Structure and reorientational dynamics of 1-F-adamantane
Résumé
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
CristallographieOrigine | Fichiers produits par l'(les) auteur(s) |
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Commentaire | Embargo imposé par l'éditeur pour le post-print |
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