Structural Investigations of Argon Hydrates at Pressures up to 10 kbar

Neutron diffraction patterns of three argon hydrates which exist at the pressures up to 10 kbar has been studied; Rietveld refinement of their structures has been done. The phase which is stable from 1 bar to 4.6 kbar appears to be typical cubic structure II gas hydrate with variable degree of filling of the large cavities. Stoichiometry of this compound under high-pressure conditions has been determined for the first time and appears to be ArW4.5H2O and Ar·4H2O at 3.4 and 4.3 kbar, respectively. Gas hydrate existing in the pressure range of 4.6–7.7 kbar has a hexagonal structure (hexagonal structure III, so-called structure H). Refinement of the structure has shown that the best agreement between calculated and experimental pattern can be reached in the case of accommodation of five (!) argon atoms in the large cavity. Indexing of the neutron diffraction pattern of the hydrate stable in the 7.7–9.5 kbar range leads to the primitive tetragonal unit cell with parameters a = 6.342 Å, c = 10.610 Å at 9.2 kbar, which does not correspond to any known type of gas hydrates. The water framework of this structure was found by idealizing the structure of pinacol semiclathrate hydrate. This hydrate belongs to a new, earlier unknown, tetragonal structural type of gas hydrates. It contains only one type of polyhedral cavities with 14 faces. This type of polyhedrons are space-filling; two argon atoms occupy each cavity. This structure gives the first example of the gas hydrate water framework which contains only one type of polyhedral cavities.