OS6A-2:PREPARATION AND CRYSTAL STRUCTURE OF THE EMPTY CLATHRATE STRUCURE TYPE II

Andrzej FALENTY1, Thomas C HANSEN2, Werner F KUHS1
1. GZG Abt.Kristallographie, Universität Göttingen, GERMANY;  2. Institut Laue-Langevin, FRANCE

    The water chemical activity of the empty clathrate structure is the reference frame for the statistical thermodynamic theory of gas hydrates. So far it is a hypothetical structure of which structural details are unknown, in particular the lattice constants and atomic positions defining the size of the empty cages. As the empty hydrates have never been obtained all their thermodynamic properties remain speculative and can only be approximately calculated by first principles methods or thermodynamic reasoning. We now have found a way to produce the crystalline empty water frame of clathrate hydrate structure type II in good quality. Its lattice constants and crystal structure determined by neutron diffraction will be presented. Similar to ice Ih the empty hydrate lattice shows negative thermal expansion at low temperature. The structure is stable up to temperatures of ~145K at which it transforms into a stacking-faulty ice I (“cubic ice”) [1]. We compare the results to theoretical predictions and discuss the implications of our findings for the statistical thermodynamic theory of gas hydrates. Finally it should be noted that by forming the empty hydrate sII we have realized for the first time experimentally the phase of water which is considered to be thermodynamically stable at negative pressures [2,3].

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    [2] 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.
    [3] Jacobson L.C., Hujo W., Molinero V. Thermodynamic stability and growth of guest-free clathrate hydrates: A low-density crystal phase of water, J.Phys.Chem. B 2009; 113: 10298-10307.