Date Published: February 01, 2017
Publisher: International Union of Crystallography
Author(s): Paul Kautny, Thomas Schwartz, Berthold Stöger, Johannes Fröhlich.
The title compound crystallizes as allotwins made up of two OD-polytypes with a maximum degree of order.
A twin is a heterogeneous edifice made up of homogeneous crystals of the same phase in different orientations related by an operation that does not belong to the point group of the individual (Friedel, 1904 ▸). The crystalline domains diffract independently and the orientation relationship is well defined (Hahn & Klapper, 2006 ▸). Allotwinning (Nespolo et al., 1999 ▸) is a generalization of twinning, where the domains are different polytypes of the same compound (Greek = different) composed of equivalent layers. In contrast to the structural characterization of classical twins, the concurrent refinement of two or more structural models against the same data set is not yet routine. It has nevertheless been performed successfully in a few cases (e.g. Friese et al., 2003 ▸; Krüger et al., 2009 ▸; Jahangiri et al., 2013 ▸; Stöger et al., 2015 ▸).
The crystals of (1) are a further addition to the growing set of known allotwins and here it should be stressed again that the phenomenon is more common than one might expect. It is therefore crucial to recognize and pursue the, often subtle, signs of allotwinning like peaks in the difference Fourier maps and weak additional reflections. Moreover, we want to emphasize the diversity of the phenomenon, making it necessary to address every allotwin as a unique problem.