High-pressure transformations in xenon hydrates

Abstract

A high-pressure investigation of the Xe⋅H₂O chemical system was conducted by using diamond-anvil cell techniques combined with in situ Raman spectroscopy, synchrotron x-ray diffraction, and laser heating. Structure I xenon clathrate was observed to be stable up to 1.8 GPa, at which pressure it transforms to a new Xe clathrate phase stable up to 2.5 GPa before breaking down to ice VII plus solid xenon. The bulk modulus and structure of both phases were determined: 9 ± 1 GPa for Xe clathrate A with structure I (cubic, a = 11.595 ± 0.003 Å, V = 1,558.9 ± 1.2 ų at 1.1 GPa) and 45 ± 5 GPa for Xe clathrate B (tetragonal, a = 8.320 ± 0.004 Å, c = 10.287 ± 0.007 Å, V = 712.1 ± 1.2 ų at 2.2 GPa). The extended pressure stability field of Xe clathrate structure I (A) and the discovery of a second Xe clathrate (B) above 1.8 GPa have implications for xenon in terrestrial and planetary interiors.

With CH₄ or CO₂, xenon is among the gases that stabilize clathrate hydrates structure I through van der Waals interactions. The unit cell of such clathrates contains two kinds of cages: two pentagonal dodecahedra (12-sided polyhedron) and six tetrakaidecahedra (14-sided polyhedron). In the case of Xe hydrates, all cavities are filled so that its formula is ideal with 46 H₂O molecules and eight guest molecules (1). Compression experiments in a piston-cylinder apparatus have shown the stability of clathrate I up to 1.7 GPa and 65°C (2, 3), but no in situ structural nor optical analysis of Xe⋅H₂O compounds have been reported at high pressure. At ambient pressure, the water molecules forming the cavities do not interact specifically with the encaged molecules, so that the structure of Xe hydrates is the same as CH₄ hydrates. On increasing pressure, the water compresses leading eventually to structural changes and expulsion of the guest molecules. Consequently, the pressure evolution of the clathrates depends on the size of their guest species (4.4 Å for Xe versus 4.1 Å for CH₄) and might differ between CH₄ hydrates and Xe hydrates. It is therefore useful to compare it to the behavior of CH₄ hydrates under pressure (4–9).

Experiments were therefore conducted in diamond-anvil cells to examine the incorporation and possible high pressure–temperature chemical reactions of Xe with H₂O. A 250-μm hole in a rhenium gasket was half-filled with distilled deionized water, and xenon was then loaded cryogenically on top of it, in an inert N₂ atmosphere. Contamination of the samples by N₂ during loading can be disregarded by the absence of N₂ vibron in the Raman spectra. Pressure was determined with the ruby fluorescence technique (10). After closing the diamond-anvil cell at about 0.5 GPa, a defocused CO₂ laser beam was used to heat the sample, and therefore completely homogenize the Xe + H₂O sample by melting both phases. Homogenization of the starting products is essential because the clathrate hydrates are formed by a surface reaction in which the hydrate structure grows and encages guest molecules occupying the partially formed cavities (11). An alternative and conventional method to synthesize clathrates is to stir powdered ice under a certain pressure of gas until it reacts with ice to give gas hydrates (2, 12).

Raman spectra were collected by a single-grating ISA HR-460 spectrometer equipped with holographic notch filters and a charge-coupled device detector. The 488-nm argon laser excitation was selected and spectra were recorded by using a 300 grooves per mm grating mode, best suited for very broad spectral features. Synchrotron x-ray diffraction experiments were also carried out on beamline X17C at the National Synchrotron Light Source (Brookhaven, NY) by using an energy dispersive setup with 2θ = 8.0078° (13). Typical recording time for diffraction experiments was 30 min, and the cell was continuously rocked around a vertical axis from −8° to +8° with respect to the x-ray beam direction.

Growth of crystals from the initial Xe + H₂O mixture was observed by increasing pressure to 0.7-0.8 GPa at room temperature (Fig. 1 Left). Raman spectra then revealed broad features in the O—H stretching vibrations region above 3,000 cm⁻¹ (Fig. 2). These features are distinct from the Raman signature of any H₂O phases but are very similar to those reported for Ar clathrates (14). The structure of this Xe⋅H₂O phase (referred to as Xe clathrate A in this paper) was determined to be a primitive cubic cell by using the treor program (15) to index the synchrotron x-ray data (a = 11.595 Å, V = 1558.9 ų at 1.1 GPa; see Fig. 3 and Table 1). Such a structure is also consistent with the low temperature-ambient pressure clathrate structure I (1).

Figure 1.

Sample photographs taken in transmitted light. In this case, the excess xenon surrounds a droplet of water in which crystals of Xe clathrates are growing. Note that the darkening in the right picture is simultaneous with the release of xenon from the clathrate and the transition of ice VI to ice VII.

Figure 2.

