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
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Kurt Leinenweber
Department of Chemistry
Arizona State University
Tempe, AZ 85287-1604
Phone: (480)-965-8853
Fax: (480)-965-2747
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-----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 1е 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
