Long-range modulation in a five-dimensional crystallographic superspace

Abstract : The intergrowth crystal of n-tetracosane/urea presents a misfit parameter, defined by the ratio γ = c h /c g (c host /c guest), that is very close to a commensurate value (γ ≅ 1/3). High-resolution diffraction studies presented here reveal an aperiodic misfit parameter of γ = 0.3369, which is found to be constant at all temperatures studied. A 2 complex sequence of structural phases is reported. The high temperature phase (phase I) exists in the four-dimensional superspace group P6 1 22(00γ). 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 (5-D) superspace group C222 1 (00γ)(10δ) with a modulation vector a o *+c m * = a o *+δ. c h *, in which the supplementary misfit parameter is δ = 0.025(1) in host reciprocal units. This corresponds to the appearance of a modulation of very long period (about 440 ± 16 Å). At T c2 = 163.0(5) K, a 5-D to 5-D phase transition leads to the crystallographic superspace group P2 1 2 1 2 1 (00γ)(00δ) with a very similar value of δ. This phase transition reveals a significant hysteresis effect.
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Physical Review B : Condensed matter and materials physics, American Physical Society, 2015, 91 (18), pp.184101. 〈10.1103/PhysRevB.91.184101〉
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Laurent Guérin, Céline Mariette, Philippe Rabiller, Michael Huard, Sylvain Ravy, et al.. Long-range modulation in a five-dimensional crystallographic superspace. Physical Review B : Condensed matter and materials physics, American Physical Society, 2015, 91 (18), pp.184101. 〈10.1103/PhysRevB.91.184101〉. 〈hal-01164047v2〉

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