title: Atomic structure and phason modes of the Sc-Zn icosahedral quasicrystal.
abstract: The detailed atomic structure of the binary icosahedral (i) ScZn7.33 quasicrystal has been investigated by means of high-resolution synchrotron single-crystal X-ray diffraction and absolute scale measurements of diffuse scattering. The average atomic structure has been solved using the measured Bragg intensity data based on a six-dimensional model that is isostructural to the i-YbCd5.7 one. The structure is described with a quasiperiodic packing of large Tsai-type rhombic triacontahedron clusters and double Friauf polyhedra (DFP), both resulting from a close-packing of a large (Sc) and a small (Zn) atom. The difference in chemical composition between i-ScZn7.33 and i-YbCd5.7 was found to lie in the icosahedron shell and the DFP where in i-ScZn7.33 chemical disorder occurs on the large atom sites, which induces a significant distortion to the structure units. The intensity in reciprocal space displays a substantial amount of diffuse scattering with anisotropic distribution, located around the strong Bragg peaks, that can be fully interpreted as resulting from phason fluctuations, with a ratio of the phason elastic constants K 2/K 1 = -0.53, i.e. close to a threefold instability limit. This induces a relatively large perpendicular (or phason) Debye-Waller factor, which explains the vanishing of 'high-Q perp' reflections.
authors.affiliation: Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University , Miyagi 980-8577, Japan.
authors.forename: Tsunetomo
authors.initials: T
authors.lastname: Yamada
authors.affiliation: Division of Applied Physics, Faculty of Engineering, Hokkaido University , Hokkaido 060-8628, Japan.
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Takakura
authors.affiliation: Institute of Materials Science and Technology, Vienna University of Technology , Vienna 1040, Austria.
authors.forename: Holger
authors.initials: H
authors.lastname: Euchner
authors.affiliation: Department of Chemistry, Ångström Laboratory, Uppsala University , Uppsala 751 21, Sweden.
authors.forename: Cesar
authors.initials: C
authors.lastname: Pay Gómez
authors.affiliation: ESRF - The European Synchrotron , Grenoble F-38043, France.
authors.forename: Alexei
authors.initials: A
authors.lastname: Bosak
authors.affiliation: Synchrotron SOLEIL , Gif-sur-Yvette F-91192, France.
authors.forename: Pierre
authors.initials: P
authors.lastname: Fertey
authors.affiliation: Université Grenoble Alpes, SIMAP, Grenoble F-38000, France; CNRS, SIMAP, Grenoble F-38000, France.
authors.forename: Marc
authors.initials: M
authors.lastname: de Boissieu
keywords: X-ray diffuse scattering; phasons; quasicrystals; structure analysis; superspace crystallography