>Sorry for the ambiguous information I gave previously. The data at >room temperature were taken with a variable wavelength (TOF) whether >at low temperature the data were recorded with constant wavelength. >There is no magnetic contribution on the pattern. The negative thermal >parameters are for the heaviest atoms in the crystal. > Then it seems that you really overlooked some instrumental issue that can affect thermal factors. As pointed out by Andreas, indeed it would be most critical for heavy atoms, with intrinsically low thermal factors at low T. For example, absorprion. It is typically not very significant for neutrons, but anyway I would calculate it (for example, using http://www.ncnr.nist.gov/instruments/bt1/neutron.html), then fix in the refinement and look if it would "cure" the thermal factors.
According to GSAS manual, "For constant wavelength data the absorption coefficient, Ah, is related to the value for 1е neutrons; the correction is indistinguishable from thermal motion effects and should not be refined. " By the way, I would be grateful if one could share the knowledge about some other important instrumental factors, relevant for thermal factors in neutrons, and ways to estimate reasonable correction values. Sincerely, Maxim. __________________________________ Maxim V. Lobanov Department of Chemistry Rutgers University 610 Taylor Rd Piscataway, NJ 08854 Phone: (732) 445-3811
