Pressure-induced amorphization of noble gas clathrate hydrates

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 I, respectively, was investigated by neutron powder diffraction (using deuterated analogues) at 95 K. Both undergo pressure-induced amorphization (PIA), indicated disappearance Bragg peaks, but rather different pressures, 1.4 above 4.0 GPa, respectively. Amorphous Ar hydrate can be recovered to ambient pressure when annealed $>1.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ 170 K is thermally stable up 120 In contrast, it impossible retain amorphous Xe pressures below 3 GPa. Molecular dynamics (MD) simulations were used obtain general insight into PIA Ng hydrates, from Ne Xe. Without a guest species, both amorphize 1.2 which very similar hexagonal ice. Filling large-sized H cages does not provide stability toward for structure II, whereas filled small-sized dodecahedral D shift successively higher with increasing size guest. For filling kinds cages, T D, acts stabilize in cooperative fashion. represents special case. MD, disordering hydration already seen around 2.5 However, coordination numbers two types guests crystalline cage are preserved, state shown produce pattern. experimentally observed 4 GPa attributed this semicrystalline state.