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

Abstract

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 watermolecules1. 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.Anatural point of reference for this large and ubiquitous family of inclusion compounds is the empty hydrate lattice1-6, 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 ofmetastability of theempty hydrate. It is the seventeenth experimentally established crystalline ice phase11, ice XVI according to the current ice nomenclature, has a density of 0.81 gramsper 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 theempty hydrate expected toimprove our understanding of the structure, properties and behaviour of these unique materials.