Diffuse scattering and disorder in urea inclusion compounds OC(NH₂)₂+C _n H_2n+2

Frustrated pretransitional phenomena in aperiodic composites

“…A large body of work has been dedicated to the phase transitions in this prototype family [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33], revealing recently a rich sequence of phases in high-dimensional crystallographic spaces [6,11,18,19,32,33]. Previously, we reported original behavior of the critical phenomena leading to a phase transition with an increase of the dimensionality of the crystallographic superspace from four to five [6,19].…”

Longitudinal positional ordering of n-alkane molecules in urea inclusion compounds

“…1). From their positions and widths they can be related to a maximum of the form factor of the alkanes and, in consequence, explained by a longitudinal and lateral disorder of the guest molecules which was previously assumed to be an intramolecular disorder of the alkanes (Forst et al, 1987) or a result of random displacements of the molecules within the channels (Welberry & Mayo, 1996). At low temperatures UICs with n-alkanes undergo a phase transition from a structure with hexagonal symmetry (space group P6 1 22) to an orthorhombic structure (space group P2 1 2 1 2 1 ± host only).…”

“…1): sharp Bragg re¯ections of the three-dimensional urea host and narrow diffuse layers (`s-layers') perpendicular to the c-axis corresponding to the one-dimensional guest structure. As the s-layers show some intensity modulations and are superimposed by weak Bragg-like re¯ections, however, this rough subdivision is not fully adequate, and lateral correlations between the guest molecules cannot be neglected (Forst et al, 1987;Fukao, 1994a,b;Weber, Boysen, Honal et al, 1996). In addition, mutual modulations of host and guest lattices give rise to three-dimensional and one-dimensional satellite scattering accompanying the Bragg re¯ections and the s-layers, respectively (Fig.…”

Organic molecular compounds with modulated crystal structures

“…While studying the crystal structures of urea-hydrocarbon complexes, Smith reported in 1952 extra spots in the diffraction pattern with fractional "l" indices [37], indicating not a random but an ordered arrangement of the hydrocarbon molecules in the channels created by the urea molecules. Those structures are now known as urea inclusion compounds (host-guest structures) and have attracted until today quite some interest [38][39][40][41][42][43][44][45]. The urea molecules act as host and form a honeycomb structure which creates at room temperature parallel tunnels.…”