> 1. Is absorption indeed the most likely cause of this problem or are > there other possibilities? > > 2. How do we model absorption in TOPAS? Which keyword do we use and what > are reasonable values for the parameters in the expression?
1. You will find discussions of negative B-factors in the Rietveld list archive http://www.mail-archive.com/rietveld_l@ill.fr/info.html Possible causes cited for X-rays include polarisation correction, surface roughness and absorption. You really do need to know how the data were collected - Rietveld refinement should not be treated as a black box. 2. If B-factors are negative, it obviously means that higher order reflections are stronger than they should be. Absorption can cause this, since absorption will be greater for transmission (low angles). For a cylindrical sample it can be shown that moderate absorption has exactly the same effect on intensities as a negative contribution to the overall B factor, and indeed you can calculate the magnitude of the effect if you know the absorption coefficient. Again, use physical information rather than simply refining more parameters. _____________________________________________ Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE <alan.he...@neutronoptics.com> +33.476.98.41.68 http://www.NeutronOptics.com/hewat ______________________________________________
