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## What to do if the gamma angle in Hexagonal / Rhombohedral does not stay at 120 degrees

### (this affects most DBW derived software)

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 Date: Fri, 15 Jun 2001 10:51:40 +0200 To: rietveld_l@ill.fr From: "A. van der Lee" [avderlee@univ-montp2.fr] Subject: rhombohedral space groups in Fullprof Hi all, I have a problem concerning the refinement of the cell parameters of a rhombohedral space group (R-3c) in Fullprof. The documentation says to use the hexagonal description of the unit cell and that is what I do: a=b, c, alpha=90, beta=90, gamma=120. a and b are the refinable parameters, the angles should be kept fixed. The program tries nonetheless to refine the gamma angle although its code is set to zero. The output shows however that the associated standard deviation is equal to zero. The total number of parameters is four in profile matching mode with constant relative intensities (jbt=3): one scale factor, one zero error and two cell parameters (a and b). The profile parameters are kept fixed for the moment. Who recognises this problem? Thanks in advance. Arie van der Lee ************************************************** Dr. A. van der Lee Institut Européen des Membranes Université de Montpellier II - cc 047 2 place E. Bataillon 34095 Montpellier CEDEX 5 Visiting address: 240 Avenue du Professeur Emile Jeanbrau Montpellier Phone: (33) 467 14 9135 Phone: (alternative) (33) 467 14 9100 FAX: (33) 467 14 9119 **************************************************

 Date: Fri, 15 Jun 2001 11:09:36 +0200 To: rietveld_l@ill.fr From: Patrick Weisbecker [Patrick.Weisbecker@mines.u-nancy.fr] Subject: Re: rhombohedral space groups in Fullprof Hello Arie, You have to set the same parameter number for gama and a. for instance for Al2O3: ! a b c alpha beta gamma 4.771113 4.771113 13.027040 90.000000 90.000000 120.000000 81.00000 81.00000 91.00000 0.00000 0.00000 81.00000 gama is kept fixed to 120°. ------------------------------------------------------------------------ Patrick Weisbecker Service commun diffraction X LSG2M Ecole des Mines de Nancy Parc de saurupt 54042 Nancy email: patrick.weisbecker@mines.u-nancy.fr tel : 03 83 58 41 99 fax : 03 83 57 63 00

 Date: Fri, 15 Jun 2001 12:17:41 -0400 To: rietveld_l@ill.fr From: "R. A. Young" [r.young@physics.gatech.edu] Subject: Re: rhombohedral space groups in Fullprof Dear Dr. Lee: This sounds like a problem you would have with DBWS. In that progra, the solution is to assign the same codeword to gamma that a and b have. The mathematical reason(in DBWS) is that the calculation is done with reciprocal space vectors.You will have ht eame problemiwth a hexagonal space group. Sincerely, Ray Young Prof. R. A. Young School of Physics Georgia Institute of Technology Atlanta, GA 30332 USA Tel: 404-894-5208 FAX 404-894-9958 e-mail r.young@physics.gatech.edu

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