15 
[le...@ipn.pt]
Sent: Wednesday, September 04, 2013 8:59 AM
To: Rietveld_L@ill.fr
Subject: Standard

Dear colleagues,
could  anybody  suggest a standard (and where i can buy), not very
expensive, like quartz or zirconia to doping a power sample  to control
the position of xrd lines  and probably later help in quantification.
we use Nist standard LaB6 to instrument performance characterization

Thanks a lot.
Nelson++
Please do NOT attach files to the whole list 
Send commands to  eg: HELP as the subject with no body text
The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/
++

RE: Anisotropic strain

[May, Frank]

To All:
 
RE:  X-RAY DIFFRACTION PROCEDURES ... SECOND EDITION, Klug & Alexander, Table 
9-2, p. 659.
 
According to this reference, the relative breadth of a peak in the powder 
diffraction pattern is related to that of every other peak.  The relationship 
is according to the shape of the crystal.
 
I have used a Scintag/Seifert diffractometer since 1982 and found its 
instrumental broadening, b(i), is of the order of 0.05 degrees 2-theta.  I used 
single crystal silicon for that determination.  It's reasonable that any peak 
breadth greater than 0.05 is due to "other factors" - principally "crystallite 
size."  NOTE:  The term "crystallite size" is really a misnomer except for the 
measurement of the externally observable dimensions of the physical particle.  
What we are really measuring is the "domain size" of the scattering entity.
 
Most of the powder patterns I obtain exhibit characteristic "anisotropic" peak 
broadening which is distinctly not due to spectral broadening caused by 
separation of K(alpha-1) and K(alpha-2) with increased angle.  How might these 
patterns be successfully modelled to yield a physically meaningful result?
 
Frank May
Research Investigator
Department of Chemistry and Biochemistry
University of Missouri - St. Louis
One University Boulevard
St. Louis, Missouri  63121-4499
 
314-516-5098 - office
314-623-4524 - cell
 
 
 
 



From: Andreas Leineweber [mailto:a.leinewe...@mf.mpg.de]
Sent: Mon 5/11/2009 9:03 AM
To: Ross H Colman
Cc: rietveld_l@ill.fr
Subject: Re: Anisotropic strain



Dear Ross,

something like this can definitely be adapted in the launch mode of
TOPAS, it has been done in the past. A few lines for orthorhombic (some
minor changes required for trigonal/hexagonal):
prm s400  8349.85430
prm s040  1967.90075
prm s004  129.62651
prm s220  2451.94087
prm s202  -921.49437
prm s022 -23.92103
prm eta  0.49540 min 0 max 1
prm  mhkl = H^4 s400 + K^4 s040 + L^4 s004 +
  H^2 K^2 s220 + H^2 L^2 s202 + K^2 L^2 s022;
  prm pp = D_spacing^2 * Sqrt(Max(mhkl,0)) / 1000;
  gauss_fwhm = 1.8/3.1415927 pp (1-eta) Tan(Th) + 0.0001;
  lor_fwhm   = 1.8/3.1415927 pp eta Tan(Th) + 0.0001;

(this way to set up the "Stephens model" in Topas originates from R.
Dinnebier or P.W. Stephens; this is the simplest way to do it if you are
not really interested in the meaning of the SHKL parameters)
 I may give detailed hints in direct exchange.

  Additionally I would like to indicate that something which looks on
the first view like anisotropic strain, may instead be anisotropic
crystallite size (is S112 positive or negative?) or stacking faults.
In particular the latter is very common for "laminar" structures and may
give to quite complex diffraction phenomena.

