Re: pseudosymmetry

[Andreas Leineweber]

Dear Patrick, dear Friedrich, the problem of pseudo(metric)symmetry leads to severe and systematic overlap of  non-equivalent reflections in powder diffraction patterns. These reflections could be measured  independently from a (non-twinned ) single crystal. The same problem exists in a non-avoidable way for low-symmetry Laue classes of a certain crystal system, e.g. 4/m in the tetragonal system. Thus the number of independent determinable reflections (e.g. by a Pawley fit) is lower than the number calculated by "counting the reflection markers". Often these overlapping reflections may even contain the decisive information about the deviation from the higher symmetry. I think at least some of the aspects are dealt with in the following papers: The number of good reflections in a powder pattern D. S. Sivia J. Appl. Cryst. (2000). 33, 1295±1301 On the number of independent reflections in a powder diffraction pattern W. I. F. David J. Appl. Cryst. (1999). 32, 654±663 Best regards Andreas Mercier, Patrick wrote: Has anyone provided comments on Friedrich W. Karau's recent e-mail: " the Rietveld refinement of pseudosymmetric structures can cause some artefacts. Does someone know some literature, where these artefacts are explained in detail? Especially problems of the Rietveld refinement of pseudosymmetric sodalites (pseudo centered structures, pseudo cubic structures, significant deviation of bond length (...)) are welcome." I would also be interested by this topic. Best regards, -Patrick ------ Patrick H.J. Mercier, Ph.D., P.Phys., ing. jr Canadian Government Laboratory Visiting Fellow National Research Council Canada Institute for Chemical Process and Environmental Technology 1200 Montreal Road, Building M-12, Room 211 Ottawa, Ontario, Canada K1A 0R6 Tel: (613) 993-7752 Fax: (613) 991-2384 E-mail: [EMAIL PROTECTED] -----Original Message----- From: Friedrich W. Karau [mailto:[EMAIL PROTECTED]] Sent: Monday, November 22, 2004 8:51 AM To: [EMAIL PROTECTED] Dear all, the Rietveld refinement of pseudosymmetric structures can cause some artefacts. Does someone know some literature, where these artefacts are explained in detail? Especially problems of the Rietveld refinement of pseudosymmetric sodalites (pseudo centered structures, pseudo cubic structures, significant deviation of bond length (...)) are welcome. Further I am interested in your opinion about the "Distance Least Squares Procedure" to avoid these artefacts. Thank you in advance Friedrich -- Dr. Andreas Leineweber Max-Planck-Institut fuer Metallforschung Heisenbergstrasse 3 70569 Stuttgart Germany Telephone: +49 (0)711 689 3365 Telefax: +49 (0)711 689 3312 [EMAIL PROTECTED] http://www.mf.mpg.de/de/abteilungen/mittemeijer/english/index_english.htm