High Resolution X-ray diffraction study of the phase transitions in an Aperiodic composite structures

Supramolecular or self-assembled materials often result from a subtle balance of weak intermolecular interactions yielding to a large variety of ordered and disordered phases with many different physical, chemical or mechanical properties. Alkane urea inclusion compounds (UIC) are molecular composite structures where a "host" framework of urea molecules accommodates "guest" alkane molecules as long chains loosely held in a honey-comb like hexagonal network of adjacent channels. Most of the alkane-UICs are incommensurate or aperiodic, which means that the ratio of the host and guest periodicities is not a rational number P/Q. As a consequence, very different types of short and long range ordering, as well as different phase transitions take place in these alkane-UICs just by changing the length of the alkane molecules. Even if these compounds are aperiodic by construction, periodicity can be recovered in superspaces of dimension greater than 3 and where full power of Fourier analysis can be used for crystallographic investigation. The measurements of the x-ray diffraction were made on a single crystals of n-alakne/urea inclusion compounds cooled down using both the nitrogen and helium flows in order to map the structural changes of these material at different low temperatures and to differentiate between the incommensurability and the long range order that occurs in these materials.