Pressure-induced amorphization of noble gas clathrate hydrates

Abstract

The high-pressure structural behavior of the noble gas (Ng) clathrate hydrates Ar • 6.5 H 2 O and Xe • 7.2 H 2 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 >1.5 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.

Structural investigation of three distinct amorphous forms of Ar hydrate

“…Fully deuterated CS-II Nat Ar hydrate and 36 Ar hydrate with a composition Ar$6.5H 2 O were synthesized at the SNAP beamline at the Spallation Neutron Source (SNS), Oak Ridge National Laboratory (ONRL), USA, as described by Brant Carvalho et al 24 The samples were virtually free from unreacted ice I h . Pressurization and heating experiments were performed using a standard VX Paris-Edinburgh (PE) pressure cell with liquid nitrogen (LN 2 )-cooled stainless steel anvils and monitored in situ by timeof-ight neutron powder diffraction (NPD).…”

“…Details on data reduction have been discussed in the supplemental material of an earlier publication. 24 The steps for extracting neutron structure factor functions S(Q) from the original diffraction data were performed with the Mantid package (MantidPlot and Workbench). 26 Diffraction patterns of crystalline samples were subjected to Rietveld renement, performed with GSAS-II.…”

“…This form obtained from initial PIA is termed HDA, in analogy to ice I h and according to the terminology used in previous works. 17,24 To prepare the VHDA state, the HDA samples were warmed to 170 K at 2 GPa and kept at the target temperature for 1 hour before cooling back to 95 K. Subsequently the VHDA samples were depressurized at 95 K. The amorphous state was retained at ambient pressure. The recovered amorphs of Ar and 36 Ar hydrates, which are crystallization-resistant up to $120 K, will be called RA.…”

“…PIA is initiated by the deformation of the 6-membered rings of the large H cages. 17,24,25 When approaching PIA, the translational motion of Ar atoms inside the cages decreased drastically. A similar hindrance of rotational motion of molecular guests was found in MD simulations of PIA of CS-II THF hydrate 17 and CS-I CH 4 hydrate.…”

“…Brant Carvalho et al attempted to provide a more unied picture on clathrate hydrate amorphization by studying the noble gas series Ne-Xe for which both types of clathrate structures are realized and the size effect of the guest can be studied without a signicant change of the host-guest interaction. 24 Major ndings include that PIA of clathrate hydrates can be expected in the pressure range 1.3 to 2.5 GPa, and that resistance toward PIA increases when both types of cages are occupied and with increasing size of the guest species.…”