>A common, but minor reason why negative displacement values occur is due to >ambiguity in the placement of the background at high Q (angle).
Another common reason why Uiso is low is because of the neglect of absorption. (We have been through this many times - see the Rietveld archive eg http://www.mail-archive.com/rietveld_l@ill.fr/msg01807.html ) >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. Hmm. That sounds doubtful for the good reasons you gave for not using constraints to "fix" parameters whose refined values seem wrong. The calculated errors are only meaningful if you don't use such constraints. Alan. _____________________________________________________________ Dr Alan Hewat, ILL Grenoble, FRANCE<[EMAIL PROTECTED]>fax+33.476.20.76.48 +33.476.20.72.13 (.26 Mme Guillermet) http://www.ill.fr/dif/AlanHewat.htm _____________________________________________________________
