Re: Literature on Rietveld limitations in nano materials

2019-06-11 Thread Frank Girgsdies

Dear Rietvelders,

Thank you for your numerous replies, which give me a bunch of references 
to read through.
Please take my apologies that I'm not able to reply and say thank you to 
all contributors individually.


With kind regards,
Frank Girgsdies

On 07.06.2019 15:36, Darren Broom wrote:

Dear Frank

With regard to accurately determining the size of small particles or nano-crystallites, 
have you seen this article on "Pitfalls in the characterization of nanoporous and 
nanosized materials" by Claudia Weidenthaler?

https://pubs.rsc.org/en/content/articlelanding/2011/nr/c0nr00561d

Best regards,

Darren


-Original Message-
From: girgs...@fhi-berlin.mpg.de
Sent: Thu, 6 Jun 2019 12:39:29 +0200
To: rietveld_l@ill.fr
Subject: Literature on Rietveld limitations in nano materials

Dear fellow Rietvelders,

Could anyone point me to some nice literature which critically discusses
the limitations of the Rietveld method when it comes to nano-crystalline
materials (specifically in the 1 to 3 nm range)?
As far as I'm aware, the core Rietveld literature seems to touch this
point only in the passing.

Background:
To the best of my knowledge, Rietveld-derived parameters (like lattice
constants or domain sizes) should not be trusted as being "physically
meaningful"  anymore when you fit the powder pattern of a material in
the few nm range with standard Rietveld tools.
My naive understanding of this problem is that the physical principles
of diffraction (or rather the best way to model it) gradually change
when you go from long-range ordered to medium-/short-range ordered
materials.
Being a Rietveld practitioner rather than a theoretician, and having no
first-hand experience with WPPM and PDF methods, I am often confronted
with the problem to explain to my "customers" why I can't extract
trustworthy lattice constants or domain sizes from their
nano-crystalline samples, especially if it seems technically possible to
fit the pattern with a Rietveld program.
I think it would be nice if I could cite some critical discussion, or
overview article with further references, to put my finger on the
problem.
Especially in the catalysis community literature, my impression is that
the applicability of the Rietveld method is sometimes overestimated,
leading to overinterpretation of the results.

Any suggestions?

Best wishes,
Frank Girgsdies



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Literature on Rietveld limitations in nano materials

2019-06-06 Thread Frank Girgsdies

Dear fellow Rietvelders,

Could anyone point me to some nice literature which critically discusses 
the limitations of the Rietveld method when it comes to nano-crystalline 
materials (specifically in the 1 to 3 nm range)?
As far as I'm aware, the core Rietveld literature seems to touch this 
point only in the passing.


Background:
To the best of my knowledge, Rietveld-derived parameters (like lattice 
constants or domain sizes) should not be trusted as being "physically 
meaningful"  anymore when you fit the powder pattern of a material in 
the few nm range with standard Rietveld tools.
My naive understanding of this problem is that the physical principles 
of diffraction (or rather the best way to model it) gradually change 
when you go from long-range ordered to medium-/short-range ordered 
materials.
Being a Rietveld practitioner rather than a theoretician, and having no 
first-hand experience with WPPM and PDF methods, I am often confronted 
with the problem to explain to my "customers" why I can't extract 
trustworthy lattice constants or domain sizes from their 
nano-crystalline samples, especially if it seems technically possible to 
fit the pattern with a Rietveld program.
I think it would be nice if I could cite some critical discussion, or 
overview article with further references, to put my finger on the problem.
Especially in the catalysis community literature, my impression is that 
the applicability of the Rietveld method is sometimes overestimated, 
leading to overinterpretation of the results.


Any suggestions?

Best wishes,
Frank Girgsdies



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Re: Dollase Paper

2014-03-21 Thread Frank Girgsdies

Hi Norberto,

just my two cents:
If you observe a discrepancy between observed and calculated intensities,
then it could also be that the calculated intensities are wrong.
Depending on the software you use for the Rietveld refinement, it may
happen that the wrong combination of space group setting and atomic
coordinates is used. Fd-3m has two alternative origin choices, which
would yield quite different calculated intensities when combined with
the same set of coordinates.
At least in Topas, which I'm using, this is one of the two most common
sources of error.