Best regards
Andreas Leineweber


Ross H Colman wrote:
> Dear Rietvelders,
>
> I am a PhD student working at UCL (UK) and was wondering if anyone out
> there could help me with a diffraction related problem:
>
> I am attempting to refine some neutron diffraction data on a powder
> sample that has a very laminar structure. The refinement is acceptable
> but close inspection shows that some peaks are modelled poorly compared
> to others. The relative intensity of each peak appears to be a good fit
> but some peaks are sharper than others whilst some are noticably broadened.
>
> The instrument responsible suggested using anisotropic strain within
> Fullprof to attempt to model the peak shape anisotropies (as an ILL
> instrument was used to collect the data). This worked quite well and
> when considering the crystallite morphologies it seems physically
> reasonable.
>
> I have also been using TOPAS to refine the some of the data for
> comparison and as a new user have not found a way of including this kind
> of anisotropic strain into the refinement.
>
> Is it possible? If so does anybody have an example?
>
> Many thanks for your help
>
> Ross Colman
>
> p.s
>
> If it helps in the discussion, I am analysing a crystal structure with P
> -3 m 1 symmetry and so the refinable parameters within Fullprof are
> s_400, s_004 and s_112. From a chemical point of view the c direction is
> only weekly hydrogen bonded and so the laminar structure seen in SEM
> should be within the a-b plane.
>
> 
>
> Ross Colman
>
> G19 Christopher Ingold Laboratories
>
> University College London
>
> Department of Chemistry
>
> 20 Gordon Street
>
> London
>
> WC1H 0AJ
>
> Phone: +44 (0)20 7679 4636
>
> Internal: 24636
>
> Email:  ross.col...@ucl.ac.uk 
>
>
>  


--
Dr. Andreas Leineweber
Max-Planck-Institut fuer Metallforschung
Heisenbergstrasse 3
70569 Stuttgart
Germany
Tel. +49 711 689 3365
Fax. +49 711 689 3312
e-mail: a.leinewe...@mf.mpg.de
home page of department: 
http://www.mf.mpg.de/de/abteilungen/mittemeijer/english/index_english.htm

Cagliotti and Other Issues

[May, Frank]

Back to basics and First Principles
 
As Alan says, the [use of the Cagliotti function is appropriate for the neutron 
case], "but not really for X-ray and other geometries."
 
My recollection is the Cagliotti function was adapted to the x-ray case when we 
had low resolution x-ray instruments and slow (or no) computers.  Now that we 
have high resolution instruments and fast computers, why does this 
inappropriate function continue to be used?
 
On another note, the world is venturing into the infinitely small realm of 
"nano-particles."  The classical rules for crystallography work very well for 
ordered structures in the macro-world (particles of the order of micron-sizes). 
 However, as the particles become smaller, does one not need to address the 
contribution of the "surface" of the particles?  The volume of the "surface" 
becomes much greater relative to the volume of the "bulk" of the crystal.  
Models today account for "stress" and "strain" in the macro-world.  As the 
relative fraction of the "bulk" becomes smaller, both the physical structure as 
well as the mathematics used to describe the bulk suffer from 
termination-of-series effect, do they not?  Does any of this make sense?  Any 
thoughts?
 
Frank May
St. Louis, Missouri  U.S.A.



From: Alan Hewat [mailto:he...@ill.fr]
Sent: Fri 3/20/2009 2:13 AM
To: rietveld_l@ill.fr
Subject: RE: UVW - how to avoid negative widths?



matthew.row...@csiro.au said:
> From what I've read of Cagliotti's paper, the V term should always be
> negative; or am I reading it wrong?

That's right. If
FWHM^2 = U.tan^2(T) + V.tan(T) + W
then the W term is just the Full Width at Half-Maximum (FWHM) squared at
zero scattering angle (2T). FWHM^2 is then assumed to decrease linearly
with tan(T) so V is necessarily negative, but at higher angles a quadratic
term (+ve W) produces a rapid increase with tan^2(T).

Cagliotti's formula assumes a minimum in FWHM^2, but if that minimum is
not well defined, U,V,W will be highly correlated and refinement may even
give negative FWHM. In that case you can reasonably constrain V by
assuming the minimum is at a certain angle 2Tm, which may be close to the
monochromator angle for some geometries. So setting the differential of
Cagliotti's equation with respect to tan(T) to zero at that minimum gives:
2U.tan(T) + V =0   at T=Tm   or   V = -2U.tan(Tm)
this approximate constraint removes the correlation and allows refinement.