(Left) Raman spectra after laser heating at 0.6 GPa and for the different phases (pressure indicated in GPa for each spectrum, letters A and B stand for clathrate A and B, respectively). (Right) Pressure shift of the most intense O—H stretching modes for Xe clathrate A (Bottom), ice VI (Middle), and ice VII (Top); the dashed line is from ref. 27 for ice VI and the triangle is from ref. 28 for ice VII.

Figure 3.

Indexed x-ray patterns of Xe clathrate A (Bottom), Xe clathrate B (Middle), and its final breakdown into solid Xe plus ice VII (Top).

Table 1.

Experimental d spacings and their fitted values for Xe clathrates A and B

Index hkl	d spacing, Å		δ,* Å
	Observed	Fitted
Xe clathrate A†
200	5.80	5.803	−0.003
210	5.19	5.194	−0.004
211	4.73	4.726	0.004
222	3.36	3.372	−0.012
320	3.21	3.205	0.005
321	3.10	3.101	−0.001
400	2.90	2.901	−0.001
410	2.82	2.827	−0.007
421	2.53	2.530	0.000
332	2.47	2.468	0.002
520	2.15	2.147	0.003
530	1.99	1.991	−0.001
600	1.93	1.928	0.002
611	1.88	1.879	0.001
630	1.73	1.731	−0.001
721	1.578	1.578	0.00
Xe clathrate B‡
110	5.94	5.996	−0.056
111	5.10	5.093	0.007
202	3.23	3.226	0.004
220	2.94	2.938	0.002
104	2.46	2.462	−0.002
320	2.31	2.312	−0.002
420	1.86	1.860	0.0
215	1.80	1.800	0.0

^*δ = d spacing_obs − d spacings_fit. 

^† Cubic unit cell; a = 11.595 ± 0.003 Å; V = 1158.88 ± 1.2 ų; P = 1.1 GPa. 

^‡ Tetragonal unit cell; a = 8.32 ± 0.004 Å; c = 10.287 ± 0.007 Å; V = 712.21 ± 1.2 ų; P = 2.2 GPa. 

With increasing pressure, the Raman-stretching modes shift to lower frequencies at about −128 cm⁻¹ per GPa (Fig. 2). As for the cell volume evolution, a fit to a second-order Birch–Murnaghan equation of state gives an isothermal bulk modulus of 9 ±1 GPa (Fig. 4). Xe clathrate A evolves with pressure similarly to structure I CH₄ clathrate as reported (8). We therefore propose that structure I clathrate is also stable with xenon as a guest for pressures ranging from 0.8 ± 0.1 GPa to 1.8 ± 0.05 GPa at room temperature, with 8 xenon atoms and 46 H₂O molecules per unit cell.

Figure 4.

Pressure dependence of the mean molecular volume (unit cell volume divided by the total number of molecules) for Xe clathrate A (●, this work; ○, CH₄-clathrates data from ref. 8) and Xe clathrate B [▴, this work; ▴, ice VI data from P. Dera (personal communication)] and ice VII (⧫, this work; ◊, data from ref. 33). All lines are second-order Birch–Murnaghan equations of state for Xe clathrates A and B (this work) and using the published K_T0 for ice VI (18) and ice VII (19, 34). (Inset) Xe clathrates phase transitions superimposed on the H₂O phase diagram.

At 1.8 GPa, though no morphological or color change was visually observed in the sample, modifications in the Raman spectra (Fig. 2) clearly indicate that Xe clathrate A transformed to a new phase (Xe clathrate B). This phase transition was confirmed by the x-ray data (Fig. 3) and is accompanied by the transition of the excess H₂O to ice VI. The x-ray pattern of Xe clathrate B is consistent with a primitive tetragonal cell (a = 8.320 ± 0.004 Å, c = 10.287 ± 0.007 Å, V = 712.1 ± 1.2 ų at 2.2 GPa; Fig. 3 and Table 1), though single-crystal x-ray diffraction experiments would be needed to reach firmer conclusions. Such displacive phase transitions are quite common among zeolites and clathrates; these transitions are nonquenchable and single crystals can be preserved through the transition. For example, the orthorhombic or tetragonal analcites become monoclinic at 0.4 GPa (16) and on increasing temperature above 65°C, the tetragonal melanophlogopite becomes cubic (17). The bulk modulus of phase B, as extracted from a second-order Birch–Murnaghan equation of state is 45 ± 5 GPa (Fig. 4), and no distinct Raman shift is observed (Fig. 2). It is worth noticing that clathrate B is stiffer than both ice VI and ice VII, the bulk modulus values of which are 17.8 and 27.8 GPa, respectively (18, 19); this probably results from the very tight packing of xenon inside the H₂O network of clathrate B.