Best wishes,
Frank



On 21.03.2014 10:59, Norberto Masciocchi wrote:

Dear Friends,

I am facing a problem in quantifying magnetite in a complex natural mixture,
using the conventional Rietveld method (in its quantitative analysis approach).
Apparently, the octahedral morphology of (difficult to grind) magnetite crystals
affects itd diffraction pattern.

To my knowledge, as certified in the original paper (JAC 1986, 267-272), the
ubiquitous and highly performing March-Dollase equation, holding for any
crystal symmetry, only applies to inequant crystallites (i.e. crystallites with
unequal sides), or, better said, to effectively rod- or disk-shaped specimens
(provided that the geometry of the experiments keeps cylindrical symmetry).
Neither octahedral nor cubic (e.g., NaCl) crystals can be considered inequant.

So, the question is:
Is there any way do get around this problem (without resorting to spherical
hamonics or to grind the specimen in a WC, SiC or BN mill)?

Thank you for your patience.

Norberto

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Re: Réf. : RE: Topas prm keyword

2010-03-12 Thread Frank Girgsdies

In the GUI mode, you simply initialize a parameter by entering
its name in into the grid on the codes tab, without the = sign!
This is equivalent to changing the line of code you've sent
from
Site OH num_posns 4 x  0 y  0 z  0.315309 occ O =oh; :  0.5 min =0; max =0.5; 
beq  1
to
Site OH num_posns 4 x  0 y  0 z  0.315309 occ O oh; :  0.5 min =0; max =0.5; 
beq  1
i.e. delete the = sign before the oh.

Thus, I think there is no need to use the prm command
(or the launch mode) in your case.

However, there may be cases where it could be convenient to
declare a parameter separately. For example, I have a solid solution
series where all lattice parameters are a known function of
the stoichiometry parameter X.
If I now want to do some restrictive fitting (instead of
four independent monoclinic lattice parameters I would
like to refine just X), then I can use the following workaround
to emulate something like the prm command in the GUI:

I simply add an additional dummy site to my structure phase.
Dummy site means that I may chose the name of the site and
the element freely, but set the occupancy to zero.
Now, I can use the x, y and z coordinates and the Beq as
containers for my parameters, because with zero occupancy,
the dummy atom will not contribute to the calculated intensities!
I enter the starting values for my parameter(s) in the Values tab
of the grid and the parameter name(s) in the Codes tab.
If a parameter is not to be refined, a ! needs to precede
the parameter name.

Maybe this little workaround is useful to some of you.
I should add that this trick works in Topas version 3,
I'm not sure if it does with other versions...

Best wishes,
Frank


Habib Boughzala wrote:

So,
 
How to initialize a variable ?
 
Cheers
 
/---Message original---/
 
/*De :*/ Kern, Arnt mailto:arnt.k...@bruker-axs.de

/*Date :*/ 12/03/2010 09:05:03
/*A :*/ 'ian.mad...@csiro.au' mailto:ian.mad...@csiro.au; 
 habib.boughz...@ipein.rnu.tn mailto:habib.boughz...@ipein.rnu.tn; 
 rietveld_l@ill.fr mailto:rietveld_l@ill.fr

/*Sujet :*/ RE: Topas prm keyword
 
The keyword prm is exclusive to so-called Launch Mode operation, 
please see also the Technical Reference manual.
 
Cheers,
 
Arnt



*From:* ian.mad...@csiro.au [mailto:ian.mad...@csiro.au]
*Sent:* Freitag, 12. März 2010 08:53
*To:* habib.boughz...@ipein.rnu.tn; rietveld_l@ill.fr
*Subject:* RE: Topas prm keyword

Try  prm   not   Prm  (ie with all lower case letters)
 

 
/Cheers/

/ /
/ooo0ooo/
/ Ian Madsen/
/ Team Leader - Diffraction Science/
/ CSIRO Process Science and Engineering/
/ Box 312,  Clayton South 3169/
/ Victoria,   AUSTRALIA/
/ Phone +61 3 9545 8785 direct/
/ +61 3 9545 8500 switch/
/ +61 (0) 417 554 935 mobile/
/ FAX+61 3 9562 8919/
/ Email //_ian.mad...@csiro.au_/ mailto:ian.mad...@csiro.au/ /
/ooo0ooo/

 



*From:* Habib Boughzala [mailto:habib.boughz...@ipein.rnu.tn]
*Sent:* Friday, 12 March 2010 6:45 PM
*To:* rietveld_l@ill.fr
*Subject:* Topas prm keyword

Hi,
 
My mail is intended to Topas users, especially to Dr. Alan Coelho
 
I am trying to determine the carbonate / hydroxide ratio in some compounds.

