....of course, we could all start refining corrected S(Q) instead of raw
data. This is less convenient, but not outrageously so with modern
computing. A bit of work on the software side would also make it more
convenient if there is interest in the community.
Simon
Brian H. Toby wrote:
I would like to know whether GSAS is able to constraint the Uiso value
so that it will be always bigger than zero.
GSAS does not have the ability to do that.
Personally, I would prefer that people not use constraints or restraints
to prevent negative displacement parameters (or even worse, have the
ability to constrain displacement factors from being too large). Both
overly large and negative atomic displacement factors are
providing diagnostic information about the quality of a refinement:
"turn it off" at the peril of missing a problem. A negative Uiso means
that there is too much scattering coming from a site. It can indicate
site substitution or vacancies are present.
If Uiso values are negative, but within 1 or 2 sigma of zero, this can
be ignored. The values are not statistically distinguishable from
small positive values.
A common, but minor reason why negative displacement values occur is due
to ambiguity in the placement of the background at high Q (angle). The
data are insufficient to determine Uiso and a background so that Uiso
values refine very low or very high. This is good to know, since the
Uiso values are then basically arbitrary (which one should note in a
publication). My usual cure is to set all Uiso values to reasonable
numbers (depending on the type of material, temperature, etc.), refine
the background, then fix the background and refine the Uiso values.
Brian
********************************************************************
Brian H. Toby, Ph.D. office: 630-252-5488
Materials Characterization Group Leader, Advanced Photon Source
9700 S. Cass Ave, Bldg. 433/D003 work cell: 630-327-8426
Argonne National Laboratory secretary (Kristy): 630-252-5453
Argonne, IL 60439-4856 e-mail: brian dot toby at anl dot gov
********************************************************************
--
Prof. Simon Billinge
Department of Physics and Astronomy
4268 Biomed. Phys. Sciences Building
Michigan State University
East Lansing, MI 48824
tel: +1-517-355-9200 x2202
fax: +1-517-353-4500
email: [EMAIL PROTECTED]
home: http://nirt.pa.msu.edu/
