Formation, stability, and structure of helium hydrate at high pressure

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Abstract
Neutron powder diffraction experiments show that, in the presence of ice and at helium-gas pressures exceeding 0.28 GPa, a gas hydrate with the composition He(6+x)H2O is formed. Its host lattice closely resembles that of ice II. However, the stability range of the helium hydrate is considerably more extended as compared to that of ice II: helium hydrate melts and recrystallizes from the liquid and is more stable than ice III/IX or ice V, at least up to 0.5 GPa. The helium content increases with increasing-pressure/decreasing-temperature, but does not appear to follow an ideal solution behavior, as do the guests in some heavier noble-gas clathrates. The lattice parameters of helium hydrate are significantly different from those of ice II, while the atomic arrangement of the host lattice is very similar, showing full orientational order of the water molecules.

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