Date: Thu, 17 May 2001 13:12:52 -0400 (EDT) From: Xiangyun Qiu xiangyun@pa.msu.edu To: rietveld_l@ill.fr Subject: One GSAS question. Good morning! To make it short, I am using space group P1 as an additional magnetic phase for manganite neutron TOF data Rietveld refinement, I still want to maintain the atomic position symmetry as I4/mmm. Can you help me how to create a constraint which makes all the atomic positions have the same shift? ( I failed after long time trial). BTW: Can I set the limits of thermal parameters and magnetic moments? Thanks a lot in advance, Xiangyun Qiu ********************* Xiangyun Qiu Dept. of Physics & Astronomy MSU East Lansing, MI Http://www.msu.edu/~qiuxiang ********************* |
From: "Radaelli, PG (Paolo) " P.G.Radaelli@rl.ac.uk To: "'rietveld_l@ill.fr'" rietveld_l@ill.fr Subject: RE: One GSAS question. Date: Thu, 17 May 2001 18:18:54 +0100 Dear Xiangyun Qiu, the easiest thing is to create a new purely magnetic phase containing only Mn and with P1 symmetry. Since Mn is in a special position, you don't need to refine the coordinates. You'll have to constrain the reciprocal lattice tensor parameters and the phase fraction between the two phases. The latter are inversely proportional to the cell volume. Tricky but a good exercise... Paolo Radaelli |
From: Xiangyun Qiu xiangyun@pa.msu.edu To: "'rietveld_l@ill.fr'" rietveld_l@ill.fr Subject: RE: One GSAS question. Thanks a lot for the answer! Apologize for not making myself very clear. Actually I am refining some layered manganites data, I have non special postion Mn atoms(4 in one unit cell for the magnetic phase of Space group P1), I need to make them in the same position as the Mn in the atomic structure phase(only one in the unit cell (I4/mmm) ). Thanks again! Best, Xiangyun Qiu On Thu, 17 May 2001, Radaelli, PG (Paolo) wrote: > Dear Xiangyun Qiu, > > the easiest thing is to create a new purely magnetic phase containing only > Mn and with P1 symmetry. Since Mn is in a special position, you don't need > to refine the coordinates. You'll have to constrain the reciprocal lattice > tensor parameters and the phase fraction between the two phases. The latter > are inversely proportional to the cell volume. Tricky but a good > exercise... > > Paolo Radaelli > Xiangyun ********************* Xiangyun Qiu Dept. of Physics & Astronomy MSU East Lansing, MI Http://www.msu.edu/~qiuxiang ********************* |
Date: Fri, 18 May 2001 10:03:02 +0200 To: rietveld_l@ill.fr From: Jonathan WRIGHT wright@esrf.fr Subject: RE: One GSAS question. Better to work with a single phase and use the red and black symmetry operators, in my opinion. You might have to lower the symmetry for some magnetic structures and add contraints on the positions accordingly, but this saves a lot of headaches, even if it creates a few by making you think about the symmetry of the magnetic structure. Prodd and fullprof both handle magnetic structures and multipattern ToF data with slightly more flexibility than GSAS in terms of the kinds and symmetry of the structures allowed. If you insist on P1 then just put all the coordinates which are related into the same constraint. The fourfold axis will relate x and y for the atoms in the P1 space group to x or y from the tetragonal. However, if you have a fourfold site in I4/mmm then it's either 0,1/2,0; 0,1/2,1/4 or 0,0,z and you only need to constrain z for the latter and fix x and y. In expedt the constraint would be something like: 1 z 1 1 ! Phase one (I4/mmm), z co-ord, atom one, value 1 2 z 1 1 ! Phase 2 (P1) atom at 0,0,z 2 z 2 -1 ! Phase 2 atom at 0,0,-z 2 z 3 1 ! Phase 2 atom at 1/2,1/2,z+1/2 2 z 4 -1 ! Phase 2 atom at 1/2,1/2,-z+1/2 Essentially theres a line added for each thing you want to constrain together. It gets a bit hair raising if you try to put the same variable in two different constraints - you can (and should) always put all the related variables into the same one, this avoids confusion with disappearing or 'ignored' constraints. Remember to enter fixes (expedt; L A F) for the x and y co-ordinates. I've assumed the magnetic cell is the same size as the crystallographic. So it could be either ferromagnetic or antiferromagnetic and the body centring involves time reversal, eg I4/mmm with k=(0,0,1). Or some evil combination of the two :) For "simple" structures you can get away with just refining the atomic positions from the nuclear phase and updating them in the magnetic phase. The magnetic model won't have a big impact on the crystal structure if you have good (any?) data at short d-spacings, unless you are looking for small structural distortions. If so, then it is indeed a tricky problem. Best of luck, Jon |