Cagliotti's formula simply describes the purely geometrical divergence of
a collimated white neutron beam hitting a monochromator, passing through a
second collimator, then scattered by a powder sample into a collimated
detector. It takes no account of other geometrical effects (eg vertical
divergence) or sample line broadening etc. This geometry is appropriate
for classical neutron powder diffractometers, but not really for X-ray and
other geometries. Still, such a quadratic expression with a well defined
minimum in FWHM, may be a good first approximation in many other cases,
requiring only a few parameters, hence its success. There are many more
ambitious descriptions of FWHM for various scattering geometries and
sample line broadening, usually allowing more parameters to be refined to
produce lower R-factors :-)

Alan
__
Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE
 +33.476.98.41.68
  http://www.NeutronOptics.com/hewat
__

RE: UVW - how to avoid negative widths?

[May, Frank]

Back to basics.  Caglioti Function is instrument function and is often times 
used inappropriately.
 
Just my two cents worth (and these days, 2-cents ain't worth much...)
 
Frank May
St. Louis, Missouri  U.S.A.



From: Jon Wright [mailto:wri...@esrf.fr]
Sent: Thu 3/19/2009 8:58 AM
Cc: Rietveld Method
Subject: Re: UVW - how to avoid negative widths?



>  According to Caglioti relation, the dimensions of U,V,W are as (angle)^2.

Quick question - does anyone have a trick to stop the Cagliotti formula
going negative? Prodd currently has a habit of bugging out on a
sqrt(negative) and I'm wondering how other folks worked around that, or
if I've got something completely wrong...?

Thanks,

Jon

Rietveld: U,V,W

[May, Frank]

To all:
 
What is the correct procedure for refining U,V,W?  It is my understanding that 
those parameters are a function of instrument geometry.  Does one use a 
standard material to determine U,V,W and then fix their values for the 
instrument you're using?or do the values of U,V,W change depending on the 
sample being examined?  If so, why do the values change?
 
Thanks in advance.
 
Frank May



From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
Sent: Sun 11/30/2008 1:32 PM
To: rietveld_l@ill.fr
Subject: Re: I am a newcome, how can I begin my rietveld refinement analysis



Hi, Li:

To me, the most wonderful tool to determine initial peakshape parameters is
CMPR.
CMPR is especially oriented to GSAS and gives you GU, GV, GW etc. And when
you use EXPGUI for GSAS, you can also try Graphs->widplt to see how FWHM
develops when parameters are tuned.
When you prefer Fullprof, you should take a factor to get U, V, W etc. i am
not sure about the facor exactly, maybe GX~100X(X=U, V, W), er..? Anyway,
just go ahead and make a try.

Faithfully
Jun Lu
--
Lst. Prof. Lijie Qiao
Department of Materials Physics and Chemistry
University of Science and Technology Beijing
100083 Beijing
P.R. China
http://www.instrument.com.cn/ilog/handsomeland/

RE: Preferred orientation?

[May, Frank]

You can check for texture effects (preferred orientation) by obtaining multiple 
patterns of the material.  It's realistic to expect some differences, but 
preferred orientation is manifest by not being able to replicate the pattern.
 
That's the simple test.  Let us know what you find.
 
Another issue for "improper intensities" is when the specimen is not 
sufficiently wide enough at low angles (typically below 20-degrees 2-Theta with 
copper radiation) and the x-ray beam does not fully impinge on the specimen.  
The observed reflections in the low angle region will be less than calculated 
by a modelling program.
 
Frank May
Research Investigator
Department of Chemistry and Biochemistry
University of Missouri - St. Louis
One University Boulevard
St. Louis, Missouri  63121-4499
 
314-516-5098



From: Gerard, Garcia S [mailto:[EMAIL PROTECTED]
Sent: Wed 5/7/2008 8:57 AM
To: rietveld_l@ill.fr
Subject: Preferred orientation?



Dear all,

I have a laboratory Bragg-Brentano X-ray (Cu) pattern that shows intensity 
mismatches only at low angles, ie 20-50 2theta or 1.8 to 4 Angstroms.
There are overestimated peaks and also underestimated peaks.I have tried to 
discard factors that might cause this problem:

The thermal parameters look sensible. Moreover, the data at high angle looks 
ok, so intensity transfer from low angle to high angle or vice versa does not 
seem to be the cause.

