title: Hidden degrees of freedom in aperiodic materials.
abstract: Numerous crystalline materials, including those of bioorganic origin, comprise incommensurate sublattices whose mutual arrangement is described in a superspace framework exceeding three dimensions. We report direct observation by neutron diffraction of superspace symmetry breaking in a solid-solid phase transition of an incommensurate host-guest system: the channel inclusion compound of nonadecane/urea. Strikingly, this phase transition generates a unit cell doubling that concerns only the modulation of one substructure by the other-an internal variable available only in superspace. This unanticipated pathway for degrees of freedom to rearrange leads to a second phase transition, which again is controlled by the higher dimensionality of superspace. These results reveal nature's capacity to explore the increased number of phases allowed in aperiodic crystals.
authors.affiliation: Universitéde Rennes 1, Institut de Physique de Rennes (IPR), 35042 Rennes Cedex, France. bertrand.toudic@univ-rennes1.fr
authors.forename: Bertrand
authors.initials: B
authors.lastname: Toudic
authors.forename: Pilar
authors.initials: P
authors.lastname: Garcia
authors.forename: Christophe
authors.initials: C
authors.lastname: Odin
authors.forename: Philippe
authors.initials: P
authors.lastname: Rabiller
authors.forename: Claude
authors.initials: C
authors.lastname: Ecolivet
authors.forename: Eric
authors.initials: E
authors.lastname: Collet
authors.forename: Philippe
authors.initials: P
authors.lastname: Bourges
authors.forename: Garry J
authors.initials: GJ
authors.lastname: McIntyre
authors.forename: Mark D
authors.initials: MD
authors.lastname: Hollingsworth
authors.forename: Tomasz
authors.initials: T
authors.lastname: Breczewski