ï
Liliana,
OK,
they should vibrate less... but are they good neutron scatterers or poor?
If they are poorly scattering, the thermal factors won't be as
meaningful.
- Kurt
-------
Kurt Leinenweber Department of
Chemistry Arizona State University Tempe, AZ
85287-1604
Phone: (480)-965-8853 Fax:
(480)-965-2747
-------
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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