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.
I hope that this is a bit clearer than the first e-mail.
Thank you,
Liliana
-----Original Message-----
From: Andreas Leineweber [mailto:[EMAIL PROTECTED]
Sent: Thu 2/26/2004 1:06 PM
To: [EMAIL PROTECTED]
Cc:
Subject: Re: thermal parameters at low temperature
Dear Liliana,
of course the remarks of Maxim are correct, the experimental
conditions are very important.
Just additionally:
Very important is also the range of d* which is covered
experimentally for the accuracy of the displacement parameters.
Of importance is also the mass of the atom. Heavy atoms (in a
soft portential) can get really small ADP whereas for light atoms a
considerable ADP should even be found at 0 K (Debye theory).
Andreas
> Marilena L Viciu schrieb:
>
> Dear all,
>
> <?xml:namespace prefix = o ns =
"urn:schemas-microsoft-com:office:office" />
>
> I am refining a powder neutron data at low temperature
(50K). I got some of the thermal parameters negative although the
model seems to be quite right (the data at room temperature behaved
well with the same model). From your experience, is this the case of a
wrong model?
>
>
>
> I really appreciate any comment!
>
> Thank you!
>
> Liliana
--
Dr. Andreas Leineweber
Max-Planck-Institut fuer Metallforschung
Heisenbergstrasse 3
70569 Stuttgart
Germany
Tel. +49 711 689 3365
Fax. +49 711 689 3312
e-mail: [EMAIL PROTECTED]
home page of department:
http://www.mf.mpg.de/de/abteilungen/mittemeijer/english/index_english.
htm
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