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Formation and properties of ice XVI obtained by emptying a type sII clathrate hydrate


Gas hydrates are ice-like solids, in which guest molecules or atoms are trapped inside cages formed within a crystalline host framework (clathrate) of hydrogen-bonded water molecules1. They are naturally present in large quantities on the deep ocean floor and as permafrost, can form in and block gas pipelines, and are thought to occur widely on Earth and beyond. A natural point of reference for this large and ubiquitous family of inclusion compounds is the empty hydrate lattice1,2,3,4,5,6, which is usually regarded as experimentally inaccessible because the guest species stabilize the host framework. However, it has been suggested that sufficiently small guests may be removed to leave behind metastable empty clathrates7,8, and guest-free Si- and Ge-clathrates have indeed been obtained9,10. Here we show that this strategy can also be applied to water-based clathrates: five days of continuous vacuum pumping on small particles of neon hydrate (of structure sII) removes all guests, allowing us to determine the crystal structure, thermal expansivity and limit of metastability of the empty hydrate. It is the seventeenth experimentally established crystalline ice phase11, ice XVI according to the current ice nomenclature, has a density of 0.81 grams per cubic centimetre (making it the least dense of all known crystalline water phases) and is expected7,12 to be the stable low-temperature phase of water at negative pressures (that is, under tension). We find that the empty hydrate structure exhibits negative thermal expansion below about 55 kelvin, and that it is mechanically more stable and has at low temperatures larger lattice constants than the filled hydrate. These observations attest to the importance of kinetic effects and host–guest interactions in clathrate hydrates, with further characterization of the empty hydrate expected to improve our understanding of the structure, properties and behaviour of these unique materials.

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Figure 1: Leaching of Ne atoms from the sII clathrate structure.
Figure 2: Lattice constants and linear expansivity of the hydrates investigated.

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We thank the Bundesministeriums für Bildung und Forschung (BMBF) for financial support in the context of the first and second phase of the German SUGAR (SUbmarine Gashydrat-Lagerstätten: Erkundung, Abbau und TRansport) project. We thank the Institut Laue-Langevin (ILL) for beam time and support. We are also grateful for the assistance of H. Bartels (Göttingen), U. Kahmann (Göttingen) and A. Daramsy (ILL), as well as for discussions with P. Lafond (Göttingen).

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Authors and Affiliations



W.F.K. and A.F. designed the study and prepared the Ne-hydrate samples; A.F., T.C.H. and W.F.K. performed the leaching and diffraction experiments; T.C.H. and W.F.K. analysed the data; and W.F.K. wrote the paper with contributions from A.F. and T.C.H.

Corresponding author

Correspondence to Werner F. Kuhs.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Cage filling as a function of time for different temperatures.

A shrinking core model of the Ne leaching process fits values from sequential Rietveld refinement. Filled data points (and solid line for the fits) present the data for small cages (Ne1); open data points (and dashed lines) show data for the large cages (Ne2). Red circles and lines represent 110 K, orange squares and lines 120 K, green triangles and lines 130 K, light blue triangles and lines 135 K, dark blue flat rhombi and lines 140 K, magenta upright rhombi and lines 145 K.

Extended Data Figure 2 Diffraction patterns of Ne-filled and empty hydrate.

a, b, Rietveld fit (obtained using FullProf software32) to diffraction pattern of empty sII D2O hydrate (a) and Ne D2O hydrate (b) taken at 5 K (λ ≈ 1.1226 Å) on D20, ILL/Grenoble. The observed intensity is represented by open black circles, the calculated intensity as a blue line, the difference of both by a green line, grey shading marks the angular regions excluded in the refinement, red lines mark the positions of Bragg peaks of the hydrate, violet lines those of the aluminium sample can and orange lines those of ice Ic.

Extended Data Table 1 Polynomial coefficients of lattice constant fits*

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This zipped file contains 2 crystallographic information files. (ZIP 13 kb)

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Falenty, A., Hansen, T. & Kuhs, W. Formation and properties of ice XVI obtained by emptying a type sII clathrate hydrate. Nature 516, 231–233 (2014).

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