This is a part of 'str' block In the input file (*.inp)
__
Str  


 Prm  oh  0.5

  Site OH num_posns 4 x  0 y  0 z  0.315309 occ O =oh; :  0.5 min =0; 
max =0.5; beq  1
  Site C num_posns 2 x  0 y  0 z  0 occ C =1 - 0.5 oh; :  1 min =0; max 
=0.5; beq  1
  Site Oc num_posns 12 x  0.040114 y  0.953411 z  0.629971 occ O =(1 - 
0.5 oh) / 3; :  1 min =0; max =0.25; beq  1

__
 
Topas 4.2 run failed...  Error: prm command in unrecognised !!!
 
Any suggestion ?
 
P.S. This keyword is unrecognised too in all tutorials inp file examples !!!
 
Thanks for help
 
Habib
  
Prof. Habib Boughzala.

*L*aboratoire de *M*atériaux et *C*ristallochimie.
*A*ssociation *T*unisienne de *C*ristallographie
I.P.E.I.N. Mrezga, 8000.
Nabeul. Tunisie.






 


Bruker AXS GmbH, Karlsruhe

HRB 107524 Amtsgericht Mannheim, Umsatzsteuer-Ident.Nr. DE812037551, 
Geschäftsführer - Dr. Frank Burgäzy, Bernard Kolodziej, Stephan Franz 
Westermann


 




 

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Re: Find-it

2010-02-10 Thread Frank Girgsdies

Dear Alan,

isn't it a little bit unfair to list just the Pros but skip the Cons?

We used to have a FINDIT subscription several years ago. Sadly,
our purchasing department canceled the subscription when we
got institutional access to the WWW version.

If I remember correctly, FINDIT had far more options to customize
and tweak a search than the WWW interface offers. You could e.g. search
for reduced cells, conduct several consecutive searches and combine
the results, select positive hits from the list and skip the rest,
export the selected hits etc. All these are features which I miss
badly in the WWW version (Or is there a way? Maybe I'm using the wrong
engine (http://icsd.fkf.mpg.de)?)

So, if you are just looking for a specific crystal structure, the
WWW version is nice because it is always up-to-date.
But if you try to condcut a systematic and as-complete-as-possible
study on a class of compounds, it can be really frustrating to
have just the WWW version instead of FINDIT.

However, this is just a personal opinion based on experience from
several years ago. Maybe the boundary conditions have changed since
then...

Best wishes,
Frank

Alan Hewat wrote:

I have problem to use FINDIT  software: after the starting windows,
sometimes it freeze. Is there anyone that know how resolve this problem?


FINDIT runs the PC version of the ICSD database. My personal opinion :-)
is that you should switch to the WWW version on
http://icsd.fiz-karlsruhe.de/ You can get a demo account and you can trade
in your FINDIT license for a WWW ICSD license, which will always be up to
date and run on any computer.

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



Re: Find-it

2010-02-10 Thread Frank Girgsdies

Dear Alan,

so it seems I am using an Interface version which is too old.
I guess I should urge the responsibles to update it.
Thanks for the information.

Best wishes,
Frank

Alan Hewat wrote:

Dear Frank,

As I wrote, I was referring to my personal opinion and the new ICSD WWW
pages on http://icsd.fiz-karlsruhe.de/ which do allow consecutive
searches, combination of results and methods of searching for similar
structures, structure-types etc.

FIZ provides both WWW and FINDIT versions under the exact same conditions,
so the choice is yours (previously FINDIT was cheaper). As I wrote, FIZ
even allow you to trade in a license for one version against another, and
provides demo versions of both to allow you to choose.

