Structure and reorientational dynamics of 1-F-adamantane

• Published in: Physical chemistry chemical physics : PCCP 2016-04, Vol.18 (16), p.1924-193
• Main Authors: Ben Hassine, B, Negrier, Ph, Romanini, M, Barrio, M, Macovez, R, Kallel, A, Mondieig, D, Tamarit, J. Ll
• Format: Article
• Language: English
• Subjects: 1-fluoroadamantane; Adamantane; Chemical Sciences; Cristallography; crystals; Diffraction; Dinàmica molecular; Disorders; Dynamic tests; Dynamics; Enginyeria química; Fluorine; Física; Física molecular; Glass transition temperature; glass-transition; Líquids; Molecular dynamics; molecular motions; Phase transformations; plasic phase; Polimorfisme; polimorphism; Polymorphism (Crystallography); Propietats físiques; Propietats tèrmiques; Química física; relaxation; Spectroscopy; Supercooled liquids; temperature; Vidre; X-rays; Àrees temàtiques de la UPC
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c6cp01144f

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 3&cmb.macr; 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, P 4&cmb.macr;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. Bimodal reorientational relaxations along the twofold ( α ) and threefold ( α ′) axes of the disordered Phase II ( P 4&cmb.macr;2 1 c ) of 1-F-adamantane.