Negative Thermal parameters

[Marco Sommariva]

Dear All and Dear Gerard, I have a very little experience on Rietveld refinement, but my professor always says to me that a negative value of isotropic factor U has no phisical meaning, and it can be a sign that your (fixed) occupation factors (frac) are too small; in fact frac and U-iso are correlated in the formula of the structure factor; viceversa the fact that U-iso  factors become too big can mean that one has fixed too big frac values. Are you sure that your fracs are correct? Otherwise maybe you could fix your U-iso at a reasonable value (for example 0.01 or minor, it depends from the kind of ions) and refine the corresponding occupation factor frac. I hope this will help you, Greetings to all, marco         Marco Sommariva Department of Materials Science University of Milano - Bicocca Via R. Cozzi 53 - 20125 MILAN, ITALY Phone +39.02.6448.5141 (5142) Fax     +39.02.6448.5400 e-mail: [EMAIL PROTECTED]