Davide, Could you perform Rietveld refinement using combined neutron and x-ray data? This would increase accuracy and the question what is better would disappear. Basically all you need is both data done at the same temperature. Also look at neutron and x-ray scattering factors. The neutron scattering is highest for Fe, lowest for Al and about the same for others, while x-ray scattering is again highest for FE but lowest for O and about the same for Mg, Al & P. Note there is significant fluorescent scattering for Fe when Cu-radiation is used. Best regards,
Peter Zavalij Director, X-ray Crystallographic Center Department of Chemistry and Biochemistry 091 Chemistry Building University of Maryland College Park, MD 20742-4454 Phone: (301)405-1861 Lab: (301)405-1861 Fax: (301)314-9121 E-mail: [EMAIL PROTECTED] http://www.chem.umd.edu/facility/xray/ ________________________________ From: Davide Levy [mailto:[EMAIL PROTECTED] Sent: Sunday, March 09, 2008 5:53 AM To: rietveld_l@ill.fr Subject: Neutron or X-ray? Dear All, I have a ”philosophic” question for you: the structure obtained by NPD are better than the ones found by XRPD? I explain a little bit more my question, I studied a MgAlFeO4 spinel both by NDP and XRPD from RT to 1000°C. The O coordinate it is different for two data set (0.2581 vs.0.25953). The found error on the oxygen coordinate is about 10-5 and 10-4 for NPD and XRPD data respectively. The coordinate calculated from cation partitioning and cation radii is similar to the XRPD result. At the moment I did not found a justification of this and I do not know which result is the ‘right one’. Do you have any suggestion? Thank you Davide
