Some small molecule crystallographers have specialized in solving and
refining structures that, exactly as you describe it, consist of two (or
more) interpenetrating, non-commensurable lattices. The usual approach is
to decribe the crystal in up to six dimensional space. The programs SAINT
and EVALCCD are able to integrate such diffraction patterns and
SADABS is able to scale them. However the case in point is probably
commensurate.
George
Prof. George M. Sheldrick FRS
Dept. Structural Chemistry,
University of Goettingen,
Tammannstr. 4,
D37077 Goettingen, Germany
Tel. +49-551-39-3021 or -3068
Fax. +49-551-39-2582
On Mon, 27 Aug 2007, Jacob Keller wrote:
> What a beautiful and interesting diffraction pattern!
>
> To me, it seems that there is a blurred set of spots with different cell
> dimensions, although
> nearly the same, underlying the ordered diffraction pattern. A possible
> interpretation occurred to
> me, that the ordered part of the crystal is supported by a less-ordered
> lattice of slightly
> different dimensions, which, because the crystal is a like a layer-cake of
> 2-d crystals, need not
> be commensurable in the short range with the ordered lattice. The
> nicely-ordered "cake" part of the
> crystal you solved, but the "frosting" between is of a different, less
> ordered nature, giving rise
> to the diffuse pattern which has slightly different lattice spacing. I would
> have to see more
> images to know whether this apparent lattice-spacing phenomenon is
> consistent, but it at least
> seems that way to me from the images you put on the web. I would shudder to
> think of indexing it,
> however.
>
> All the best,
>
> Jacob Keller
>
> ps I wonder whether a crystal was ever solved which had two interpenetrating,
> non-commensurable
> lattices in it. That would be pretty fantastic.
Jacob,
Some small molecule crystallographers have specialized in solving and
refining structures that, exactly as you describe it, consist of two
interpenetrating, non-commensurate lattices. The usual approach is
to index the diffraction pattern in multiple dimensional space
('superspace'). The programs SAINT and EVALCCD are able to integrate
diffraction patterns in up to six dimensions, SADABS is able to scale
them and the refinement is almost always performed with Petricek's
program JANA2000:
http://www-xray.fzu.cz/jana/Jana2000/jana.html
However the case in point is probably commensurate.
George
Prof. George M. Sheldrick FRS
Dept. Structural Chemistry,
University of Goettingen,
Tammannstr. 4,
D37077 Goettingen, Germany
Tel. +49-551-39-3021 or -3068
Fax. +49-551-39-2582
