Structure and reorientational dynamics of 1-F-adamantane

Bacem Ben Hassine (1, 2) , Philippe Négrier (2) , M. Romanini (3) , M. Barrio (3) , R Macovez (3) , A. Kallel (1) , Denise Mondieig (2) , Josep Ll Tamarit (3)

1 LaMaCoP - Laboratoire des Matériaux Composites céramiques et Polymères [Université de Sfax]
2 LOMA - Laboratoire Ondes et Matière d'Aquitaine
3 ETSEIB - Departament de Fisica i Enginyeria Nuclear [UPC, Barcelone]

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, Fm 3m, 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, P 42 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 threefold axes. In the dielectric loss spectra, the cooperative (a) 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.

Domaines: Cristallographie

Bacem Ben Hassine, Philippe Négrier, M. Romanini, M. Barrio, R Macovez, et al.. Structure and reorientational dynamics of 1-F-adamantane. Physical Chemistry Chemical Physics, 2016, 18 (16), pp.10924-10930. ⟨10.1039/c6cp01144f⟩. ⟨hal-01341793⟩