title: Long-range modulation of a composite crystal in a five-dimensional superspace.
abstract: The intergrowth crystal ofn-tetracosane/urea presents a misfit parameter, defined by the ratioγ=ch/cg(chost/cguest), that is very close to a commensurate value (γ≅ 1/3). High-resolution diffraction studies presented here reveal an aperiodic misfit parameter ofγ= 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 groupP6122(00γ). AtTc1= 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 groupC2221(00γ)(10δ) with a modulation vectorao*+cm*=ao*+δ·ch*, in which the supplementary misfit parameter isδ= 0.025(1) in host reciprocal units. This corresponds to the appearance of a modulation of very long period (about 440 ± 16 Å). AtTc2= 163.0(5) K, a 5D to 5D phase transition leads to the crystallographic superspace groupP212121(00γ)(00δ) with a very similar value ofδ. This phase transition reveals a significant hysteresis effect.
authors.affiliation: Institut de Physique de Rennes, UMR UR1-CNRS 6251, Université de Rennes 1, 35042 Rennes, France.
authors.forename: Laurent
authors.initials: L
authors.lastname: Guérin
authors.affiliation: Institut de Physique de Rennes, UMR UR1-CNRS 6251, Université de Rennes 1, 35042 Rennes, France.
authors.forename: Céline
authors.initials: C
authors.lastname: Mariette
authors.affiliation: Institut de Physique de Rennes, UMR UR1-CNRS 6251, Université de Rennes 1, 35042 Rennes, France.
authors.forename: Philippe
authors.initials: P
authors.lastname: Rabiller
authors.affiliation: Institut de Physique de Rennes, UMR UR1-CNRS 6251, Université de Rennes 1, 35042 Rennes, France.
authors.forename: Michael
authors.initials: M
authors.lastname: Huard
authors.affiliation: Synchrotron SOLEIL, L'Orme des merisiers, Saint-Aubin BP 48, 91192 Gif-sur-Yvette cedex, France.
authors.forename: Sylvain
authors.initials: S
authors.lastname: Ravy
authors.affiliation: Synchrotron SOLEIL, L'Orme des merisiers, Saint-Aubin BP 48, 91192 Gif-sur-Yvette cedex, France.
authors.forename: Pierre
authors.initials: P
authors.lastname: Fertey
authors.affiliation: Department of Chemistry, 213 CBC Building, Kansas State University, Manhattan, Kansas 66506-0401, USA.
authors.forename: Shane M
authors.initials: SM
authors.lastname: Nichols
authors.affiliation: Department of Chemistry, 213 CBC Building, Kansas State University, Manhattan, Kansas 66506-0401, USA.
authors.forename: Bo
authors.initials: B
authors.lastname: Wang
authors.affiliation: Center for Advancing Electronics Dresden, Dresden University of Technology, 01062 Dresden, Germany.
authors.forename: Stefan C B
authors.initials: SC
authors.lastname: Mannsfeld
authors.affiliation: Laboratory of Crystallography, Eidgenössische Technische Hochschule (ETH) Zurich, CH-8093 Zurich, Switzerland.
authors.forename: Thomas
authors.initials: T
authors.lastname: Weber
authors.affiliation: Department of Chemistry, 213 CBC Building, Kansas State University, Manhattan, Kansas 66506-0401, USA.
authors.forename: Mark D
authors.initials: MD
authors.lastname: Hollingsworth
authors.affiliation: Institut de Physique de Rennes, UMR UR1-CNRS 6251, Université de Rennes 1, 35042 Rennes, France.
authors.forename: Bertrand
authors.initials: B
authors.lastname: Toudic