Is it possible the data was collected with variable slit aperture? You could try to convert the data to fixed slit aperture intensities and see what happens to the B values.
Regards, Jeff Nicolich W.R. Grace & Co. - Conn. 62 Whittemore Ave, Cambridge, MA 02140 +1-617-498-3816 [email protected] -----Original Message----- From: Alan Hewat [mailto:[email protected]] Sent: Wednesday, February 10, 2010 5:43 AM To: Jacco van de Streek Cc: [email protected]; [email protected] Subject: Re: Negative Biso / absorption in TOPAS. > 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/[email protected]/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 <[email protected]> +33.476.98.41.68 http://www.NeutronOptics.com/hewat ______________________________________________
