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Tutorials and Examples

LMGP suite for Windows by Jean Laugier and Bernard Bochu

(Laboratoire des Materiaux et du Génie Physique de l'Ecole Supérieure de Physique de Grenoble http://www.inpg.fr/LMGP/)

Basic Demonstration of celref Unit Cell Refinement software on a Monoclinic singlephase system

The CCP14 Homepage is at http://www.ccp14.ac.uk

[Tutorials page] | [LMGP Suite Index]

CELREF: Graphical Unit Cell refinement.

7th September 1999 E-mail from Jean Laugier
"Generally I should like to make some remarks about the cell parameters measurements:
-The precision of the refinement of the cell parameters are not proportionnal to the number of peaks treated by the program. It is always better, when it is possible, to use peaks above 45°, mainly because the vertical divergence changes at this value. For a cubic structure, for example, it is better to measure only one peak at the maximum Theta angle than to use Celref with a lot of peaks, if the zero shift is weak or known!
-In the Least Square method it is assumed that the errors are random and in an experimental diagram the errors are totally systematic! It is necessary to be suspicious about the parameters precisions given by the program.
-The mathematical method does not work very well with cubic structures, if the starting a value is very different of the true value. it is always better to refine the zero shift in this case. (The batch bersion works in ILL since 20 years and nobody saw that! Probably they never refine cubic cells!).
Perhaps it would be necessary to make a special algorithm for the cubic case..."


CELREF can be obtained off the web at the the LMGP (Laboratoire des Materiaux et du Génie Physique de l'Ecole Supérieure de Physique de Grenoble http://www.inpg.fr/LMGP/) program suite site at http://www.ccp14.ac.uk/ccp/web-mirrors/lmgp-laugier-bochu/


The example data file is of monoclinic Zirconia used in the Rietveld Round Robin (Hill and Cranswick)
Wavelength:  1.54056 1.54439   (Cu K alpha - Philips Diffractometer)
Monoclinic ZrO2
SG: P 21/C  
Cell:  5.1471  5.2129  5.3141 90.0000 99.2224 90.0000
ZR  ZR+4         0.27566 0.04011 0.20906 0.08229 0.98650
HF  HF+4         0.27566 0.04011 0.20906 0.08229 0.01350
O1  O-2          0.07027 0.33247 0.34197 0.72209 1.00000
O2  O-2          0.45011 0.75639 0.47740 0.59420 1.00000


Click on the CELREF icon or run the program via the windows explorer/windows file manager to bring up the CELREF starting interface.


  • You can then open a Powder diffraction data file using either the File, Open, Profile menu option or the the bottom left hand set of ICONs (Celref can presently use Siemens/Bruker RAW, CPI or RIET7 DAT raw datafiles). In this case we open a mzro2.dat file. CELREF will prompt for the Wavelength of the peak file. Select Other and type in the wavelength: 1.54056 Angstrom (Cu k Alpha 1).


  • Either type the reflections in by hand or open a peak find/peak profile file either via the File, Open, Peaks File or the ICON under the Measured Reflections tab. The peaks will be displayed on the bottom window. (Celref can presently import Siemens/Bruker DIF, Winfit *.DAT, or XFIT *.TXT peak find/peak profile output files). If loading an XFIT or related file (in this case we open a Y2O3.dat file), when prompted for wavelength select OTHER and confirm the wavelength being used (1.54056 Angstrom) and continue.


  • Select the Initial Cell Parameters tab.
    Input the (approximate) Cell, Spacegroup and calculation range information then press the Calc Button to generate the HKL listing. It is also possible to load this from a file if this information exists. Just click the File tab under the "Selected" Initial Cell Parameters tab and load the relevant *.cry file).


  • Select the Selection of the Reflections tab.
    To select the corresponding peaks, this can be done manually or automatically (with a controllable angular tolerance). Click on the Automatic Selection button to automatically select the corresponding peaks. Check both the top peak list and the bottom graphical representation to see that the selection has occured appropratiately. Peaks can be added or deleted manually. Be careful to check that Celref hasn't duplicated HKLs due to poor quality input data.


  • Select the Cell Refinement tab.
    At this point, recheck that the wavelength is correct, CELREF may be using the average alpha wavelength when you are profiled the peaks on the Alpha-1 wavelength. If you wish to change the wavelength, you can do this in the top left Current Values option area and click on the Red Change button.

    Click on the Compute button to perform the unitcell refinement. Two-theta offset can also be refined. If you click on this, Celref will prompt whether you wish to use model a constant zero shift or a sample exentricity. If you keep the unitcell constants at known values, the wavelength can be refined.


  • You can see all the results in a file using the Print ICON. Which is normally outputted in a file called celref.out

    ----------------------------------------------------
    No Title
    ----------------------------------------------------
     Number of reflections     :    21
     Max. number of refinement cycles   10
     Refinement constraints    : NO   
    
    Initial values    :
       Zero  Lambda     a       b       c     alpha   beta  gamma
     .00000  1.54056  5.1471  5.2129  5.3141  90.00  99.22  90.00
       1.    0.       1.      1.      1.       0.     1.     0.
    
    Recipr.lattice :   .1968   .1918   .1906  90.00  80.78  90.00
    Volume (a**3)  :   140.741
    
     Number of refinement cycles       :     5
    
    
    Final values      : (Standard errors on 2nd line)
       Zero  Lambda     a       b       c     alpha   beta  gamma
     .00174  1.54056  5.1491  5.2140  5.3165  90.00  99.23  90.00
     .00059   .00000   .0002   .0002   .0002    .00    .00    .00
    
    Recipr.lattice :   .1968   .1918   .1906  90.00  80.77  90.00
    Volume (a**3)  :   140.887
    
        H    K    L   2Th(obs)   2Th-Zero    2Th(Calc)     diff.
    
        1    0    0   17.532      17.530      17.532       -.002
        0    1    1   24.143      24.141      24.138        .003
        1    1    0   24.537      24.535      24.535        .000
       -1    1    1   28.270      28.268      28.263        .005
        1    1    1   31.550      31.548      31.552       -.004
        0    0    2   34.242      34.240      34.241       -.001
        0    2    0   34.464      34.462      34.468       -.006
        2    0    0   35.388      35.386      35.387       -.001
       -1    0    2   35.988      35.986      35.979        .007
        0    2    1   38.629      38.627      38.625        .002
        2    1    0   39.511      39.509      39.510       -.001
       -2    1    1   40.811      40.809      40.806        .003
        1    0    2   41.230      41.228      41.233       -.004
       -1    2    1   41.449      41.447      41.452       -.005
        1    1    2   44.906      44.904      44.903        .002
        2    1    1   45.599      45.597      45.593        .004
       -2    1    2   48.992      48.990      49.003       -.013
        0    2    2   49.327      49.325      49.327       -.002
        2    2    0   50.186      50.184      50.183        .001
       -1   -2    2   50.638      50.636      50.631        .005
       -2    2    1   51.269      51.267      51.262        .005
    
    Sqrt(Sum(Th O-C)**2)/(Nref-Npar)) =    .001343
    
    Sqrt(Sum(2Th O-C)**2)/(Nref-Npar)) =    .000671


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