Filling Ices with Helium and the Formation of Helium Clathrate Hydrate

We have formed the long-sought He-clathrate. This was achieved by refilling heliuminto ice XVI, opening a new synthesis route for exotic forms of clathrate hydrates. The process was followed by neutron diffraction; structures and cagefillings were established. The stabilizing attractive van der Waals interactions are enhanced by multiple cagefillings with theoretically up to four helium atomsper large cageand up to one per small cage; He- clathrate hydratescan be considered as a solid-state equivalent of the clustering of small apolar entities dissolved in the liquid state of water. Unlike most other guests, heliumeasily enters and leaves the water cagesat temperatures well below 100 K, hampering applications as a gas storage material. Despite the weak dispersive interactions, the inclusion of heliumhas a very significant effect on lattice constants; this is also established for heliuminclusion in ice Ihand suggests that lattice parameters are arguably the most sensitive measure to gauge dispers...