The chemical formula per unit-cell of Xe clathrate B can be approximated from the following observations. (i) The Xe/H₂O content of Xe clathrate B can be estimated relative to Xe clathrate A from the ratio of Xe clathrate diffraction peaks heights (plane [210] of clathrate A and plane [110] of clathrate B) to xenon fluorescence peak heights. For both sets of xenon fluorescence bands the resulting Xe/H₂O measured ratio is twice that of Xe clathrate B. (ii) The orientation of the clathrates cristallites does not change on phase transition, therefore the (110) diffraction peak of phase B should be twice more intense than the (210) diffraction peak of phase A. (iii) No xenon is released from the clathrate, which is consistent with the absence of any crystalline xenon diffraction peak, and it is assumed than the volume of free H₂O does not change either. It follows that the Xe/H₂O molecular ratio is the same for both clathrates. Because the cell volume of phase B is about half that of phase A, phase B contains four xenon molecules per unit cell.

Combining the above observations leads to a 4Xe⋅24H₂O formula per unit cell in Xe clathrate B. Knowing the number of molecules for both phases, one can trace the pressure evolution of the mean molecular volume of both clathrate phases along with ice VI and VII as observed in x-ray data (Fig. 4). In contrast to CH₄-clathrates, the guest/host molecular ratio does not increase with pressure and no Xe clathrate has been observed beyond 2.55 GPa, whereas up to three phase transitions have been reported for CH₄ clathrates (5, 6, 8). Indeed, with further increase in pressure above 2.5 GPa, both Raman and x-ray features of the Xe clathrate B vanish with simultaneous appearance of solid xenon and ice VII peaks in x-ray data (Fig. 3). Experiments were repeated up to 9 GPa and always only solid xenon and ice VII were observed.

We noted that the Kα and K_β1 xenon fluorescence peaks are not only very intense in spectra from the Xe clathrate zones of the samples but are also present in patterns obtained from both the liquid H₂O and ice VI regions (Fig. 5). It is known that xenon readily dissolves in water and has the highest water solubility among hydrate-forming species (20), with a solubility of 110.9 (in c.c. at standard conditions, P = 0.1 MPa) at 19.6°C. In contrast, the solubility of argon is only 34.6 at 18.2°C (21); this result might apply as well for solid H₂O phases such as ice VI. The hypothesis that a significant amount of xenon exists in microinclusions in ice VI can be ruled out because the stability field of the phase is expanded. The larger stability field of both liquid H₂O (transition to ice VI at 1.8 GPa instead of 0.9 GPa for pure H₂O) and ice VI (transition to ice VII at 2.5 GPa instead of 2.2 GPa) can then be explained by the solubility of xenon in these phases and the lack of xenon incorporation in ice VII.

Figure 5.

Energy-dispersive x-ray data collected in the H₂O-rich regions of the samples as a function of pressure. Vertical lines show the position of the Xe K_α K_β1 fluorescence peaks (esc., escape peaks); major ice VI and ice VII diffraction peaks are off-scale.

Examining the extent to which xenon can be hosted in Earth materials is an important geochemical problem. Xenon is depleted by a factor of 20 in the atmospheres of Earth and Mars relative to the other rare gases neon, argon, and krypton (22); this is the so-called “missing Xe problem.” One possible explanation is the incorporation of xenon within rocks and minerals, perhaps under pressure. In fact, xenon compounds have been synthesized at ambient pressure (23, 24), but none apparently so far with major terrestrial materials without photolysis (ref. 24; see also ref. 25). In the ideal Xe clathrate I structure, each cage is occupied by 1 xenon atom. Meanwhile, in natural occurrences of clathrates, the maximum reported concentration of xenon is only ≈2 parts per million (e.g., for CH₄ clathrates obtained from the southeast coast of the United States; ref. 26). Although Xe clathrates have been thought to be a potentially important sink of xenon on both Earth and Mars (27, 28), there appears to be insufficient xenon in the sampled hydrates to account for its depletion in the terrestrial atmosphere. Similarly, field studies have confirmed that polar ices do not form an important sink for xenon on Earth (29).

The stability of Xe clathrate B at high pressure shown here reopens the discussion. Pressures of 1.8–2.5 GPa correspond to depths of 50–75 km, that is the upper mantle of Earth. It is useful to consider the uptake of terrestrial xenon in hydrated high-pressure rocks in the upper mantle rather than in marine sediment clathrates. Because such depths correspond to a temperature range of 540–750°C, the thermal stability of Xe clathrate B needs to be determined to reach a firmer conclusion. As for the depletion of martian xenon, the storage depth would have to be translated from 160 to 225 km because the gravity field is one-third the terrestrial value. Why then is only xenon depleted in terrestrial and martian atmospheres because argon also enhances the formation of clathrates up to 0.6 GPa at room temperature and up to 3.0 GPa at 140°C (14)? Previous studies of phase equilibria in rare gas–water systems under pressure (3, 30) have led to the conclusion that the hydrate stability diminishes from xenon to neon. Argon can enter or leave the cavity relatively easily; the enthalpy change (when 1 mol of inert gas is sorbed within the clathrate cavity) is −2.82 kcal/mol whereas it is −5.37 kcal/mol for xenon (11), which is larger (4.56-Å diameter). Thus, Xe clathrates are expected to be thermodynamically more stable than Ar clathrates at high pressures and temperatures, but this needs to be explored directly.