The creation of modulated monoclinic aperiodic composites in n-alkane/urea compounds

Céline Mariette (1) , Laurent Guérin (1) , Philippe Rabiller (1) , Yu-Sheng Chen (2) , Alexei Bosak (3) , Alexander Popov (3) , Mark D Hollingsworth (4) , Bertrand Toudic (1)

1 IPR - Institut de Physique de Rennes
2 ANL - Argonne National Laboratory [Lemont]
3 ESRF - European Synchrotron Radiation Facility
4 Kansas State University

n-Dodecane/urea is a member of the prototype series of n-alkane/urea inclusion compounds. At room temperature, it presents a quasi-one dimensional liquid-like state for the confined guest molecules within the rigid, hexagonal framework of the urea host. At lower temperatures, we report the existence of two other phases. Below Tc=248 K there appears a phase with rank four superspace group P61 22(00γ), the one typically observed at room temperature in n-alkane/urea compounds with longer guest molecules. A misfit parameter, defined by the ratio γ=ch/cg (chost/cguest), is found to be 0.632±0.005. Below Tc1=123 K, a monoclinic modulated phase is created with a constant shift along c of the guest molecules in adjacent channels. The maximal monoclinic space group for this structure is P121 1(α0γ). Analogies and differences with n-heptane/urea, which also presents a monoclinic, modulated low-temperature phase, are discussed

Mots clés: diffraction; phase transitions; aperiodic crystals; crystallographic superspace; alkane/urea crystals

Domaines: Physique [physics]

Céline Mariette, Laurent Guérin, Philippe Rabiller, Yu-Sheng Chen, Alexei Bosak, et al.. The creation of modulated monoclinic aperiodic composites in n-alkane/urea compounds. Zeitschrift für Kristallographie, 2015, 230 (1), pp.5--11. ⟨10.1515/zkri-2014-1773⟩. ⟨hal-01168329⟩