Atomic positions also look sensible. And again, data at high angle looks ok. Is 
the scattering angle dependence of the atomic positions the same as for the 
thermal parameters? (I cannot remember that, but i am pretty sure it is not).

Following the advice published in J. Appl. Cryst. 32, 36 (1999), the other 
factor that might cause this problem is preferred orientation:
I have tried to find a hkl dependence in the overestimated and underestimated 
peaks but i could not find any. If i try to model preferred orientation with 
spherical harmonics the problems disappears nicely. The problem is how to 
justify the existence of preferred orientation. The crystal system is 
orthorhombic. But i have no other information that supports the existence of 
preferred orientation.

Is there any other problem that I cannot think of?Is the preferred orientation 
correction masking any of these other problems I cannot think of?

Regards

Gerard





Heriot-Watt University is a Scottish charity registered under charity number 
SC000278. 

RE: Question on Rietveld with x-ray

[May, Frank]

To all:
 
The mathematical description of "crystals" is valid for the bulk of the volume. 
 However, the description suffers from a physical "termination of series" 
effect at the surface of the crystal.  For very large crystals, the amount of 
surface is much greater relative to the total volume of the crystal.  However, 
when the crystalline material approaches what we now refer to as the "nano" 
state (perhaps less than 40 Angstroms - 8 to 10 unit cells), the amount of 
surface becomes significant.  The chemical composition - oxidation state of 
surface atoms - is intuitively different at the surface of a material than 
within the bulk.  How does one deal with that situation?  The effects are 
present in the powder pattern; how does one model them?
 
Frank May
Research Investigator
Department of Chemistry & Biochemistry
University of Missouri - St. Louis
One University Boulevard
St. Louis, Missouri  63121-4499
U.S.A
 
314-516-5098 - office
314-623-4524 - cell



From: Larry Finger [mailto:[EMAIL PROTECTED]
Sent: Wed 4/2/2008 9:47 AM
To: Jae-Ho Chung
Cc: Rietveld_L@ill.fr
Subject: Re: Question on Rietveld with x-ray



Jae-Ho Chung wrote:
> Dear experts on Rietveld with x-ray,
>
> 
>
> When it comes to Rietveld, I have exclusively been working with
> neutrons. Now I am just trying Rietveld with x-rays, and immediately
> came up with a question. Since x-ray scatters from electrons, it seems
> to me that we always have to use, for instance, O-2 instead of O, and so
> on, for ionic compounds. But I have an impression that O is still used
> in many examples, including certain example files given in GSAS, such as
> Y2O3.EXP. Why is it?

I suggest that you plot the scattering curves for O and O-2, and then
decide how much difference it makes. In addition, you should also
postulate a theory concerning the differences. Hint: The electron
cloud is not a "point object".

> As a naturally following question, since no material is perfectly ionic,
> the oxidation numbers may become another "refinable" parameters. But I
> suppose they are never refinable in GSAS or in FullProf. Therefore, I
> suppose the fractional occupation may be compromised instead? Is this in
> any way discussed among x-ray experts?

See above. What experiment would you propose to make the oxidation
number be refinable?

Larry

RE: % Crystallinity

[May, Frank]

As Pamela Whitfield indicates below, this is not a straightforward issue.  
Reading the reference in Dr. DM's posting, one see that it is necessary to have 
"100% crystalline" or "100% amorphous" or "50% crystalline/50% amorphous" 
standards.  The rub is that it is necessary to have a means to determine if 
something is precisely "100" or "50" or another amount.  In order to do that, 
one must have a standardwhich is circular logicand which illustrates at 
least part of the problem.
 
Further, the concept of "para-crystallinity" also includes materials which are 
not well-behaved and 3-dimensionally ordered substances.  This is 
well-recognized in "high polymer" systems, but it also occurs in organic and 
inorganic materials.  How does one assess the "crystallinity" of a material 
which is not 3-dimensionally ordered when a "reference" for that material has 
3-dimensional ordering?
 
