Pressure-induced amorphization of noble gas clathrate hydrates

The high-pressure structural behavior of the noble gas (Ng) clathrate hydrates $\mathrm{Ar}·6.5{\mathrm{H}}_2\mathrm{O}$ and $\mathrm{Xe}·7.2{\mathrm{H}}_2\mathrm{O}$ 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 $>1.5\mathrm{GPa}$ 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.

Figure 1

Unit cell of the cubic clathrate hydrate structures. (a) CS-I ($Pm\text{−}3n, a\approx 12\AA$), composed of 46 water molecules forming a hydrogen-bonded network around six large T cages (gray) and two small D cages (black). (b) CS-II ($Fd\text{−}3m$ space group, lattice parameter $a\approx 17\AA$), composed of 136 water molecules which build up 16 D cages and eight H cages (cyan). (c) The three different cages in cubic structured clathrate hydrates: D cages (pentagon dodecahedra, $5^{12}$), common for both CS-I and CS-II; T cages (tetrakaidecahedra, $5^{12}{6}^2$), large cages in CS-I; and H cages (hexakaidecahedra, $5^{12}{6}^4$), large cages in CS-II (and largest among all). T cages are represented in (c) with their two different types of pentagon faces: those constituting H cages only (black edges), and those constituting both H and D cages (white edges). Red spheres on cage vertexes represent oxygen atoms.

Figure 2

Experimental neutron powder diffraction (NPD) profiles of (a) CS-I Xe and (b) CS-II Ar clathrate hydrate. The diffractograms were measured at 95 K and at the lowest pressure point 0–0.1 GPa. Profiles obtained upon Rietveld refinement are shown as solid lines. The difference curves are plotted beneath. Tick marks show Bragg positions of allowed reflections for the fitted phases. Xe hydrate: $a=11.8662\left(6\right)\AA , R_{\mathrm{wp}}=3.73\%$, GoF $\left({\chi }^2\right)=3.04$. Ar hydrate: $a=17.040\left(3\right)\AA , R_{\mathrm{wp}}=1.37\%$, GoF $\left({\chi }^2\right)=2.90$.

Figure 3

Experimental pathways followed in this paper. The phase diagram of pure ice (from Salzmann, Ref. [54]) is underlaid as background. Ar hydrate (AH) experiments (green), AH1: Compression to PIA (1.4 GPa); AH2: Compression to 2.6 GPa; AH3: Compression to PIA and up to 3.15 GPa, temperature cycling to 170 K, cooling, decompression, and heating to 200 K at ambient pressure; AH4: Compression to 2 GPa, temperature cycling to 170 K, cooling, and decompression; AH5: Compression to 1.9 GPa, temperature cycling to 170 K, cooling, and decompression. Xe hydrate (XH) experiments (navy blue), XH1: Compression to 4.5 GPa; XH2: Compression to 4.8 GPa; XH3: Compression to 5.1 GPa, decompression to atmospheric pressure, and heating to 150 K; XH4: Compression to 4.9 GPa, temperature cycling to 170 K, cooling, decompression to 0.4 GPa, and heating to 200 K; XH5: Compression to 4 GPa, decompression to 2.5 GPa, heating to 170 K, and cooling back to 95 K, decompression to 0.48 GPa.

Figure 4

Evolution of the neutron powder diffraction (NPD) profiles of (a) CS-I Xe and (b) CS-II Ar hydrate upon compression. Tick marks show Bragg positions of allowed reflections for the fitted phases. Stars mark peaks from ice (111 reflection of ice VII or 112 reflection of ice VIII), which can be seen in (a) after recrystallization of high-density amorphous (HDA) above 2.5 GPa. Cross marks the 110 peak of iron as the steel anvil shades part of the beam at high pressures. Patterns were treated for background (including diffuse scattering from the samples) and smoothed for clarity.

Figure 5

Equations of state at 95 K of (a) Xe hydrate and (b) Ar hydrate. The line is a fit of the Murnaghan equation $V_0/V={[p(B^{\prime }/B)+1]}^{1/B^{\prime }}$ with $V_0,B$, and $B^{\prime }$ being the volume at zero pressure, the bulk modulus, and its first pressure derivative, respectively. The dashed line in (a) marks a transition to a semicrystalline state, as suggested from molecular dynamics (MD) simulations.

Figure 6

Density vs pressure comparison from neutron powder diffraction (NPD) experiments and molecular dynamics (MD) simulations. CS-I Xe hydrate (top) and CS-II Ar hydrate (bottom). MD data were rescaled for ${\mathrm{D}}_2\mathrm{O}$, from simulations of cage occupancies ideally fully occupied (100%) and approaching the experimental composition: 80% of D and T cages filled in Xe hydrate, 80% of D and H cages filled in Ar hydrate.

Figure 7

Pressure dependence of the pair-distribution functions from the simulated molecular dynamics (MD) Ar and Xe hydrate systems at 95 K. (a) and (b) Oxygen-oxygen pair distribution functions (PDFs) from the water structures. (c)–(f) O-Ng PDFs with respect to guests inside small (D) and large (T and H) cages. The PDFs after transitions (crystalline → amorphous and crystalline → semicrystalline for Ar and Xe, respectively) are highlighted with broken lines. The insets in (b), (c) and (e), (f) show coordination numbers for (${\mathrm{Ar}}^{\mathrm{H}}, {\mathrm{Ar}}^{\mathrm{D}}$) and (${\mathrm{Xe}}^{\mathrm{T}}, {\mathrm{Xe}}^{\mathrm{D}}$), respectively.

Figure 8

Neutron powder diffraction (NPD) patterns showing the structural progression of CS-I Xe hydrate with pressure in experiments and molecular dynamics (MD) snapshots. Black lines represent experimental data (experiment XH2, SM [29]). Red-dotted lines represent simulated patterns from MD snapshots. Patterns show no sudden change in the long-range structure, and the final pattern with still crystalline features is in good agreement with experimental data.

Figure 9

Molecular dynamics (MD) snapshots showing the major structural features of Ar and Xe hydrate prior and after discontinuous densification at 95 K. (a) Crystalline CS-II Ar hydrate at 1.4 GPa. (b) Crystalline CS-I Xe hydrate at 2.0 GPa. (c) Guest environments in Ar hydrate at 1.4 GPa (crystalline) and 1.7 GPa (amorphous). (d) Guest environments in Xe hydrate at 2.4 GPa (crystalline) and 2.6 and 2.9 GPa (semicrystalline with disordered large cages). Red spheres: O, blue: Xe, green: Ar, white: H.

Figure 10

Molecular dynamics (MD) derived density-pressure relations for (a) CS-I and (b) CS-II noble gas clathrate hydrates at 95 K. Both types of cages in each structure are completely filled, i.e., the compositions correspond to $\mathrm{Ng}·5.75{\mathrm{H}}_2\mathrm{O}$ for CS-I and $\mathrm{Ng}·5.67{\mathrm{H}}_2\mathrm{O}$ for CS-II.

Figure 11

Molecular dynamics (MD) derived density-pressure relations for CS-I and CS-II noble gas clathrate hydrates at 95 K considering different cage fillings. (a) Empty CS-I and Ne-Xe hydrate with only T or only D cages filled. (b) Empty CS-II (ice XVI) and Ne-Xe hydrate with only D or only H cages filled. CS-II Ne hydrate with only H cages doubly occupied (red line) and D cages singly occupied and H cages doubly occupied (olive line).