RE: Re: Problems using TOPAS R (Rietveld refinement)

2007-03-21 Thread Whitfield, Pamela
Makes sense with ultra-fines.  My portlandite grains were 5 microns upwards.
I'm working to avoid ultra-fines even harder than the bigger stuff :-)
 
Pam



From: Omotoso, Oladipo [mailto:[EMAIL PROTECTED]
Sent: Wed 21/03/2007 10:47 AM
To: rietveld_l@ill.fr
Subject: Re: Problems using TOPAS R (Rietveld refinement)



My limited experience with X-ray capillary measurements of ultra fine clay 
minerals suggests that you could have significant preferred orientation along 
the b* axis.  It is actually a good way of determining aspect ratios in 
phyllosilicates. 

 

Dipo Omotoso



From: Whitfield, Pamela [mailto:[EMAIL PROTECTED] 
Sent: Wednesday, March 21, 2007 7:51 AM
To: rietveld_l@ill.fr
Subject: RE: Re: Problems using TOPAS R (Rietveld refinement)

 

I have to disagree with that; at least on a practical front with lab XRD.  I 
have done measurements myself with samples containing large portlandite plates 
(granted, not a silicate but lovely-looking plates in a SEM) for quantitative 
analysis.  The whole point of the work was to see if capillary measurements 
would be worth it if the normal sample prep techniques would change the nature 
of the sample.  The reflection measurements had awful orientation, but the 
portlandite spherical harmonics PO coefficients for the capillary data gave a 
texture index of 1, i.e. an ideal powder.  The diffraction optics and detector 
were identical for reflection and transmission.  The capillary data actually 
gave better quantitative results than the reflection.  A reason might be that 
if the grains are large enough to orientate significantly they might be big 
enough to cause microabsorption effects that are best avoided (the Brindley 
correction assumes spherical particles so plates are a bit of a headache).

 

It's just a thought, but the orientation in neutron and X-ray data might differ 
due to the difference in sample container size and orientation.  Neutron cans 
are often mounted vertically and are pretty big so there won't be much 
advantage over reflection as the material settles.  Lab capillaries are usually 
mounted horizontally and the capillary diameter is often quite small in 
relation to the grain size (compared to neutron sample cans). All bets are off 
for wollastonite though!

 

I will shut up at this point as I trying to avoid doing clay analysis!

 

Pam

-Original Message-
From: David L. Bish [mailto:[EMAIL PROTECTED] 
Sent: March 21, 2007 9:11 AM
To: rietveld_l@ill.fr
Subject: Re: Problems using TOPAS R (Rietveld refinement)

One often hears of attempts to "eliminate" preferred orientation in 
diffraction patterns of layer silicates using transmission measurements.  Keep 
in mind that if PO is a problem in reflection geometry, it will also affect 
transmission measurements, in a manner potentially similar to flat-plate 
samples.  We did some TOF neutron measurements on phyllosilicates a few years 
ago with what amounts to capillary sample holders, and preferred orientation 
was a significant problem.  If a material orients, it will do so in all mounts 
unless steps are taken to minimize it.

Dave

At 07:50 AM 3/21/2007 +0100, you wrote:



Gentlemen,
I've been listening for a week or so and I am really wondering what do 
you
want to get ... Actually you are setting up a "refinement", whose 
results
will be, at least, inaccurate. I am always surprised by attempts to 
refine
crystal structure of a disordered sheet silicate from powders, 
especially
when it is known it hardly works with single crystal data. Yes, there 
are
several models of disorder, but who has ever proved they are really 
good ?
I do not mean here a graphical comparison of powder patterns with a
calculated trace, but a comparison of structure factors or integrated
intensities. (Which ones are to be selected is well described in the 
works
of my colleague, S.Durovic and his co-workers.)
As far as powders are concerned, all sheet silicates "suffer" from
prefered orientation along 001. Until you have a pattern taken in a
capillary or in transmission mode, this effect will be dominating and 
you
can forget such noble problems like anisotropic broadening.

Last but not least : quantitative phase analysis by "Rietveld" is (when 
only
scale factors are "on") nothing else but multiple linear regression. 
There
is a huge volume of literature on the topic, especially which variables
must, which should and which could be a part of your model.
I really wonder why the authors of program do not add one option called
"QUAN", which could, upon convergence of highly sophisticat

RE: Re: Problems using TOPAS R (Rietveld refinement)

2007-03-21 Thread Whitfield, Pamela
I have to disagree with that; at least on a practical front with lab XRD.  I 
have done measurements myself with samples containing large portlandite plates 
(granted, not a silicate but lovely-looking plates in a SEM) for quantitative 
analysis.  The whole point of the work was to see if capillary measurements 
would be worth it if the normal sample prep techniques would change the nature 
of the sample.  The reflection measurements had awful orientation, but the 
portlandite spherical harmonics PO coefficients for the capillary data gave a 
texture index of 1, i.e. an ideal powder.  The diffraction optics and detector 
were identical for reflection and transmission.  The capillary data actually 
gave better quantitative results than the reflection.  A reason might be that 
if the grains are large enough to orientate significantly they might be big 
enough to cause microabsorption effects that are best avoided (the Brindley 
correction assumes spherical particles so plates are a bit of a headache).
 
