Critical phenomena in higher dimensional spaces: The hexagonal-to-orthorhombic phase transition in aperiodic n-nonadecane/urea

PACS number(s): 61.44.Fw, 64.60.−i, 61.05.cf.-- et al.Upon cooling, the aperiodic inclusion compound n-nonadecane/urea presents a hexagonal-to-orthorhombic group-subgroup phase transition at Tcl that increases the structure's superspace dimensionality from four to five. This paper reports on pretransitional phenomena in such a high-dimensional space, generalizing the critical results previously reported at a lower dimensionality. Very high-resolution diffraction data reveal anomalously large correlation lengths along the aperiodic direction, with all correlation lengths diverging at Tcl. This could be explained by low-frequency phason excitations that soften at Tcl at the critical wave vector, in accordance with an increase in the critical diffuse scattering intensity. © 2013 American Physical Society.Peer Reviewe

Frustrated pretransitional phenomena in aperiodic composites

Citation: Mariette, C., Frantsuzov, I., Wang, B., Guerin, L., Rabiller, P., Hollingsworth, M. D., & Toudic, B. (2016). Frustrated pretransitional phenomena in aperiodic composites. Physical Review B, 94(18), 9. doi:10.1103/PhysRevB.94.184105This paper reports on symmetry breaking in the aperiodic inclusion compound n-octadecane/urea and its isotopomer n-octadecane/urea-d(4). The high-symmetry phase is described by a hexagonal rank-4 superspace group. Pretransitional phenomena in this crystallographic superspace reveal competing short-range-ordering phenomena within the high-symmetry phase. Very high-resolution diffraction data show 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 4. 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 low-symmetry phases that arise are described by rank-5 superspace groups

Neutron Laue and X-ray diffraction study of a new crystallographic superspace phase in n-nonadecane-urea

International audienceAperiodic composite crystals present long-range order without translational symmetry. These materials may be described as the intersection in three dimensions of a crystal which is periodic in a higher-dimensional space. In such materials, symmetry breaking must be described as structural changes within these crystallographic superspaces. The increase in the number of superspace groups with the increase in the dimension of the superspace allows many more structural solutions. This is illustrated in n-nonadecane-urea, revealing a fifth higher-dimensional phase at low temperature

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

Citation: Mariette, C., Guerin, L., Rabiller, P., Chen, Y. S., Bosak, A., Popov, A., . . . Toudic, B. (2015). The creation of modulated monoclinic aperiodic composites in n-alkane/urea compounds. Zeitschrift Fur Kristallographie, 230(1), 5-11. doi:10.1515/zkri-2014-1773n-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 T-c=248 K there appears a phase with rank four superspace group P6(1)22(00 gamma), the one typically observed at room temperature in n-alkane/urea compounds with longer guest molecules. A misfit parameter, defined by the ratio gamma=C-h/C-g (C-host/C-guest), is found to be 0.632 +/- 0.005. Below T-c1=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 P12(1)1(alpha 0 gamma). Analogies and differences with n-heptane/urea, which also presents a monoclinic, modulated low-temperature phase, are discussed

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

Citation: Guerin, L., Mariette, C., Rabiller, P., Huard, M., Ravy, S., Fertey, P., . . . Toudic, B. (2015). Long-range modulation of a composite crystal in a five-dimensional superspace. Physical Review B, 91(18), 7. doi:10.1103/PhysRevB.91.184101The intergrowth crystal of n-tetracosane/urea presents a misfit parameter, defined by the ratio gamma = c(h)/c(g) (c(host)/c(guest)), that is very close to a commensurate value (gamma congruent to 1/3). High-resolution diffraction studies presented here reveal an aperiodic misfit parameter of gamma = 0.3369, which is found to be constant at all temperatures studied. A complex sequence of structural phases is reported. The high temperature phase (phase I) exists in the four-dimensional superspace group P6(1)22(00 gamma). At T-c1 = 179(1) K, a ferroelastic phase transition increases the dimension of the crystallographic superspace. This orthorhombic phase (phase II) is characterized by the five-dimensional (5D) superspace group C222(1)(00 gamma)(10 delta) with a modulation vector a(o)* + c(m)* = a(o)* + delta . c(h)*, in which the supplementary misfit parameter is delta = 0.025(1) in host reciprocal units. This corresponds to the appearance of a modulation of very long period (about 440 +/- 16 angstrom). At T-c2 = 163.0(5) K, a 5D to 5D phase transition leads to the crystallographic superspace group P2(1)2(1)2(1)(00 gamma)(00 delta) with a very similar value of delta. This phase transition reveals a significant hysteresis effect

L'écologie en livres et en images

Pour les enseignants et pour les élèves, de l'école élémentaire à la classe de seconde : des livres, des diapositives et des films permettant de se former et de s'informer sur tous les domaines de l'écologi

Dynamics in the supercooled liquid

The temperature and wave number dependences of the dynamical structure factor obtained in a nonfragile glass-forming liquid by Brillouin scattering and coherent neutron scattering experiments are for the first time described by the solutions of a schematic mode coupling theory model over a wide frequency range. In intermediate regime, the crossover between vibrational excitations and fast relaxational processes observed above the susceptibility minimum in neutron spectra is well reproduced by the theoretical model. At lower Q-values, the temperature dependence of the Brillouin lineshape is also accurately described. Besides, the dynamical transition is shown to occur between 601 K and 637 K ($T_{\rm g} = 515\;{\rm K}$, $T_{\rm m} = 749\;{\rm K}$) in this supercooled liquid

BRILLOUIN SCATTERING STUDY OF PHASE TRANSITIONS IN p-POLYPHENYLS

A Brillouin scattering study of the various phase transitions in p-polyphenyls reveals different kinds of elastic anomalies depending on the nature of these transitions