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
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
-------
Kurt Leinenweber
Department of Chemistry
Arizona State University
Tempe, AZ 85287-1604
Phone: (480)-965-8853
Fax: (480)-965-2747
-------
-----Original Message-----
From: Maxim V. Lobanov [mailto:[EMAIL PROTECTED]
Sent: Thursday, February 26, 2004 12:59 PM
To: [EMAIL PROTECTED]
>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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