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Tutorials and Examples

LHPM-Rietica Rietveld for Win95/NT by Brett Hunter

Setting thermal constraints on shared occupancy sites

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[Tutorials page] | [LHPM-Rietica Tutorial Index]

When you have shared occupancies on a single site in LHPM-Rietica Rietveld, you must manually set the constraints so the atoms move identically. In the present versions of Rietica, shared atom positions will be constrained automatically, but you must constrain the thermal positions manually.

The following gives an example of how to do this with a shared occupancy site.

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

In the following screen dump, the Ti1 and Fe1 share a single site; and the Ti2 and Fe2 share another site. Thus any non-special atom positions would normally be made to move / shift together, as well as the B's (thermal parameters) as well. (special positions being special, are not set to refine)

The constraints to be set in this example are to constrain the thermals for each site as being the same. (The z position for the Ti2/Fe2 shared site will be constrained automatically by Rietica to move in an identical fashion.)

Structural co-ordinates in Rietica

Before you can constrain parameters, you must first set them to refine. Thus go into Model, Phases and set to refine on the atomic parameters on non-special positions and the thermal parameter.

Parameters to refine are selected

Now to set the constraint, enter Model, Constraints. You will note that Rietica has set the z position for Ti1/Fe2 to be constrained to move in an identical fashion.

Model Constraints dialog box

To set the thermals for Ti1/Fe1 to be constrained to refined together; click and drag the P 1: B (TI1) from the top middle Phase box to the bottom left Parameter box. Then click and drag the P 1: B (FE1) from the top middle Phase box to the bottom right Parameter box.

Now repeat the above with the P 1: B (FE2) / P 1: B (FE2) combination.

Done.

Model Constraints dialog box - setting constraints

Now when you perform refinement cycles in Rietica - these parameter pairs will be restrained to move together.

You will note that the Ti1/Fe2 pair thermals have gone negative. This implies either a problem with the refinement sequence or the model.

Structural co-ordinates in Rietica

If you manually peruse the INP input file; you will see the codeword pairs (11, 21, 31) causing the refined parameters to be constrainted. (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 0.39172 0.08333
 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
BA2 BA           0.33333 0.66667 0.09062 0.38826 0.16667
 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
TI1 TI           0.00000 0.00000 0.00000-0.27095 0.08333
 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
FE1 FE           0.00000 0.00000 0.00000-0.27095 0.00000
 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
TI2 TI           0.33333 0.66667 0.84830 0.57789 0.00000
 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
FE2 FE           0.33333 0.66667 0.84830 0.57789 0.16667
 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
O1  O            0.52066 0.04117 0.25000 0.60000 0.50000
 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
O2  O            0.83423 0.66846 0.08313 0.60000 0.50000
 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
2.469710E-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  11.000   0.000
   0.000   0.000   0.000   0.000   0.000   0.000
   0.000   0.000   0.000  11.000   0.000
   0.000   0.000   0.000   0.000   0.000   0.000
   0.000   0.000  21.000  31.000   0.000
   0.000   0.000   0.000   0.000   0.000   0.000
   0.000   0.000  21.000  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   0.000   0.000   0.000   0.000
   0.000   0.000   0.000   0.000   0.000   0.000
  41.000           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

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