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
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 really
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
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
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,
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
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
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
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?
-
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
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
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