Frank May
Research Investigator
Department of Chemistry & Biochemistry
University of Missouri - St. Louis
One University Boulevard
St. Louis, Missouri  63121-4499
 
314-516-5098
[EMAIL PROTECTED]
 



From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
Sent: Wed 2/27/2008 9:58 PM
To: rietveld_l@ill.fr
Subject: Fw: % Crystallinity




(See attached file: XRD Percent Crystallinity Determination in
Polymers.pdf)
PERCENT CRYSTALLINITY DETERMINATION BY XRD THE MOST ACCURATE ONE AS IT
CONSIDERS AMORPHOUS PART DURING CALCULATION , IN OTHER METHODS IT IS
IGNORED , HAPPY TO GET VALUE ADDED COMMENT FROM ALL RESERACHER WHO ARE
EXPERT ON THIS FIELD.
Regards,
Dr. DM




- Forwarded by Debabrata Mukhopadhyay/hpl/IN on 02/28/2008 09:08 AM
-
 
  




This is a can of worms that keeps cropping up now and again.  Rather than
go into masses of detail I'd suggest looking at the Rietveld mailing list
archive for some fairly detailed discussions on the subject.  I'd go with
Bob's route as it's applicable in a laboratory environment but foolproof it
isn't and good sample preparation is vital.

Pam



From: Von Dreele, Robert B. [mailto:[EMAIL PROTECTED]
Sent: Wed 27/02/2008 11:52 AM
To: rietveld_l@ill.fr
Subject: RE: % Crystallinity



John (& others),
The best way to determine %crystallinity (or %amorphous) is to do "spiking"
experiments with material with known (preferably ~100%) crystallinity. The
change in the apparent phase fractions by Bragg intensities as compared to
that expected from the masses can be used to estimate the fraction that
doesn't Bragg scatter (i.e. the amorphous content). The spiking material
should also be chosen to have nearly matching absorption to avoid
systematic effects (e.g. Brindley corrections). It does not have to be the
same phase as the crystalline phase in your sample. There are literature
references to this but I can't recall the exact ones to look at. Perhaps
others might know them better. (BTW - this is not a PDF problem).
Bob Von Dreele



From: john o callaghan [mailto:[EMAIL PROTECTED]
Sent: Wed 2/27/2008 5:58 AM
To: rietveld_l@ill.fr
Subject: % Crystallinity


Dear Users,
I'm trying to determine the percentage crystallinity in a
crystalline/amorphous mixture, could someone point me in a foolproof
direction.

Thanks,
John




Free upgrade for your Windows Live Messenger! Click here! <
http://get.live.com/>



(See attached file: winmail.dat) 

<>

RE: % Crystallinity

[May, Frank]

Regarding Crystallinity -
 
As a technicality, the concept of "% crystallinity" is not only an x-ray 
analysis issue.  One can measure "% crystallinity" by other methods - and the 
findings (numbers) from the different techniques are not necessarily the same.  
I'm not sure if there is a technique which claims to give an absolutely correct 
number for "% crystallinity."
 
For an "absolute" figure, one needs a reference which is either "100 percent" 
crystalline (or 100% non-crystalline).  How does one prove that is the case?  
One must have an analytical standard..
 
When I have a researcher who wants an assessment of crystallinity by XRD, I 
provide a "crystallinity index" instead of a "percent".  It provides a relative 
estimate of crystallinity.
 
If one assumes a 100% crystalline reference, one might find the situation 
within a system of samples (in an environment where numerous samples are 
generated) that a sample calculates to be 110% crystalline.  Such a number is 
physically impossible.  However, using a reference which is assigned a 
crystallinity index equal to 1.00, it is perfectly reasonable to find a 
crystallinity index of 1.10 for a sample and report that figure relative to the 
chosen standard.  The higher figure merely illustrates the fact the original 
standard wasn't an end member along the amorphous-to-crystalline line.
 
There are many reasons for subsequent encounters of a crystallinity index 
greater than 1.00.  That might be the subject of another posting if you want it.
 
