Re: Uiso constraints

[Brian H. Toby]

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  ********************************************************************