Abstract
The high-pressure structural behavior of the noble gas (Ng) clathrate hydrates $\mathrm{Ar}\ifmmode\cdot\else\textperiodcentered\fi{}6.5\phantom{\rule{0.16em}{0ex}}{\mathrm{H}}_{2}\mathrm{O}$ and $\mathrm{Xe}\ifmmode\cdot\else\textperiodcentered\fi{}7.2\phantom{\rule{0.16em}{0ex}}{\mathrm{H}}_{2}\mathrm{O}$ featuring cubic structures II and I, respectively, was investigated by neutron powder diffraction (using the deuterated analogues) at 95 K. Both hydrates undergo pressure-induced amorphization (PIA), indicated by the disappearance of Bragg diffraction peaks, but at rather different pressures, at 1.4 and above 4.0 GPa, respectively. Amorphous Ar hydrate can be recovered to ambient pressure when annealed at $g1.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and 170 K and is thermally stable up to 120 K. In contrast, it was impossible to retain amorphous Xe hydrate at pressures below 3 GPa. Molecular dynamics (MD) simulations were used to obtain general insight into PIA of Ng hydrates, from Ne to Xe. Without a guest species, both cubic clathrate structures amorphize at 1.2 GPa, which is very similar to hexagonal ice. Filling of large-sized H cages does not provide stability toward amorphization for structure II, whereas filled small-sized dodecahedral D cages shift PIA successively to higher pressures with increasing size of the Ng guest. For structure I, filling of both kinds of cages, large-sized T and small-sized D, acts to stabilize in a cooperative fashion. Xe hydrate represents a special case. In MD, disordering of the guest hydration structure is already seen at around 2.5 GPa. However, the different coordination numbers of the two types of guests in the crystalline cage structure are preserved, and the state is shown to produce a Bragg diffraction pattern. The experimentally observed diffraction up to 4 GPa is attributed to this semicrystalline state.
Highlights
Clathrate hydrates are crystalline compounds consisting of a framework of tetrahedrally coordinated water molecules forming polyhedral cages which include guest species [1]
Molecular dynamics (MD) simulations were used to obtain general insight into pressure-induced amorphization (PIA) of noble gas (Ng) hydrates, from Ne to Xe. Both cubic clathrate structures amorphize at 1.2 GPa, which is very similar to hexagonal ice
The composition of the hydrates was assumed to be as earlier determined by Flacau et al [23], Xe · 7.2 H2O (D 76% filled and T 81%), and Yang et al and Brant Carvalho et al [6,24], Ar · 6.5 H2O (D 89% filled and H 83%)
Summary
Clathrate hydrates are crystalline compounds consisting of a framework of tetrahedrally coordinated water molecules forming polyhedral cages which include guest species [1]. Since the tetrahedral nearest neighbor arrangement of water molecules in the host structure of cubic clathrate hydrates is identical to ice Ih and the concentration of the guest is rather small, their pressure-induced amorphization (PIA) should follow the same or a very similar mechanism. CS-II Ar hydrate (AH) transforms successively into a hexagonal, tetragonal, and orthorhombic clathrate structure variant The latter is stable up to 6 GPa, after which decomposition into solid Ar and ice VII occurs [8,9,10,11]. The PIA behavior was examined with molecular dynamics (MD) simulations, addressing the complete series of Ng atoms, Ne through Xe
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