Obviously, there is no "foolproof" way to determine "percent crystallinity."
 
Frank May
Research Investigator
Department of Chemistry & Biochemistry
University of Missouri - St. Louis
One University Boulevard
St. Louis, Missouri  63121-4499
 
314-516-5098
[EMAIL PROTECTED]
 



From: Von Dreele, Robert B. [mailto:[EMAIL PROTECTED]
Sent: Wed 2/27/2008 10:52 AM
To: rietveld_l@ill.fr
Subject: RE: % Crystallinity



John (& others),
The best way to determine %crystallinity (or %amorphous) is to do "spiking" 
experiments with material with known (preferably ~100%) crystallinity. The 
change in the apparent phase fractions by Bragg intensities as compared to that 
expected from the masses can be used to estimate the fraction that doesn't 
Bragg scatter (i.e. the amorphous content). The spiking material should also be 
chosen to have nearly matching absorption to avoid systematic effects (e.g. 
Brindley corrections). It does not have to be the same phase as the crystalline 
phase in your sample. There are literature references to this but I can't 
recall the exact ones to look at. Perhaps others might know them better. (BTW - 
this is not a PDF problem).
Bob Von Dreele



From: john o callaghan [mailto:[EMAIL PROTECTED]
Sent: Wed 2/27/2008 5:58 AM
To: rietveld_l@ill.fr
Subject: % Crystallinity


Dear Users,
I'm trying to determine the percentage crystallinity in a crystalline/amorphous 
mixture, could someone point me in a foolproof direction.

Thanks,
John







<>

RE: advice on new powder diffractometer

[May, Frank]

I've watched this thread develop over the past few days.  Apparently there is 
little advice forthcoming about the original inquiry.

Just one person's observation.

Frank May - University of Missouri-St. Louis

<>

RE: Stephens/More Caglioti U V W parameters

[May, Frank]

Peter et al.,
 
If the reason for their use is "historic and somewhat convenient, but their 
usual application is based on no theory whatsoever," why does their use 
continue?
 
It is generally recognized that adding parameters to a least squares fitting 
process causes refinement to proceed to a lower residual (R-factor), even if 
the results are not physically meaningful.  Would this not be the case when one 
adds U,V,W in Rietveld structure refinement?
 
Frank May
University of Missouri - St. Louis



From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
Sent: Mon 6/25/2007 3:56 PM
To: rietveld_l@ill.fr
Subject: Re: More Caglioti U V W parameters



In my opinion, the short answer (regarding use of Caglioti parameters) is that 
their use is historic and somewhat convenient, but their usual application is 
based on no theory whatsoever, and they can be quite troublesome to apply. 

They came from a paper (Nuc. Instrum. & Methods, 1958) on the resolution of a 
neutron powder diffractometer using mosaic crystals and S\"{o}ller (that's an 
umlaut over the o; please, not solar) collimators, which gives precise 
expressions for U, V, and W in terms the various geometric parameters of the 
diffractometer.  If (as was true of most samples on neutron powder 
diffractometers at the time) the instrument dominated the peak shape, they give 
a good representation of the observed linewidth.  Maybe you could tweak them up 
a bit to account for sample broadening.  Accordingly, they were ideally suited 
to Rietveld's method which was first developed for CW neutron powder 
diffractometers.  Historically, they seem to have overstayed their welcome, I 
mean their theoretical justification.  This is especially so for high 
resolution x-ray powder diffractometers at synchrotrons and elsewhere where the 
peak width is almost entirely from the sample, not the instrument. 

One problem with them is that for inappropriate choices of U, V, and W, the 
linewidth can become an imaginary number over a certain range of diffraction 
angles.  This leads to some unpleasant instabilities in refinement programs 
that use them. 

The fundamental parameters approach would have you model the instrument and the 
sample separately, and for any other kind of diffractometer, U, V, and W are 
probably not a very good model of either.  You can learn about fundamental 
parameters e.g., from the Bruker Topas documentation, or from Klug and 
Alexander, chapter 6.   

