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-----
From: Von Dreele, Robert B. [mailto:[EMAIL PROTECTED]
Sent: Thu 2/26/2004 2:59 PM
To: [EMAIL PROTECTED]
Cc:
Subject: RE: Re: thermal parameters at low temperature
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 will be a small positive value. You didn't
mention the composition but there must be a neutron absorber of some
sort in your sample. BTW one always should refine ABS for neutron TOF
data - it is almost always there (might be negative since the incident
spectrum is frequently not corrected for it's own absorption).
Bob Von Dreele
-----Original Message-----
From: Marilena L Viciu [mailto:[EMAIL PROTECTED]
Sent: Thu 2/26/2004 2:30 PM
To: [EMAIL PROTECTED]
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-----
From: Maxim V. Lobanov [mailto:[EMAIL PROTECTED]
Sent: Thu 2/26/2004 1:59 PM
To: [EMAIL PROTECTED]
Cc:
>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 heaviest atoms in the crystal.
>
Then it seems that you really overlooked some instrumental
issue that can
affect thermal factors. As pointed out by Andreas, indeed it
would be most
critical for heavy atoms, with intrinsically low thermal
factors at low T.
For example, absorprion. It is typically not very significant
for neutrons,
but anyway I would calculate it (for example, using
http://www.ncnr.nist.gov/instruments/bt1/neutron.html), then
fix in the
refinement and look if it would "cure" the thermal factors.
According to GSAS manual,
"For constant wavelength data the absorption coefficient, Ah,
is related to
the value for 1? neutrons; the correction is indistinguishable
from thermal
motion effects and should not be refined. "
By the way, I would be grateful if one could share the
knowledge about some
other important instrumental factors, relevant for thermal
factors in
neutrons, and ways to estimate reasonable correction values.
Sincerely, Maxim.
__________________________________
Maxim V. Lobanov
Department of Chemistry
Rutgers University
610 Taylor Rd
Piscataway, NJ 08854
Phone: (732) 445-3811
<<winmail.dat>>