Certainly each has pros and cons, and I have not tried to list all of
them. In particular ICSD is good at drawing structures using Jmol such
that it is usually easy to see immediately the chemical interest of the
structure. ICSD was the first to use this kind of drawing for inorganic
structures, but it has now been adopted by all IUCr journals
http://journals.iucr.org/ as well as other databases such as the Cambridge
Organic database http://www.ccdc.cam.ac.uk/ the Zeolite database
http://www.iza-structure.org/databases/ etc

Alan.

Frank Girgsdies said:

Dear Alan,

isn't it a little bit unfair to list just the Pros but skip the Cons?

We used to have a FINDIT subscription several years ago. Sadly,
our purchasing department canceled the subscription when we
got institutional access to the WWW version.

If I remember correctly, FINDIT had far more options to customize
and tweak a search than the WWW interface offers. You could e.g. search
for reduced cells, conduct several consecutive searches and combine
the results, select positive hits from the list and skip the rest,
export the selected hits etc. All these are features which I miss
badly in the WWW version (Or is there a way? Maybe I'm using the wrong
engine (http://icsd.fkf.mpg.de)?)

So, if you are just looking for a specific crystal structure, the
WWW version is nice because it is always up-to-date.
But if you try to condcut a systematic and as-complete-as-possible
study on a class of compounds, it can be really frustrating to
have just the WWW version instead of FINDIT.

However, this is just a personal opinion based on experience from
several years ago. Maybe the boundary conditions have changed since
then...

Best wishes,
Frank

Alan Hewat wrote:

I have problem to use FINDIT  software: after the starting windows,
sometimes it freeze. Is there anyone that know how resolve this
problem?

FINDIT runs the PC version of the ICSD database. My personal opinion :-)
is that you should switch to the WWW version on
http://icsd.fiz-karlsruhe.de/ You can get a demo account and you can
trade in your FINDIT license for a WWW ICSD license, which will always
be up to date and run on any computer.

Alan.

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



Re: Source and comparison of X-ray scintillator screens

2009-06-17 Thread Frank Girgsdies

Dear Liang,

I'm not sure if we are talking about the same Z, but I'll
give it a try.
To the best of my knowledge, Z simply denotes the number
of formula units per unit cell.
In order to determine Z, we have to distinguish two
cases:
1) The crystal structure is known.
 In this case, you simply count the atoms in the unit cell
 and divide by the contents of one formula unit.
 Note that often the definition of a formula unit may be
 arbitrary (i.e. a matter of choice), which means that
 defining formula unit and Z always need to go together.
2) The crystal structure is unknown and you are trying to
 determine it. In order to do so successfully, you need
 to have a good idea about how many atoms of which type
 are in the unit cell. Thus, you will need additional
 information like elemental analysis etc.
 Once you have an idea about what the formula unit
 would be, you should guess Z in order to know the
 total contents of the unit cell. Based on the formula
 unit, Z is usually an integer (or, depending on the
 symmetry of the cell, at least something like 3/2 or so).
 To estimate Z, you need the unit cell volume (i.e. the
 cell dimensions need to be determined first).
 If you know the measured density, than you may deduce
 Z from that, because it should be similar to the theoretical
 density of the crystal structure. However, the real (measured)
 density is often somewhat lower than the calculated density.
 If you do not know the real density, there are other ways
 to estimate Z. For example, for organic or organometallic
 compounds you may guess Z by assuming that every non-hydrogen
 atom in the formula contributes about 17 cubic Angstroms
 to the unit cell volume. This may be a crude estimate (especially
 if the structure contains several heavy atoms), but as Z
 is typically integer, there won't be too many choices.
 Alternatively, you may try to calculate Z by using the
 density of a very similar compound instead.
 If Z turn out to be ambiguous and you do not succeed in solving
 the structure, you should give another choice of Z a try.
 Example: your estimate calculations lead to a Z of
 3.7. In this case, assuming Z=4 would be the first choice.
 If the structure determination fails, Try Z=3 next.
 Of course, considering the space group symmetry ususually
 limits the number of choices further, so it would be a
 good idea to have a look into the International Tables
 of Crystallography and see which Wyckoff multiplicities
 may actually occur in the space group of your choice.

