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Twin cif

• Subject: Twin cif
• From: [email protected]
• Date: Tue, 3 Jul 2001 02:23:27 +0100 (BST)

Twin cif:

I think we should think carefully about cif for twins. I think we should
start by considering what we want from a database of twinned structures. I
would like the following:

1. The ability to identify the twin interface. I find it interesting to look
down or along the twin interface to determine what kind of bonds hold the
twin components together. I do this with XSHELL (I know, David Watkin, there
are other programs that can do the same thing).

2. The ability to determine what the distribution of merohedral and
non-merohedral twins fall in specific space groups (I think most merohedrals
are in high symmetry space groups and that most non-merohedrals are in low
symmetry space groups).

3. Do non-merohedral racemic twins exist? I think so, but I don't know of
any. I would like to search for potential candidates.

4. What percentage of twins are parallel twins (rotation about a crystal
edge) and what percentage are perpendicular (called normal by minerologists)
(rotation about the perpendicular to a face) and what percentage are
composite (rotation about a direction in the twin interface perpendicular to
an edge)? I don't know of any composite twins.

5. How do crystallographers refine twins? Do they use or throw out the
partial or exact overlaps?

The .res file of SHELXL doesn't contain enough information to answer these
questions. A narrative type cif almost surely will not contain enough
information. In fact, I suspect that with a narrative type cif the only
useful information will be that "it is twinned". Trying to extract the useful
information from a narrative cif may require a lot of programming.

I am not a invent_cif_names_type_of_person so I'll let someone else do that.
Here is the list of items I would like to see in a cif file of twinned
structures:

1. The number, n, of different twin component orientations.
2. The orientation matrix for each twin component or if the twin law is known
3. The Twin Laws for components two thru n and the orientation matrix for
component one.
4. The L.S. refined fraction (the relative sample volume) of each twin
component present and its esd.
5. For each twin component the fraction of observed non-overlaps used in the
refinement.
6. The fraction of observed partial and exact overlaps used in the
refinement. Sometimes the crystallographer uses only non-overlaps for one
component to refine the structure (I recommend using all measured reflections
- overlaps and non-overlaps).
7. Was the structure refined as a merohedral or a non-merohedral twin?
8. Was the structure refined as a racemic twin? What is the Flack parameter
value and its esd? Or better, what are the relative volumes and esds of the
entantiamorph components?
9. A list of the indices of crystal faces for each component and how they are
positoned with respect to the phi axis. This information may be difficult to
obtain and may be left out.
10. Was the reflection data corrected for absorption? What method or program
was used?

With the orientation matrices (or twin laws) it is usually possible to
determine the indices of the twin interface for a perpendicular twin. For
parallel twins it is only possible to determine that the twin interface is
one of a set of faces (identification of crystal faces - as in absorption
correction procedures - may pin down the twin interface). For a merohedral
racemic twin it is usually not possible to determine the twin interface.

What other things can be gotten from a database with twin cif items? What
other twin cif items would be useful?

Bob Sparks
e-mail: [email protected]

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