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:
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 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 1Ð 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
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