Hope I could help.

Cheers,
Frank




Liang wrote:

Dear all,
could anyone give me some knowledge about how to determine the Z  of 
the unite cell. I understand it can be determined from the density 
measured from the sample. In some papers, I find it was deduced from the 
electron diffraction patterns and I have not completely understanding 
about how to calculate the Z just from the electron diffraction 
patterns. Which is better In this two method, calculated from the 
measured density or deduced from the electron diffraction patterns?  And 
are there any other way to determine the Z ?


Thanx for your comments!




Re: Example of simultaneous fitting in TOPAS Academic

2009-03-30 Thread Frank Girgsdies

Hi Shishir,

I'd like to help, but I am not exactly sure what you want.
First, you are asking for example code.
 Does this mean that you want to work in launch mode instead
 of using the GUI? If so, why?
Second, it would be important to know why exactly you want to
 use simultaneous refinement.
 This option only makes sense if you have parameters which should
 be the same for different data sets. If, for example, you have
 measured the same sample at different temperatures, what would be
 the common refined parameters? The lattice parameters are of course
 different for different temperatures. If you are doing a Rietveld
 fit, will you refine the atomic coordinates? If not, there should
 be no need for simultaneous refinement. If yes, I would argue that
 the atomic coordinates should differ slightly with temperature.
 Again, no need for simultaneous refinement. The only parameter
 I could think of whould be some intrumental parameter which shouldn't
 change with temperature or the sample measured (e.g. zero shift).
 The same arguments would apply for a doping series.
Simultaneous refinement makes sense if you have measured the same
 sample with different wavelengths or methods. Whether it is reasonable
 to refine a common structure model on a combination of neutron and
 XRD data simultaneously is open for discussion. Multiple wavelength
 synchrotron data might be a good example for simultaneous refinement.
Another possibility would be a series of phase mixtures composed of
 the same phases. If you are interested in refining the structures
 of these phases and can't get them phase pure, you might try to
 correlate the peak shape and structure parameters to partially
 compensate the effects of peak overlap between the different phases.

Best wishes,
Frank

sisir ray wrote:

Hi,
Does any body have an working example code for TOPAS Academic  for 
simultaneous fitting of different diffractograms obtained at different 
temperatures or for different doping concentrations .I really appreciate 
your help.


--
thankyou,
Shishir Ray
Graduate Student


Re: U iso and U aniso

2008-11-05 Thread Frank Girgsdies

Dear Ana,

maybe I am a little bit picky here, but actually you
cannot convert U(ij) into U(iso), because U(ij) result from
a fit with anisotropic displacement parameters, while
U(iso) results from a fit with isotropic displacement
parameters. Thus, to convert one into the other means
repeating the fit with a different model.

What you probably mean is how to convert U(ij) into
U(eqiv), which is something like simulating the
change in model mentioned above by calculating an
equivalent isotropic atomic displacement parameter
from the anisotropic one, for the sake of comparison
between isotropic and anisotropic parameters.

But as I said, I am very picky about the terminology
here, so please don't take that too serious. ;)

Details on the calculation can be found in the CIF
core dictionary of the IUCr:

http://ww1.iucr.org/cif/cifdic_html/1/cif_core.dic/Iatom_site_U_iso_or_equiv.html

Cheers,
Frank

Ana Isabel Becerro Nieto wrote:

Dear All,

 

How can I convert the “anisotropic temperature factors” (u_ij ) into an 
“isotropic temperature factor” (U_iso )?


 


Thank you!

 


ana

 

 

 


Dr. Ana Isabel Becerro

Instituto de Ciencia de Materiales de Sevilla -

Dpto. Química Inorgánica (CSIC-US)

C/ Américo Vespucio, 49

41092 Sevilla

Tel: 954489545

 

 





Re: Anisotropic peak broadening with TOPAS

2008-10-31 Thread Frank Girgsdies
 as a separate
RAW file, then I delete the range again.
5) I repeat steps 3 and 4 for c0k0 and c00l.
6) I delete the additional convolution
again and re-activate the Cry Size L parameter
instead. I make sure that the check box for
calculating LVol-IB is activated.
7) I replace the measured range with one of
the previously exported calculated ranges
and start the refinement. After convergence,
I obtain the LVol-IB value for this particular
crystal direction and write it down.
  [Of course, this fit will be a perfect one,
  which unfortunately means that the
  LVol-IB(hkl) parameter obtained will have
  an error calculated as zero.]
8) I repeat step 7 for the remaining two
calculated patterns/crystal directions.

