Dear Mario,
Negative isotropic displacement parameters are physically meaningless and are
indicative of problems in either the model or the data (or both).
Possible reasons:
low sensitivity of refinement to displacement parameters due to insufficient
data quality;
incorrect refinement of peak shape and/or FWHM parameters;
improperly corrected or neglected 2teta-dependent alterations of diffraction
intensities due to instrument/sample misalignment, polarization, absorption,
surface roughness etc.;
unaccounted partial occupation of atomic positions by heavier atoms.
The list of possible reasons is incomplete, of course……
Best regards,
Leonid
*******************************************************
Leonid A. Solovyov
Institute of Chemistry and Chemical Technology
K. Marx 42
660049, Krasnoyarsk
Russia
Email: [EMAIL PROTECTED]
www.icct.ru/eng/content/persons/Sol_LA
www.geocities.com/l_solovyov
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--- On Tue, 9/23/08, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote:
> From: [EMAIL PROTECTED] <[EMAIL PROTECTED]>
> Subject: Beta iso
> To: Rietveld_l@ill.fr
> Date: Tuesday, September 23, 2008, 3:34 PM
> Dear all rietvelders,
>
> Can I obtain negative value of Beta isotropic?
>
> I have obtained good value of refinements with negative
> value of Beta
> isotropic. Is that possible? I mean, the Beta isotropic
> depend of
> quadratic displacement of the atom.
>
> Thanks a lot for your help,
>
> Mario Macías
> UIS, Colombia