If you are not going to try to separately model instrument and sample, you can 
get a pretty good line through your data points and relative intensities 
suitable for Rietveld analysis with U, V, and W (and some of their extensions, 
such as Lorentzian X and Y in, e.g., GSAS)  Toward that end note that if you 
forget V, the (Gaussian) FWHM is $(U \tan^2 \theta + W)^{1/2}$, which suggests 
that U is kind of like strain broadening and W is kind of like size broadening, 
coming together in quadrature.  I have had generally OK luck leaving V set to 
zero and refining U and W.  That has the advantage of being more robust than 
refining the three (or more) parameters.  I guess once your refinement is 
pretty much under control, you could let V vary to see if the fit improves.  
Just be careful not to believe that the refined values of U, V, and W have any 
meaning in such a refinement. 

^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~
Peter W. Stephens
Professor, Department of Physics and Astronomy
Stony Brook University
Stony Brook, NY 11794-3800
fax 631-632-8176

<>

More Caglioti U V W parameters

[May, Frank]

To all:
 
OBSERVATION:  Most of the reported X-ray Rietveld analyses I've seen include 
refined values for U, V, & W which are dependent on the particular sample of 
interest.
 
As you say below regarding U, V, & W: "They can be a bit hard to determine 
unless a very high quality pattern is used for the calibration."
 
Therefore, I am puzzled why it is current practice to allow U, V & W to vary 
during Rietveld refinement.  If U, V, & W are really INSTRUMENT parameters, 
shouldn't they be determined independently using "a very high quality [specimen 
to determine the] pattern [which] is used for the calibration" -- and then 
FIXED FOR FUTURE ANALYSES?
 
Can anyone explain why U, V, & W are refined?
 
Frank May
Research Investigator
Department of Chemistry and Biochemistry
University of Missouri - St. Louis
One University Boulevard
St. Louis, Missouri  63121-4499
 
314-516-5098
[EMAIL PROTECTED]
 



From: Von Dreele, Robert B. [mailto:[EMAIL PROTECTED]
Sent: Mon 6/25/2007 12:27 PM
To: rietveld_l@ill.fr
Subject: RE: Caglioti U V W parameters



Dear Stefan (& all, I suppose),
>From the original formulation by Caglioti, et al. U>0, V<0 & W>0 for a
"nonfocusing" neutron CW instrument and describes a parabolic curve with
the minimum at roughly the 2-theta angle that matches the monochromator
"take-off" (really 2-theta) angle. For a Bragg-Brentano powder
diffractometer the curve is usually quite flat yielding very much
smaller FWHM especially for the low angle portion rising only at high
angles. I suppose the minimum is about the 2-theta for the analyser
crystal. Thus, U, V & W are usually quite small but the relationship of
U>0, W>0 & V<0 still holds. They can be a bit hard to determine unless a
very high quality pattern is used for the calibration. The expression
using U, V & W yields (FWHM)^2 so it REALLY can't ever have a negative
result as Yaroslav noted in his message. Rietveld refinement codes will
protect against the possibility of a negative square root in various
ways (if not they can crash on a "negative sqrt" error).
Bob Von Dreele

R.B. Von Dreele
IPNS Division
Argonne National Laboratory
Argonne, IL 60439-4814



-Original Message-
From: Stefan Berger [mailto:[EMAIL PROTECTED]
Sent: Monday, June 25, 2007 11:18 AM
To: rietveld_l@ill.fr
Subject: Caglioti U V W parameters


Dear All,

i am a little bit confused about the magnitudes of the U V W profile
parameters of the caglioti function for instrumental broadening. Is it
true, that U and W are always positiv (larger 0) and V is always smaller
0 (negativ)?

Thanks for your hints and explanations.

Best Regards,

Stefan



<>

RE: Re: Strange peaks from grainy Si

[May, Frank]

If you've eliminated the possibility of target or sample impurities, the 
so-called Renninger Reflections would also be my guess.  However, I also don't 
have the (picture of) data to observe.
 
If the "continuous ring" [re:  Andrew Payzant below] refers to a transmission 
photo of a powder, then it's not reasonable to suspect multiple diffraction 
from a very large crystal.  Such a large crystal would manifest itself in a 
spot, and not a ring, wouldn't it?
 
