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Tutorials and Examples

LHPM-Rietica Rietveld for Win95/NT by Brett Hunter

Setting atom position constraints in LHPM-Rietica [x, 2x, z sites] (relevant to DBWS, Fullprof, as well)

The CCP14 Homepage is at http://www.ccp14.ac.uk
[Tutorials page] | [LHPM-Rietica Tutorial Index]

LHPM-Rietica Rietveld (like DBWS and Fullprof Rietvelds) does not automatically handle symmetry constraints. Instead the user must manually implement the symmetry constraints. The following gives an example of how to do this with x, 2x, z positions.

Click here to download the ZIPPED example data file and input file.

In the following screen dump, the O1 and O2 atoms are both x, 2x, z positions. To correctly refine the Oxygen atom positions, y must shift double to that of x to satisfy the constraints of symmetry for this particular Titanate structure.

Structure where the O1 and O2 are x 2x z positions

To set the atom shift constraints via the GUI (Graphical User Interface), first the atom positions must have the refinement flag set (via the Model, Phases menu).

Thus the following screen image shows the atom position flags being set to refine for the Oxygens.

Note that the 0.25 z position for O1 is not set to refine as this is a special position. You would normally check in the IUCr International Tables as to find out the symmetry constraints of each position.

Structure where the O1 and O2 are x 2x z positions

To set x, 2x atom constraints, now enter Model, Constraints. Click and drag via the Mouse is now used to set the constraints from the list of refined parameters.

Model Constraints dialog windows

As an example for O1 (Oxygen 1).

  1. Click and drag the P 1: x(O1) from the top-middle box to the bottom left box.

  2. Then click and drag the P 1: y(O1) from the top-middle box to the bottom left box.

However, we are not finished as this sets a linear 1:1 (x, x, z) constraint on the two parameters whereas we need a 1:2 constraint (x, 2x, z).

Model Constraints dialog windows

Now, in the bottom right /value box. Set this to 2. This has now set the x, 2x, z constraint.

Model Constraints dialog windows

Now do the same for the O2 position.

Model Constraints dialog windows

If you manually peruse the INP input file; you will see the 11/12 code words as the 21/22 codewords that show how these constraints are represented in the input file. (near the bottom of the input file in bold) 

Oxygen Deficient Titanate                                                       
   0   6   1  -5   0   0   0   0   0   1   0   0
0011100002120010000    0.000   0.000 0.00000
 1.54051 1.54433 0.50000    1.00  7.0000  1.0000    0.00 50.0000  0.0000   0.000
  300.100.900.900.900.90  15.000   0.025 150.000   0.000   0.000
       4
   0.000   0.000
 -0.047648   0.000
  131.700000   -1.085000    0.001540   0.0000E+0   0.0000E+0   0.0000E+0
       0.000       0.000       0.000       0.000       0.000       0.000
A new phase                                                                     
   8     1   0.0 0.0 1.0         0.0 0.0 1.0
P 63/M M C          
BA1 BA           0.00000 0.00000 0.25000 0.60000 0.08333
 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
BA2 BA           0.33333 0.66667 0.09104 0.00000 0.16667
 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
TI1 TI           0.00000 0.00000 0.00000 0.00000 0.08333
 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
FE1 FE           0.00000 0.00000 0.00000 0.00000 0.00000
 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
TI2 TI           0.33333 0.66667 0.84793 0.00000 0.00000
 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
FE2 FE           0.33333 0.66667 0.84793 0.00000 0.16667
 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
O1  O            0.52066 0.04117 0.25000 0.00000 0.50000
 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
O2  O            0.83423 0.66846 0.08313 0.00000 0.50000
 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
2.105470E-04      0.0000
 0.00900 0.00680 0.00190 0.00000
  5.6925  5.6925 13.9557 90.0000 90.0000120.0000
 1.00000 0.00000 0.06624 0.00000
 0.54430 0.00250 0.00000 0.00000
   0.000   0.000   0.000   0.000   0.000
   0.000   0.000   0.000   0.000   0.000   0.000
   0.000   0.000   0.000   0.000   0.000
   0.000   0.000   0.000   0.000   0.000   0.000
   0.000   0.000   0.000   0.000   0.000
   0.000   0.000   0.000   0.000   0.000   0.000
   0.000   0.000   0.000   0.000   0.000
   0.000   0.000   0.000   0.000   0.000   0.000
   0.000   0.000   0.000   0.000   0.000
   0.000   0.000   0.000   0.000   0.000   0.000
   0.000   0.000   0.000   0.000   0.000
   0.000   0.000   0.000   0.000   0.000   0.000
  11.000  12.000   0.000   0.000   0.000
   0.000   0.000   0.000   0.000   0.000   0.000
  21.000  22.000  41.000   0.000   0.000
   0.000   0.000   0.000   0.000   0.000   0.000
  31.000           0.000
   0.000   0.000   0.000   0.000
   0.000   0.000   0.000   0.000   0.000   0.000
   0.000   0.000   0.000   0.000
   0.000   0.000   0.000   0.000

Now that these atom positions are appropriately constrained, you can refine them.

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