Liliana, You can calculate the absorption coefficient for your CW neutron experiment. GSAS wants mu*R/lambda. Find a table of "true" neutron absorption cross sections for your elements & calculate mu from them in cm-1. R is the radius of your sample can in cm. Divide by 2 to correct for packing efficiency. Finally divide by your CW neutron wavelength. Put it into GSAS but don't refine it. Your scale factor will shift and so will the Uiso's. Bob Von Dreele
-----Original Message----- From: Marilena L Viciu [mailto:[EMAIL PROTECTED] Sent: Thu 2/26/2004 3:14 PM To: [EMAIL PROTECTED] Dear Dr Von Dreele, The structure at room temperature was refined from neutron TOF. Now, I am using this model to refine low temperature data from neutron CW. My sample has Cu-Cl-La-Nb-O. Sincerely, Liliana -----Original Message----- From: Von Dreele, Robert B. [mailto:[EMAIL PROTECTED] Sent: Thu 2/26/2004 2:59 PM To: [EMAIL PROTECTED] Cc: Liliana, GSAS will refine the value of the absorption coefficient for neutron TOF data quite easily (I recall one message of yours mentioned that your data was of this "flavor"). Just turn on the flag. Your Uiso's should rise for all atoms making those for the heavy atoms positive. The ABS value will be a small positive value. You didn't mention the composition but there must be a neutron absorber of some sort in your sample. BTW one always should refine ABS for neutron TOF data - it is almost always there (might be negative since the incident spectrum is frequently not corrected for it's own absorption). Bob Von Dreele -----Original Message----- From: Marilena L Viciu [mailto:[EMAIL PROTECTED] Sent: Thu 2/26/2004 2:30 PM To: [EMAIL PROTECTED] Maxim, I looked quickly in GSAS manual and I colud not find the path of setting the absorption correction. Do you know the way of fixing these terms, or is there anybody out there willing to tell me the way of doing it? Thank you very much, Liliana -----Original Message----- From: Maxim V. Lobanov [mailto:[EMAIL PROTECTED] Sent: Thu 2/26/2004 1:59 PM To: [EMAIL PROTECTED] Cc: >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 1? 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
