With regard to "constrained" refinement: Should one first refine lattice parameters and then "fix" them before proceeding with structure refinement? As I see it, lattice parameters are directly observable in the xrd pattern; if one allows lattice parameters to change during structure refinement, they will no longer be correct. I got this notion from using Vegard's Law to establish stoichiometry of electronic materials over 30 years ago. Please enlighten me if this is not correct. Thanks. Frank May Research Investigator Department of Chemistry - Benton 315 University of Missouri - St. Louis One University Boulevard St. Louis, Missouri 63121-4499 314-516-5098
________________________________ From: Alan Hewat [mailto:[EMAIL PROTECTED] Sent: Mon 7/3/2006 9:52 AM To: rietveld_l@ill.fr; rietveld_l@ill.fr >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 _____________________________________________________________
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