Les clathrates hydrates

Desmedt, Arnaud

doi:10.1051/sfn/2010013

Cited by 1 publication

(1 citation statement)

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“…Clathrate hydrates are crystals formed by means of a framework of H-bonded water molecules containing inclusion cavities within which guest molecules are located. 1,2 The natural abundance of this composite as well as its peculiar structure and composition variability, which lead to various specific physical− chemical properties, 3 have given birth to a huge interest from the scientific community. 4,5 Ionic clathrate hydrates constitute one interesting class of these materials.…”

Section: Introductionmentioning

confidence: 99%

Proton Diffusion in the Hexafluorophosphoric Acid Clathrate Hydrate

et al. 2014

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The hexafluorophosphoric acid clathrate hydrate is known as a "super-protonic" conductor: its proton conductivity is of the order of 0.1 S/cm at ca. room temperature. The long-range proton diffusion and the associated mechanism have been analyzed with the help of incoherent quasi-elastic neutron scattering (QENS) and proton pulsed-field-gradient nuclear magnetic resonance ((1)H PFG-NMR). The system crystallizes into the so-called type I clathrate structure (SI) at low temperature and into the type VII structure (SVII) above ca. 230 K with a melting point close to room temperature. While, in the SI phase, no long-range proton diffusion is observed (at least faster than the present measurement capabilities, i.e., 10(-7) cm(2)·s(-1)) with respect to the probed time scale, both techniques evidence a long-range proton diffusion process in the SVII phase (3.85 × 10(-6) cm(2)·s(-1) at 275 K with an activation energy of 0.19 ± 0.04 eV). QENS experiments lead to modeling the microscopic mechanism of the long-range proton diffusion by means of a Chudley-Elliot jump diffusion model with a characteristic jump distance of 2.79 ± 0.17 Å. In other words, the long-range diffusion occurs through a Grotthus mechanism with proton jumping from one water-oxygen site to another. Moreover, the analysis of the proton diffusion for hydration numbers greater than 6 (i.e., in the SVII structure) reveals that the additional water molecules coexisting with the SVII structure act as a "structural defect" barrier for the proton diffusivity, responsible for the conductivity.

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