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Are “virtual ices” really virtual ? Formation and properties of Ice XVI - the lowest density crystalline form of ice

Vendredi 13 mars 2015 14:00 - Duree : 1 heure
Lieu : Amphi Chadwick, ILL 4 - 71 avenue des Martyrs - Grenoble

Orateur : Andrzej FALENTY (University of Göttingen)

Ice XVI belongs to a group of hydrogen bonded, open framework structures predicted to be stable low-temperature configurations of water at negative pressures [1]. These “virtual ices” are topologically equivalent to the empty clathrate structures ; a reference frames for the statistical thermodynamic theory of gas hydrates on which our understanding of stability limits, composition of clathrates is based with their vast implications to chemical- and geo- engineering problems. As these water-host frameworks are stabilized by guest molecules at normal p-T conditions the empty hydrates were considered experimentally difficult to access and could only be approximately calculated using thermodynamic reasoning. Recently we have managed to overcome this limitation and prepared one of these phases in a region of thermodynamic metastability by pumping on small particles of sII Ne-clathrate at temperatures of 140 K [2]. The leaching process has been observed in-situ by neutron diffraction between 100 and 145K providing activation energies and diffusion constants of Ne moving in the clathrate framework. The obtained empty water frame, ice XVI is so far the least dense of all known crystalline phases and is topological identical to SiO2-, Si- and Ge-clathrates (Si136, Ge136) and a hypothetical carbon clathrate (C136). The open water framework shows a marked negative thermal expansion at low temperature. Ice XVI, similar to ice Ih ; this can be attributed to the increased low-energy framework-bending modes for the empty structure. We have also observed a considerable lattice expansion upon gas removal quantifying the importance of attractive interactions of water and small gas molecules in clathrate hydrates and beyond. The structure is stable up to temperatures of 145 K at which it transforms into ice Ich [3].

[1] Conde M.M, Vega C., Tribello G.A., Slater B. The phase diagram of water at negative pressures : Virtual ices, J.Chem.Phys. 2009 ; 131 : 034510.

[2] Falenty A., Hansen T.C. and Kuhs W.F., Formation and properties of ice XVI obtained by emptying a type sII clathrate hydrate, Nature 2014 ; 516, 231-233, doi:10.1038/nature14014

[3] Kuhs W.F., Sippel C., Falenty, A., Hansen T.C. Extent and relevance of stacking disorder in ”ice Ic”. PNAS 2012 ; 109(52) : 21259-21264.

Contact : goarin@ill.eu

Discipline évènement : (Physique)
Entité organisatrice : (ILL)
Nature évènement : (Séminaire)
Evènement répétitif : (ILL Colloquium)
Site de l'évènement : Polygone scientifique

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