Negative U's in Rietveld can arise from several causes, so that there is not one single answer. Some of the reasons are 1. The structural model is simply incorrect. 2. High absorption means that the low-angle data are weaker than they should be, or conversely that the high-angle data appear stronger than they should be. Abnormally strong high-angle data give rise to a decrease in U's, even making them appear negative 3. Correlation between the refinement parameters. For instance U's will tend to be highly correlated with site occupation parameters, often making it difficult to separate them. 4. In general, many of the errors that one encounters tend to end up in the refined U's, and this is why their precise values have to be treated with caution. Rietveld refinement (as opposed to single-crystal refinement) is in fact refinement of degraded data (it is one-dimensional instead of three-dimensional) and so the errors will be more significant.
Mike Glazer -----Original Message----- From: [email protected] [mailto:[email protected]] Sent: 03 March 2010 16:08 To: [email protected] Subject: Negative Uiso in GSAS Dear all, could someone explain to me the meaning of obtaining a negative Uiso in GSAS? I thought it was always positive...(p. 123 manual) thanks Carolina _-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_- Dr. Carolina Ziparo Istituto dei Sistemi Complessi - sezione di Firenze, C.N.R. - Consiglio Nazionale delle Ricerche via Madonna del Piano, 10 I-50019 Sesto Fiorentino Italy tel.: +39 055 5226693 fax: +39 055 5226683 e-mail: [email protected]
