Elucidating the guest disorder in structure II argon hydrate – A neutron diffraction isotopic substitution study

Clathrate hydrates with the cubic structure II (CS-II) form typically with large guest molecules, such as tetrahydrofuran, trimethylamine oxide, or propane. However, CS-II is also realized for argon hydrate despite the comparatively small van der Waals diameter of the guest (around 3.8 ​Å). Here, the structure of deuterated argon hydrate was studied at ambient pressure in the temperature range 20–95 ​K using neutron diffraction and comparing natural Ar with ³⁶Ar, which scatters neutrons more than 13 times more efficiently. The procedure allowed to unambiguously establish the positional disorder within the large cages of CS-II, while simultaneously refining host and guest structures. These cages are singly occupied and off-centered argon atoms distribute on two tetrahedron-shaped split positions with a ratio 3:1. Molecular dynamics (MD) simulations revealed that the crystallographic positional disorder structure is due to mobile argon atoms even at 20 ​K. The MD potential energy distribution confirmed the diffraction model. It is noted that the unit cell volumes of argon hydrate in the investigated temperature range are virtually identical to N₂ hydrate, which has a similar composition at ambient pressure, indicating a very similar (slightly attractive) host-guest interaction.