title: Structure and reorientational dynamics of 1-F-adamantane.
abstract: The polymorphism and the dynamics of a simple rigid molecule (1-fluoro-adamantane) have been studied by means of X-ray powder diffraction and broadband dielectric spectroscopy. At temperatures below the melting point, the molecule forms an orientationally disordered Phase I with a cubic-centered structure (Phase I, Fm3¯m, Z = 4). This phase possesses eight equilibrium positions for the fluorine atom, with equal occupancy factors of 1/8. A solid-solid phase transition to a low-temperature tetragonal phase (Phase II, P4¯2(1)c, Z = 2) reduces the statistical disorder to only four possible equivalent sites for the fluorine atom, with fractional occupancies of 1/4. The dynamics has been rationalized under the constraints imposed by the space group of the crystal structure determined by powder X-ray diffraction. The dielectric spectroscopy study reveals that the statistical disorder in Phase II is dynamic in character and is associated with reorientational jumps along the two- and three-fold axes. In the dielectric loss spectra, the cooperative (α) relaxation exhibits a shoulder on the high-frequency side. This remarkable finding clearly reveals the existence of two intrinsic reorientational processes associated with the exchange of the F atom along the four sites. In addition to such "bimodal" relaxation, a secondary Johari-Goldstein relaxation is detected at lower temperatures.
authors.affiliation: Univ. Bordeaux, LOMA, UMR 5798, F-33400 Talence, France and Laboratoire des Matériaux Céramiques Composites et Polymères, Département Physique, Faculté des Sciences de Sfax, 3000 Sfax, Tunisie.
authors.forename: B
authors.initials: B
authors.lastname: Ben Hassine
authors.affiliation: Univ. Bordeaux, LOMA, UMR 5798, F-33400 Talence, France.
authors.forename: Ph
authors.initials: P
authors.lastname: Negrier
authors.affiliation: Grup de Caracterització de Materials, Department de Física, ETSEIB, Diagonal 647, Universitat Politècnica de Catalunya, 08028 Barcelona, Catalonia, Spain. josep.lluis.tamarit@upc.edu.
authors.forename: M
authors.initials: M
authors.lastname: Romanini
authors.affiliation: Grup de Caracterització de Materials, Department de Física, ETSEIB, Diagonal 647, Universitat Politècnica de Catalunya, 08028 Barcelona, Catalonia, Spain. josep.lluis.tamarit@upc.edu.
authors.forename: M
authors.initials: M
authors.lastname: Barrio
authors.affiliation: Grup de Caracterització de Materials, Department de Física, ETSEIB, Diagonal 647, Universitat Politècnica de Catalunya, 08028 Barcelona, Catalonia, Spain. josep.lluis.tamarit@upc.edu.
authors.forename: R
authors.initials: R
authors.lastname: Macovez
authors.affiliation: Laboratoire des Matériaux Céramiques Composites et Polymères, Département Physique, Faculté des Sciences de Sfax, 3000 Sfax, Tunisie.
authors.forename: A
authors.initials: A
authors.lastname: Kallel
authors.affiliation: Univ. Bordeaux, LOMA, UMR 5798, F-33400 Talence, France.
authors.forename: D
authors.initials: D
authors.lastname: Mondieig
authors.affiliation: Grup de Caracterització de Materials, Department de Física, ETSEIB, Diagonal 647, Universitat Politècnica de Catalunya, 08028 Barcelona, Catalonia, Spain. josep.lluis.tamarit@upc.edu.
authors.forename: J Ll
authors.initials: JL
authors.lastname: Tamarit