Figure 1

Diffraction data from the initial sample as loaded in the Paris-Edinburgh press, the initial sample is shown to be a single phase of sI clathrate. The inset shows the sample after in situ annealing and recystallization into ice VIII and hexagonal clathrate at 32 kbar and 226 K.Reuse & Permissions

Figure 2

The experimental (a) and simulated (b) structure factor functions of the collapsed amorphous clathrate forms. The experimental data has been corrected for background, empty instrument, normalized to vanadium, and scaled at high-momentum transfer to the sum of the weighted scattering lengths of the constituent atoms, and the final $S$($Q$) is made to oscillate about one at high $Q$. The vertical arrows in (b) indicate the positions of the two fitted peaks in the simulation. An expanded region in the vicinity of the first diffraction peak is shown in (c), (d), and (e) and illustrates the effect of annealing the sample at the conditions given, the solid vertical bars mark the peak positions.Reuse & Permissions

Figure 3

The partial $S{\left(Q\right)}_{{\mathrm{D}}_{\mathrm{w}}{-\mathrm{D}}_{\mathrm{w}}}$ and $S{\left(Q\right)}_{{\mathrm{D}}_{\mathrm{w}}{-\mathrm{D}}_{\mathrm{m}}}$ function in (a) and (b), respectively, determined by molecular dynamic simulation. Here D${}_{\mathrm{w}}$ denotes water deuterium and D${}_{\mathrm{m}}$ denotes methane deuterium. Each panel shows data for 1 bar (sI crystalline), 20 kbar (sI crystalline), and 40 kbar (amorphous) forms. The vertical lines illustrate that the first peaks overlap in the crystalline phase but differ significantly in the amorphous phase.Reuse & Permissions

Figure 4

Experimentally determined total radial distribution functions after initial collapse of the crystal at 100 K and 32 kbar, upon further compression to 39 kbar at 100 K, and upon annealing at 39 kbar to 220 K (a). The neutron weighted total radial distribution function determined by molecular dynamics simulation at 39 kbar at 100 K (b). The time scale of the MD simulations precluded modeling the annealing process.Reuse & Permissions

Figure 5

The partial oxygen-to-oxygen radial distribution function $g$($r$)${}_{\mathrm{O}-\mathrm{O}}$ determined from the MD simulation (a). These data show that after collapse, at 40 kbar, the water network exhibits only limited short-range order. The partial carbon-to-carbon radial distribution function $g$($r$)${}_{\mathrm{C}-\mathrm{C}}$ determined from MD simulation (b). Even in the amorphous form there remains a significant degree of intermediate range ordering of methane within the amorphous water network.Reuse & Permissions

Figure 6

The partial carbon-to-water deuterium radial distribution functions $g{\left(r\right)}_{{\mathrm{C}-\mathrm{D}}_{\mathrm{w}}}$, as determined by the MD simulation, for both the small and large cage. Here C represents the carbon atom of the methane molecule and D${}_{\mathrm{w}}$ represents the deuterium atoms of the water molecules. Each panel shows partials from both small and large cages for 1 bar (a), 20 kbar (b), and 40 kbar (c).Reuse & Permissions

Figure 7

Experimental data showing the stability limits of various forms of water. The (dilatometric) glass transformations (red curve) and crystallization data (heavy black curve) of various forms of high-density amorphous ice over a broad pressure range. The reported glass transition temperatures (open diamonds) and crystallization temperature (solid circle) of low-density amorphous ice (LDA) at 1 bar. The melt line of both H${}_2$O and D${}_2$O are plotted below 3 kbar. The supercooling limit of liquid water upon cooling for both H${}_2$O and D${}_2$O are also plotted below 3 kbar (the arrow pointing downward indicates crystallization upon cooling). The extension of this curve up to 6 kbar is not experimental data but is taken to be parallel to the crystalline melt line over this pressure range. Also plotted here are the crystallization data for a binary mixture of amorphous water and methane from the current experiments (squares connected by dashed curve and the three arrows pointing upward indicate crystallization upon heating). The solid black squares are samples that crystallized into ice VIII, methane, and hexagonal hydrate, and the open square is the sample that crystallized into ice VI and hexagonal hydrate. These samples transformed from an amorphous binary methane water mixture to crystalline ice VIII + sH hydrate over 2 h between 220 and 225 K. Analysis of the ice VIII cluster size indicates the methane could have diffused a maximum of 116 Å through the water network at temperatures near 220 K.Reuse & Permissions