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Winplotr Graphics and Fullprof Rietveld (and Single Crystal) Software

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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Via Rietveld Mailing List

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

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
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.


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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