The true structural periodicities and superspace group descriptions of the prototypical incommensurate composite materials: Alkane/urea inclusion compounds

¹ Université de Bordeaux, CNRS, ISM UMR 5255 - 351 cours de la Libération, F-33405 Talence Cedex, France
² School of Chemistry, Cardiff University - Park Place, Cardiff CF10 3AT, Wales, UK
³ Department of Structural Biology, Weizmann Institute of Science - Rehovot 7610001, Israel
⁴ Diamond Light Source, Harwell Science and Innovation Campus - Didcot, Oxfordshire OX11 0DE, England, UK
⁵ Inorganic Chemistry Laboratory - South Parks Road, Oxford OX1 3QR, England, UK

^(a) michel.couzi@u-bordeaux.fr
^(b) francois.guillaume@u-bordeaux.fr
^(c) HarrisKDM@cardiff.ac.uk

Received: 27 July 2016
Accepted: 9 December 2016

Abstract

The prototypical family of incommensurate composite materials are the n-alkane/urea inclusion compounds, in which n-alkane guest molecules are arranged in a periodic manner along one-dimensional tunnels in a urea host structure, with an incommensurate relationship between the periodicities of the host and guest substructures along the tunnel. We develop interpretations of the structural periodicities, superspace group descriptions and symmetry properties of the low-temperature phases of n-alkane/urea inclusion compounds, based in part on a high-resolution synchrotron single-crystal X-ray diffraction study of n-nonadecane/urea. Specifically, we prove that, on passing from phase I to phase II, the C-centering of the orthohexagonal unit cell is lost for both the host and guest substructures, and that the symmetries of all phases I, II and III are described completely by (3 + 1)-dimensional superspace groups.