Frank May
XRD since 1972
University of Missouri - St. Louis



From: Whitfield, Pamela [mailto:[EMAIL PROTECTED]
Sent: Wed 1/24/2007 1:05 PM
To: rietveld_l@ill.fr
Subject: RE: Re: Strange peaks from grainy Si


For some reason the original email never made it to me so I can't see the data 
- so apologies if this is way off base.  However, it's not impossible for 
perfect single crystal material to produce forbidden reflections from multiple 
diffraction effects if the crystallites are big enough.
 
Pam



From: Andrew Payzant [mailto:[EMAIL PROTECTED]
Sent: Wed 24/01/2007 12:58 PM
To: Rietveld list
Subject: Re: Strange peaks from grainy Si


Peter,

That is my guess too - either additional (weak) characteristic lines such as 
Mo-K beta, or contaminant lines such as W L, or, as you point out, some 
wavelength in the Bremmstrahlung that meets the Bragg condition for some 
silicon crystallites.

However, I would not have expected a few large crystallites to yield a 
continuous ring, as appears in his CCD image, so perhaps there is another 
explanation.

Andrew
-- 
E. Andrew Payzant
Senior R&D Staff Member
High Temperature Materials Laboratory

Oak Ridge National Laboratory
1 Bethel Valley Road
PO Box 2008, MS 6064
Building 4515, Room 113
Oak Ridge, TN, 37831-6064

ph: (865) 574-6538   FAX: (865) 574-3940
web:  
 







From: <[EMAIL PROTECTED]>
Reply-To: 
Date: Wed, 24 Jan 2007 12:51:57 -0500
To: 
Subject: Re: Strange peaks from grainy Si


I'll mail you some fine Si powder if you send me your address. 

My best guess is that you have a relatively big lump of Si in your sample that 
happens to be lined up to make a bright spot from the bremstrahlung part of the 
spectrum.  It happens to meet some Si Bragg reflection condition for some 
wavelength in the brems. spectrum. 

^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~
Peter W. Stephens
Professor, Department of Physics and Astronomy
Stony Brook University
Stony Brook, NY 11794-3800
fax 631-632-8176


<>

Re: Uiso constraints

[May, Frank]

With regard to "constrained" refinement:  Should one first refine lattice 
parameters and then "fix" them before proceeding with structure refinement?  As 
I see it, lattice parameters are directly observable in the xrd pattern; if one 
allows lattice parameters to change during structure refinement, they will no 
longer be correct.
 
I got this notion from using Vegard's Law to establish stoichiometry of 
electronic materials over 30 years ago.
 
Please enlighten me if this is not correct.  Thanks.
 
Frank May
Research Investigator
Department of Chemistry - Benton 315
University of Missouri - St. Louis
One University Boulevard
St. Louis, Missouri 63121-4499
314-516-5098
 



From: Alan Hewat [mailto:[EMAIL PROTECTED]
Sent: Mon 7/3/2006 9:52 AM
To: rietveld_l@ill.fr; rietveld_l@ill.fr




>A common, but minor reason why negative displacement values occur is due to 
>ambiguity in the placement of the background at high Q (angle).

Another common reason why Uiso is low is because of the neglect of absorption. 
(We have been through this many times - see the Rietveld archive eg 
http://www.mail-archive.com/rietveld_l@ill.fr/msg01807.html )

>My usual cure is to set all Uiso values to reasonable numbers (depending on 
>the type of material, temperature, etc.), refine the background, then fix the 
>background and refine the Uiso values.

Hmm. That sounds doubtful for the good reasons you gave for not using 
constraints to "fix" parameters whose refined values seem wrong. The calculated 
errors are only meaningful if you don't use such constraints.

Alan.


_
Dr Alan Hewat, ILL Grenoble, FRANCE<[EMAIL PROTECTED]>fax+33.476.20.76.48
+33.476.20.72.13 (.26 Mme Guillermet) http://www.ill.fr/dif/AlanHewat.htm
_



<>