The disordered crystal structure of urea inclusion compounds OC(NH2)2 + CH2n+2

Forst, Rainer; Jagodziñski, H.; Boysen, H.; Frey, Friedrich

doi:10.1107/s0108768189009912

Cited by 43 publications

(51 citation statements)

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“…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].…”

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confidence: 99%

Frustrated pretransitional phenomena in aperiodic composites

et al. 2016

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This Letter reports on symmetry breaking in the aperiodic inclusion compound noctadecane/urea and its isotopomer n-octadecane/urea-d 4 . The high-symmetry phase is described by a hexagonal rank four superspace group. Pretransitional phenomena in this crystallographic superspace reveal competing short-range ordering phenomena within the high-symmetry phase. Very high-resolution diffraction data shows that critical scattering appears at inequivalent points within the four-dimensional Brillouin zone, although the first phase transition at T c1 near 158 K implies the condensation at only one of those points. The resulting superspace group remains of dimension four. Two other phase transitions are reported at T c2 = 152.8(4) K and T c3 = 109(4) K in n-octadecane/urea-d 4 . The two lowsymmetry phases that arise are described by rank five superspace groups.

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Section: Fig 1 (Color Onlinementioning

confidence: 99%

Frustrated pretransitional phenomena in aperiodic composites

et al. 2016

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“…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.…”

Section: Organic and Organometallic Compoundsmentioning

confidence: 99%

Organic molecular compounds with modulated crystal structures

Schoenleber¹

2011

Zeitschrift Für Kristallographie

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Abstract. Even though the higher-dimensional superspace approach is an established method to describe aperiodic crystal structures, it is not yet fully considered and applied by a larger community of scientists. In this contribution the structural description of incommensurately and commensurately modulated molecular compounds is discussed and it is shown, that the higher-dimensional superspace approach is an elegant way for an exact structural description and an exact crystal-chemical analysis of such structures. While discussing several modulated molecular compounds, the idea behind the higher-dimensional superspace approach is shown. On the examples treated in the discussion it is explained, how to understand and how to interprete modulated molecular compounds. Also a short introduction is given to the higher-dimensional superspace approach itself, to the basic principles and to the applied nomenclature.

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“…In the hk0 layer, therefore, the guests contribute directly to the Bragg peaks, the remaining diffuse scattering being due to the rotational disorder of the guests about the tunnel axis. The guests in alkane-urea and similar compounds are thought to be in approximately a trans conformation, with the plane of the molecules lying in any of six energetically favorable orientations parallel to the 11 0 and equivalent directions (4). The hk0 diffuse scattering is highly structured and is consistent with local ordering of these guest orientations into a herringbone arrangement like that of the low-temperature phase ( Fig.…”

Section: Structural Featuresmentioning

confidence: 67%