Re: Defining the origin of P2/n

[Kurt Leinenweber]

Thanks Andreas.  I realized I was wrong when I drew a picture of how the glide plane and the 2-fold axis interact with the translational symmetry.  They make 4 points at +x +y +z, -x -y +z, +x +y -z, -x -y -z (2 pairs definitely related by a center of symmetry).  I should have done that before I posted to the list!   I am at home with newborn twins and have not had my hands on the blue book for several days.                               - Kurt ------- Kurt Leinenweber Department of Chemistry Arizona State University Tempe, AZ  85287-1604 Phone:  (480)-965-8853 Fax: (480)-965-2747 ------- -----Original Message----- From: Andreas Leineweber [mailto:[EMAIL PROTECTED] Sent: Wednesday, March 31, 2004 11:09 AM To: [EMAIL PROTECTED] Subject: Re: Defining the origin of P2/n This problem can actually quite simply be solved without tables: There is the old rule (which crystrallographers a definitely able to prove) to obtain the crystal class: 1.delete the Bravais type symbol (delete P in case of P2/n) 2.substitute screw axes by the correspondingly oriented simple rotations (we already have a rotation, 2(1) becomes 2) 3.substitute glide planes by mirror planes with the same orientation.  (2/n becomes 2/m) This gives you the symbol of the crystal class/crystallographic point group. Thus you can see that P2/n belongs to the crystal class 2/m, which has a centre of inversion. All space group types belonging to a centrosymmetric crystal class have one or several Wyckoff sites with centre of inversion. Therefore, there must be a position (there are several ones) with fixed z coordinate. Andreas Leineweber Kurt Leinenweber wrote: Yes, but the atom doesn't have to sit there, unlike in the case of 2/m. So, GSAS has to fix z for one atom... I don't have my International Tables with me, please correct me if I'm wrong (there should be no sites with a fixed z parameter in P2/n). - Kurt ------- Kurt Leinenweber Department of Chemistry Arizona State University Tempe, AZ 85287-1604 Phone: (480)-965-8853 Fax: (480)-965-2747 ------- -----Original Message----- From: Magnus H. Sørby [mailto:[EMAIL PROTECTED]] Sent: Wednesday, March 31, 2004 12:24 AM To: [EMAIL PROTECTED] Stephen and Kurt, P2/n has a 2-fold axis perpendicular to a glide plane, so it does have fixed points (unlike e.g. R3c). Best regards, Magnus H. Sørby -----Opprinnelig melding----- Fra: Kurt Leinenweber [mailto:[EMAIL PROTECTED]] Sendt: 31. mars 2004 08:55 Til: [EMAIL PROTECTED] Emne: Defining the origin of P2/n Stephen, I think that P2/n has no fixed points, and in order to keep the whole structure from shifting along the c-axis, one of the atoms has to be arbitrarily fixed. GSAS automatically fixes the first atom z-parameter (was your atom on the 2-fold symmetry axis?). I remember this from refining the structure of high pressure FeTiO3 which had space group R3c. - Kurt ------- Kurt Leinenweber Department of Chemistry Arizona State University Tempe, AZ 85287-1604 Phone: (480)-965-8853 Fax: (480)-965-2747 ------- -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] Sent: Tuesday, March 30, 2004 11:03 PM To: [EMAIL PROTECTED] Dear Rietvelders, I used GSAS to refine my structure in P2/n, however the program automatically fixed the the XYZ positions of the first atom during refinement. To my surprise, this atom is not seen in the "fixed atom list" in the atom parameters menu of GSAS. Please advise and many thanks, stephen