Confined linear molecules inside an aperiodic supramolecular crystal: The sequence of superspace phases in n-hexadecane/urea

Abstract

High-resolution studies of the host-guest inclusion compound n-hexadecane/urea are reported at atmospheric pressure, using both cold neutrons and x-ray diffraction. This intergrowth crystal presents a misfit parameter, defined by the ratio c(h)/c(g) (c(host)/c(guest)), which is temperature independent and irrational (γ = 0.486 ± 0.002) from 300 to 30 K. Three different structural phases are reported for this aperiodic crystal over this temperature range. The crystallographic superspaces are of rank 4 in phases I and II, whereas phase III is associated with an increase in rank to 5, with a supplementary misfit parameter (δ = 0.058 ± 0.002) that is constant throughout this phase. The superspace group of phase I is hexagonal P6(1)22(00γ) down to T(c1) = 149.5 ± 0.5 K; phase II, which persists down to T(c2) = 127.8 ± 0.5 K is orthorhombic P2(1)2(1)2(1)(00γ), and phase III is orthorhombic P2(1)2(1)2(1)(00γ)(00δ).

Long-range modulation of a composite crystal in a five-dimensional superspace

“…n -Tetracosane/urea is thus aperiodic at all temperatures from room temperature to 100 K. In the case of n -alkane/urea inclusion compounds, the superspace group is conventionally described by using the mean structure of the host (urea) subsystem. According to our analysis of the room temperature data, the crystal is described by the same crystallographic superspace group P 6 1 22(00 γ ) as previously determined at room temperature for other n -alkane/urea crystals [20,36,37]. Although the diffuse layers signify substantial displacive disorder of the guests along the c axis, in ordered regions of the crystal, the host and guest share hexagonal symmetry, and the offset between guest molecules in adjacent channels (Δ g ) [17] is 0 Å.…”

“…n -Alkane/urea [ n -C n H 2 n +2 /(CO(NH 2 ) 2 )] crystals are typically incommensurate materials, and indeed diffraction patterns with the four types of Bragg peaks have been reported in these materials [18–21]. Structural instabilities have been extensively studied in these materials [20–32]. High pressure studies demonstrate that the guest-guest interactions along the aperiodic direction can be manipulated [33], enhancing the interest in further studying the competition between forces in favor of or against commensurability.…”

“…These include n -heptane/urea, where γ is very close to unity ( γ = 0.981 at ambient conditions) [21] and n - h exadecane/urea, where γ is close to 1/2 ( γ = 0.486 at ambient conditions) [20]. In n - h exadecane/urea, the misfit parameter was found to be constant as a function of temperature at ambient pressure; these composite crystals were therefore aperiodic at all temperatures measured.…”

The true structural periodicities and superspace group descriptions of the prototypical incommensurate composite materials: Alkane/urea inclusion compounds

“…A recent series of papers [11,[36][37][38][39][40][41][42][43] has argued against this simple generalized description of the phase transitions in n-alkane/urea inclusion compounds. First, for n-nonadecane/urea [11,36,38,42], in addition to the "classical" I↔II phase transition (discussed above) at temperature T 1 , another phase transition at a lower temperature T 2 was reported, corresponding to a weak thermal event in DSC data [59].…”

Symmetry breakings in aperiodic composite crystals

“…Studies performed by X-ray diffraction using synchrotron sources reveal multiple structural solutions implying or not changes of the dimension of the superspace. Most fascinating ones concern phase transitions which increase the dimension of the crystallographic superspace [6][7][8]. We will present the characterization of the order parameter and of the critical pretransitionnal phenomena associated to these phase transitions of group/subgroup types.…”