It's just a thought, but the orientation in neutron and X-ray data might differ 
due to the difference in sample container size and orientation.  Neutron cans 
are often mounted vertically and are pretty big so there won't be much 
advantage over reflection as the material settles.  Lab capillaries are usually 
mounted horizontally and the capillary diameter is often quite small in 
relation to the grain size (compared to neutron sample cans). All bets are off 
for wollastonite though!
 
I will shut up at this point as I trying to avoid doing clay analysis!
 
Pam

-Original Message-
From: David L. Bish [mailto:[EMAIL PROTECTED] 
Sent: March 21, 2007 9:11 AM
To: rietveld_l@ill.fr
Subject: Re: Problems using TOPAS R (Rietveld refinement)


One often hears of attempts to "eliminate" preferred orientation in 
diffraction patterns of layer silicates using transmission measurements.  Keep 
in mind that if PO is a problem in reflection geometry, it will also affect 
transmission measurements, in a manner potentially similar to flat-plate 
samples.  We did some TOF neutron measurements on phyllosilicates a few years 
ago with what amounts to capillary sample holders, and preferred orientation 
was a significant problem.  If a material orients, it will do so in all mounts 
unless steps are taken to minimize it.

Dave

At 07:50 AM 3/21/2007 +0100, you wrote:


Gentlemen,
I've been listening for a week or so and I am really wondering 
what do you
want to get ... Actually you are setting up a "refinement", 
whose results
will be, at least, inaccurate. I am always surprised by 
attempts to refine
crystal structure of a disordered sheet silicate from powders, 
especially
when it is known it hardly works with single crystal data. Yes, 
there are
several models of disorder, but who has ever proved they are 
really good ?
I do not mean here a graphical comparison of powder patterns 
with a
calculated trace, but a comparison of structure factors or 
integrated
intensities. (Which ones are to be selected is well described 
in the works
of my colleague, S.Durovic and his co-workers.)
As far as powders are concerned, all sheet silicates "suffer" 
from
prefered orientation along 001. Until you have a pattern taken 
in a
capillary or in transmission mode, this effect will be 
dominating and you
can forget such noble problems like anisotropic broadening.

Last but not least : quantitative phase analysis by "Rietveld" 
is (when only
scale factors are "on") nothing else but multiple linear 
regression. There
is a huge volume of literature on the topic, especially which 
variables
must, which should and which could be a part of your model.
I really wonder why the authors of program do not add one 
option called
"QUAN", which could, upon convergence of highly sophisticated 
non-linear
L-S, fix all parameters but scale factors and run standard 
tests or factor
analysis. One more diagonalization is not very time consuming, 
is it ? To
avoid numerical problems, I'd use SVD.
This idea is free and if it helps people reporting 0.1% MgO 
(SiO2) in a
mixture  of 10 phases to think a little of the numbers they are 
getting, I
would only be happy :-)
Lubo

P.S. Hereby I declare I have never used Topas and I am thus not 
familiar
with all its advantages or disadvantages compared to other 
codes.


  

RE: Re: Problems using TOPAS R (Rietveld refinement)

2007-03-15 Thread Omotoso, Oladipo
Leandro,

You probably should consult the references suggested by Alan Hewat and Reinhard 
Kleeberg before you read anything into your "reasonable" Rwp.  Kaolinite is 
grossly over-parametized in your refinement strategy.  If you are stuck with 
TOPAS, you may want to contact Arnt Kern (Bruker) about last year's TOPAS 
workshop.  I recall that there was a paper on refinement strategies for 
disordered clays. 

Dipo Omotoso
CANMET Energy Technology Centre - Devon
Energy Technology and Programs Sector
Natural Resources Canada
#1 Oil Patch Drive, Devon, AB. Canada
Groupe des techniques perfectionnées de séparation
Centre de la technologie de l'énergie de CANMET - Devon
Secteur de la technologie et des programmes de l'énergie
Ressources naturelles Canada


-Original Message-
From: Leandro Bravo [mailto:[EMAIL PROTECTED] 
Sent: Thursday, March 15, 2007 4:15 PM
To: rietveld_l@ill.fr
Subject: Re: Problems using TOPAS R (Rietveld refinement)

Ok, I´m starting to have sucess in the kaolinite refinement, the 
quantification is giving me reasonable values. I´m refining the thermal 
factors, all the atoms positions in the kaolinite, the lattice parameters 
and the cystallite size. Lattice parameters and crystallite size are giving 
me very good numbers, with very low errors (about 0,09). In the thermal 
factors, I realized that alll of them tend to 20, so after all refinements I 
put them to 20, and refine all over again. I don´t care that much for atoms 
positions, I´m only using them because refining only lattice, thermal and 
cry size wasn´t enough to make a good calculated pattern to compare with the 
measured one.
In the calcite and dolomite I refine: lattice parameters, cry size and 
thermal factors. And use on both a preferred orientation correction 
(spherical harmonics 4 th order). The RWP is about 16.

I´d to hear some opinions about this strategy of refinement, if you think 
that I can spare some refining cycles or even fix some values to reduce 
erros in the refinement.

_
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