Finally, I have three parameters
LVol-IB(h00), LVol-IB(0k0) and LVol-IB(00l),
which I freely interpret as anisotropic
equivalents of the isotropic LVol-IB, which
is exactly what I originally wanted.

PLEASE NOTE: I definitely do NOT claim that
these parameters have any real physical
meaning. I just use them to parametrize
sample XRDs exhibiting anisotropic size
broadening. I have chosen this particular
approach because I consider it intuitive
(and I'm an intuitive guy).

Of course, I will try in the future to see
if there are correlations between these
anisotropic size parameters and other
experimental results, especially from
the shape analysis of electron microscopy.

I hope that my lengthy explanations are
clear enough and maybe useful for some
of you.

Thanks again to all people who sent
replies, especially to Peter Stephens!

Of course, If someone could teach me
how to replace my clumsy GUI workaround
with some nice launch mode code, I'd
really appreciate that...

Cheers,
Frank


Frank Girgsdies wrote:

Dear Topas experts,

this is my first email to the list, so if you would like
to know something about my background, please refer to
the about me section at the end of this mail.

My question is concerning advanced modeling of anisotropic
peak broadening with Alan Coelhos program Topas.

I'm working on a transition metal mixed oxide phase of
orthorhombic symmetry. Composition, lattice parameters,
crystallite size etc. may vary from sample to sample.
I'm using Topas to fit the powder patterns with a
structure phase. If the peaks exhibit more or less
homogeneous peak widths, I refine the Cry Size L
and/or Cry Size G parameters to model the peak
shapes. Thus, I can obtain the LVol-IB as a measure
for the average crystallite size.

In some cases, however, I observe strongly anisotropic
peak broadening, with the 00l series of reflections
being much sharper then the hk0 and hkl reflections.
This observation fits nicely with the electron
microscopy results, where the crystals are needles of
high aspect ratio, the long axis being the c-axis of
the crystal (thus, I assume that the peak broadening
is dominated by the crystallite size effect, so
let us ignore the possibility of strain etc.).
In such case, I leave the GUI and switch to launch mode,
where I can successfully model the anisotropic peak
broadening with a second order spherical harmonics
function, following section 7.6.2. of the Topas (v3.0)
Technical Reference. So far, so good.

However, since the peak width is now primarily a
function of hkl (i.e. the crystallographic direction)
instead of a function of 1/cos(theta), I lose the size
related information. Of course, I'm aware of the
fact that the LVol-IB parameter is based on the
1/cos(theta) dependence and thus cannot be calculated
for a spherical harmonics model.
But the peaks still have a width, so it should be
possible somehow to calculate hkl-dependent size
parameters. And this is the point where I'm hoping
for some input from more experienced Topas users.

I could imagine three directions of approach:

A) The refined spherical harmonics functions
yields a set of coefficients. I'm not a mathematician,
so how to make use of these coefficients for my
purpose is beyond my comprehension.
I imagine the refined spherical harmonics function
as a 3-dimensional correction or scaling function,
which yields different values (scaling factors)
for different crystallographic directions.
Thus, it should be possible to calculate the
values for certain directions, e.g. 001 and 100.
I would expect that the ratio of these two values
is somehow correlated with the physically observed
aspect ratio of the crystal needles, or at least a
measure to quantify the degree of anisotropy.
Is there a recipe to re-calculate (or output) these
values for certain hkl values from the set of
sh coefficients?

