Dr Andrzej Falenty (GZG Abt. Kristallographie Uni Göttingen)
Formation of gas hydrates from ice is generally a slow process due to the intrinsically sluggish solid-state mass transfer (Salamatin et al. 2015). Small size Ne, H2 and He are an exception to this rule by being able to penetrate into the lattice of the ordinary ice Ih with ease. Upon further compression ice Ih presaturated with He-gas was found to transform rapidly into gas-filled ice II. Interestingly, H2 and Ne were found also to form a cubic sII clathrate hydrate at intermediate pressures. In planetary science, such ices filled with Ne, H2 and He are seen as potent materials selectively trapping and fractionating these primordial gases. Moreover, in the context of fundamental clathrate research Ne- and H2-hydrates have played the unorthodox role as starting point for the formation of ice XVI, the empty clathrate structure sII (Falenty et al. 2014). Here we report on the gas loading into ice Ih spheres. The pressure in the system was increased stepwise to 320MPa for D2 and 400MPa for He and Ne. All guest types were found to rapidly penetrate into ice yet with marked differences. Pressurization of ice with deuterium lead to a rapid formation of sII D2-hydrate initiated already at 110 MPa. The sII structure remained stable at least up to 320MPa. Ice loaded with Ne remained stable up to 210MPa, i.e. to approximately the stability boundary between pure Ih and unfilled ice II. After crossing this threshold, the sample adopted the sII Ne-hydrate structure and remains stable at least up to 350MPa. Further compression up to ~400MPa resulted in a new Ne-hydrate, topologically similar to the H2-filled chiral C0-structure and the recently discovered C0 structure of CO2 (Amos et al. 2017). Upon decompression to 350MPa C0 structure reverted back to sII Ne-hydrate. Upon recovery at liq. N2 temperature to ambient pressure, cage occupancies in Ne- and D2-hydrates rapidly readjust by decreasing the number of guests in all cages. A similar pattern can be seen upon guest-removal under vacuum where larger cages empty much faster than the smaller ones; curiously D2 molecules at comparable conditions seems to leave the sII structure slower than the larger Ne atoms. Smaller cavities seem to be much more difficult to empty and may require the presence of water vacancies for the cage-to-cage hopping. The He-water system has proven to be the most peculiar one as the compression up to 300MPa did not yield any structural change. He-loaded ice Ih remained stable in the whole investigated pressure range. Still, for the first time we succeeded to form unequivocally sII He-clathrate by filling ice XVI at 120K with 150MPa of He gas; both, the crystal structure and the He-uptake and release, was studied as a function of pressure. Amos et al.(2017) J.Phys.Chem.Lett. 8, 4295-4299 Falenty, Hansen & Kuhs (2014) Nature 516, 231-233 Salamatin, Falenty, Hansen & Kuhs (2015) Energy & Fuels 29, 5681-5691
We represent two new high pressure phases in small gas - water systems: A Ne-hydrate with a chiral water structure and the long-searched Helium form of sII clathrate hydrate. These new structural forms are manifestations of the importance of van der Waals interactions between water and small gas molecules.
Prof. Werner F Kuhs (University of Göttingen)
Dr Andrzej Falenty (GZG Abt. Kristallographie Uni Göttingen) Dr Dirk Wallacher (Helmholtz-Zentrum für Materialien & Energie Berlin) Dr Thomas C Hansen (Institut Laue-Langevin Grenoble)