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Transformation and/or reduction of the unit cell

TRCELL will always end up with a primitive triclinic unit cell. To increase the symmetry of the selected and refined unit cell, you can use the command lattice .. .. step .., for example to get a primitive orthorhombic cell:


fix all fit orientation fit beam fit distance lattice P orthorhombic step 10

The program will do 10 steps of refinement, adjusting the cell parameters to force them to the symmetry of the selected lattice between each step.

It may sometimes be necessary to transform the unit cell to a different setting first, before running lattice .. .. step ... The program lepage is an excellent program for determining the transformation matrix between the current primitive triclinic unit cell and a potential unit cell with higher symmetry. The transformation matrix can be given in LAUECELL in the following way:


transformation 1.0 0.0 0.0 0.0 0.0 1.0 0.0 1.0 0.0

Only right-handed cell settings are allowed: make sure that the determinant of the transformation matrix is positive.

If lepage suggests a transformation to an A, B, C, I or F Bravais lattice, you can use the following procedure to obtain the correct lattice:


lattice P triclinic fix all fit orientation fit beam fit distance transformation 1.0 0.0 -1.0 1.0 0.0 1.0 -1.0 -2.0 -1.0 lattice F triclinic run INDXPAT ...

Intermezzo - we realize that the lattice system F triclinic does not exist. It is just used in order to be able to simulate the old diffraction pattern using the new cell parameters that still have to be adjusted, but with using the correct extinction criteria. -

... lattice F cubic step 10

If the match between the predicted and observed pattern is still O.K., the unit cell was transformed successfully.



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Next: Saving and exporting Up: INDXPAT: Indexing and Previous: Refinement



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