B) As far as I understand the spherical harmonics
approach as given in the Topas manual, it REPLACES
the Cry Size approach. However, it might be possible
to COMBINE both functionalities instead. Within a
given series of reflections (e.g. 00l) the
1/cos(theta) dependence might still be valid.
I could imagine that the spherical harmonics model
might be used as a secondary correction function
on top of a 1/cos(theta) model.
I

Re: cif files for austenite

2008-10-31 Thread Frank Girgsdies

Dear Antonio,

as far as I know, austenite is just a solid solution
of a few mole-% of carbon in gamma-iron.
Thus, you essentially need the structure of gamma-Fe,
which is very simple, as it is an fcc metal
(space group Fm-3m, a = 3.6468 A, just one
Fe atom located on 0,0,0).
I found one ICSD entry for austenite.
However, the dissolved carbon is placed sharing
the Fe site in this structure, which I doubt makes
sense from the chemical point of view.
I would bet the carbon atoms are actually located
on interstitial sites instead.
Anyway, as we are talking about a few MOLE-%
of carbon here, and carbon is much lighter than
iron, I think neglecting the carbon will not
make much of a difference (both for the accuracy
of the calculated relative intensities and
the quantification in wt-%.
Thus, I would just use gamma-Fe as a replacement
for austenite here.

Cheers,
Frank

--
Frank Girgsdies
Department of Inorganic Chemistry
Fritz Haber Institute (Max Planck Society)
--

antonio josé wrote:

Dear All.

 

I trying analyze phases in a steel sample using   X-Ray technique. The 
analysis of X-Ray pattern shows that the main phases are ferrite and 
austenite. The next step would be quantify these phases by Rietveld 
method using TOPAS 3.0. I am looking for cif files for austenite and I 
didn’t find it in WEB free database or in ICSD database.


Do anyone has this cif file or could tell me where I can find?

 


Thanks a lot,

Antonio José

[EMAIL PROTECTED]





Re: Anisotropic peak broadening with TOPAS

2008-10-30 Thread Frank Girgsdies

Dear Matthew,

thanks for your reply.
I hope to look a bit deeper into it (and try the code)
a little later today.
At the first glance, however, I'm not sure whether
treating hk0 and hkl the same way would be appropriate
from the theoretical point of view.
But as I'm a practical guy, I will just give it a try.

Thanks again!
Frank

[EMAIL PROTECTED] wrote:

Sorry, pressed the wrong button...


If you just want to try fitting the peaks, you could try something like this:

str
phase_name Metal_oxide
local broad 100 'crys size for hk0 and hkl
local sharp 2000 'crys size for 00l
local csL =
IF (And(H == 0, K == 0, L  0)) THEN
sharp
ELSE
broad
ENDIF;

CS_L(csL)
'insert remainder of structure...


I don't know much about Lvol, but isn't an average crystallite size for a 
highly asymmetric crystal not all that meaningful? I am willing to be educated here, as I 
haven't had much need to get accurate crystallite size from diffraction data before



Cheers

Matthew


Matthew Rowles

CSIRO Minerals
Box 312
Clayton South, Victoria
AUSTRALIA 3169

Ph: +61 3 9545 8892
Fax: +61 3 9562 8919 (site)
Email: [EMAIL PROTECTED]
-Original Message-
From: Frank Girgsdies [mailto:[EMAIL PROTECTED]
Sent: Wednesday, 29 October 2008 22:05
To: Rietveld_l@ill.fr
Subject: Anisotropic peak broadening with TOPAS

Dear Topas experts,
C) One could leave the spherical harmonics approach
and go to a user defined model, which refines different
Cry Size parameters for different crystal directions.
In my case, two parameters would probably be sufficient,
one for the c-direction, and a common one for the a- and
b-direction.
The Topas Technical Reference, section 7.6.3. gives a
similar example of a user defined peak broadening function,
depending on the value of l in hkl.
I could probably come up with an analogous solution
which has a 1/cos(theta) dependence and two parameters,
one for the 00l and one for the hk0 case.
My problem with this approach is how to treat the
mixed reflections hkl. I suppose they should be
scaled with a somehow weighted mix of the two
parameters, where the weighting depends on the
angle between the specific hkl and the c-axis.
However, I no idea how a physically reasonable
weighting scheme (and the corresponding Topas syntax)
should look like.
--
Frank Girgsdies
Department of Inorganic Chemistry
Fritz Haber Institute (Max Planck Society)
--