Forst, R.; Boysen, Hans; Frey, Friedrich; Jagodzinski, H.; Zeyen, C.
Phase transitions and ordering in urea inclusion compounds with n-paraffins.
In: Journal of Physics and Chemistry of Solids, Vol. 47, Nr. 11: S. 1089-1097
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Urea inclusion compounds consist of a pseudo-hexagonal framework of urea with \"infinite\" channels parallel to c, into which paraffins are embedded. The compound with hexadecane exhibits phase transitions at ≅ 365, ≅ 148 and ≅ 120 K (phases I-IV), which have been investigated by X-ray and neutron scattering. Phase I is hexagonal with a (longitudinal and orientational) random distribution of the included molecules. In phase II, which uniquely occurs in the adduct with C16H34, a doubling of the lattice constant c0 of the host structure indicates partial longitudinal ordering. This is assigned to the approximately equal length of the hexadecane molecule and 2co leading to mutual deformations of both constituents. The transition II→ III is connected with a lateral orientational ordering of the guest molecules in adjacent channels and a simultaneous orthorhombic deformation of the host. Correlated rotational motions of the included molecules around their long axes are deduced from the increasing critical scattering above Tc In phase IV a crystal-like ordering tendency of the paraffins plays the dominant role, while the host breaks into a heavily disordered domain structure. Experimental results indicate a transformation process as no thermal equilibrium was reached during the measurements. The different ordering mechanisms are discussed in terms of interactions between host and guest, guest and guest along each channel, and guest and guest in adjacent channels.