Hi All,
This is just to introduce myself to the members of the
mailing list. My name is Nandini Devi Radhamonyamma
and I am currently working in St. Andrews University,
Scotland with Dr. Paul Wright. I am relatively new to
structure solution from powder data and hope to get
all the help I can to g
Hi All,
How important is Rbragg in powder data structure
solution? Does GSAS suit give this parameter?
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Title: RE: Re: thermal parameters at low temperature
ï
Liliana,
OK,
they should vibrate less... but are they good neutron scatterers or poor?
If they are poorly scattering, the thermal factors won't be as
meaningful.
- Kurt
---Kurt LeinenweberDepartment
Thank you.
In parallel with single crystal analysis, is there an
accepted limit for R(F2) or any other parameter like
Rexp to follow? I'm using synchrotron data.
nandini
--- "Von Dreele, Robert B." <[EMAIL PROTECTED]> wrote:
> Nandini,
> GSAS gives R(F^2) which is sometimes defined for
> single c
Liliana,
GSAS will refine the value of the absorption coefficient for neutron TOF data quite
easily (I recall one message of yours mentioned that your data was of this "flavor").
Just turn on the flag. Your Uiso's should rise for all atoms making those for the
heavy atoms positive. The ABS value
Dear all,
I am refining a powder neutron
data at low temperature (50K). I got some of the thermal parameters negative
although the model seems to be quite right (the data at room temperature behaved
well with the same model). From your experience, is this the case of a wrong
model?
I reall
Dear Dr Von Dreele,
The structure at room temperature was refined from neutron
TOF. Now, I am using this model to refine low temperature data from
neutron CW. My sample has Cu-Cl-La-Nb-O.
Sincerely,
Liliana
-Original Message-
> ... the path of
> setting the absorption correction.
>
In expedt (after choosing the file): L, O, A, C
In EXPGUI: input window is behind "Globally Edit Absorption" buttom in the
Histogram panel.
Best Regards,
Markus
--
Markus Valkeapää, [EMAIL PROTECTED]
Inorganic Chemistry, Göteborg Univer
Dear Liliana,
if the model is correct, and variable site occupancies can be excluded
and the chosen scattering lengths are correect,
it is likely that some 2theta-dependent or time-flight-dependent (if
time of flight diffractometer was used) effects (e.g. absorption, Lorentz
factors) are not corr
Nandini,
I suggest you look at the paper "Rietveld refinement guidelines", J. Appl. Cryst.
(1999) 32, 36-50.
Bob Von Dreele
-Original Message-
From: Nandini Devi Radhamonyamma [mailto:[EMAIL PROTECTED]
Sent: Thu 2/26/2004 8:38 AM
To: [EMAIL PROTECTED]
Thank you.
In parallel with single
Hello,
Can I insert a single crystal data set into GSAS for a refinement without
having to reduce the data? Can I start the refinement from the hkl
intensity data file (ASCII), without entering the intrument specifics? ie
orientation angles, attenuation and wavelength? My data set has al
Dear Liliana,
To guess what could be a possible answer one would probably need a little
more information.
Is the RT dataset collected with the same instrument and setup? Is it
constant-wavelength of TOF? Do you have any magnetic contribution at 50K,
and how do you treat it?
Sincerely,
In comparing refinements with different powder datasets there is no
"number of observations" in common usage and no 10:1 rule of thumb.
Broadly, this means "crappy" data with a chemically unreasonable model
can sometimes give much better figures of merit than a good structure
with "good" data.
I tend to use bond lengths to check refinements. If bond lengths - and
thermal parameters too - aren't physically reasonable, it can mean that the
structure is correct but the refinement is bad, that the structure is
somehow wrong, or the atom assignments are wrong.
Bond-valence sums are great fo
>Sorry for the ambiguous information I gave previously. The data at
>room temperature were taken with a variable wavelength (TOF) whether
>at low temperature the data were recorded with constant wavelength.
>There is no magnetic contribution on the pattern. The negative thermal
>parameters are for
Jennifer,
Yes. Start with SXTLDATA to read in the hkl file. It will ask for lots of stuff but
you will only really need h,k,l,Fo & sig(Fo). Then go to EXPEDT to put in the rest of
the stuff (atom positions, etc.).
Bob Von Dreele
-Original Message-
From: Jennifer Anderson [mailto:[EMAIL
Liliana,
You can calculate the absorption coefficient for your CW neutron experiment. GSAS
wants mu*R/lambda. Find a table of "true" neutron absorption cross sections for your
elements & calculate mu from them in cm-1. R is the radius of your sample can in cm.
Divide by 2 to correct for packing
Dear Liliana,
of course the remarks of Maxim are correct, the experimental conditions are very
important.
Just additionally:
Very important is also the range of d* which is covered experimentally for the
accuracy of the displacement parameters.
Of importance is also the mass of the atom. Heavy a
Wait! I have to mention - the "heaviness" of atoms (atomic number Z) is
important in x-ray diffraction, but is uncorrelated with the neutron
scattering factor for the nuclei. When you say heavy, do you mean the
highly scattering ones for neutrons, or the high-Z ones?
- Ku
Kurt,
I mean heavy in the atomic number.
Liliana
-Original Message-
From: Kurt Leinenweber [mailto:[EMAIL PROTECTED]
Sent: Thu 2/26/2004 2:21 PM
To: [EMAIL PROTECTED]
Cc:
Subject: RE: Re: thermal parameters at low temperature
Maxim,
I looked quickly in GSAS manual and I colud not find the path of
setting the absorption correction. Do you know the way of fixing these
terms, or is there anybody out there willing to tell me the way of
doing it?
Thank you very much,
Liliana
-Original Message-
In EXPGUI it is in the "Histogram" panel. Or, in "Expedt" go to l -
o(overall)-a(absorption). Choose function number 0 (Debye-Scherrer)...
>
>I looked quickly in GSAS manual and I colud not find the path of
>setting the absorption correction. Do you know the way of fixing these
>terms, or is ther
At 16:28 26/02/2004, Jon Wright wrote:
>Broadly, this means "crappy" data with a chemically unreasonable model can sometimes
>give much better figures of merit than a good structure with "good" data.
True, with enough parameters you can fit an elephant, but the calculated errors in
your structur
Sorry for the ambiguous information I gave previously. The data at
room temperature were taken with a variable wavelength (TOF) whether
at low temperature the data were recorded with constant wavelength.
There is no magnetic contribution on the pattern. The negative thermal
parameters are for the h
Nandini,
GSAS gives R(F^2) which is sometimes defined for single crystal structure analysis. It
can be used in the same way as RBragg. Neither is statistically related to the
minimization process in a Rietveld least squares refinement, but can be a useful
diagnostic for the quality of the struct
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