High-pressure transformations in xenon hydratesChrystele Sanloup, Ho-kwang Mao, and Russell J. Hemley*

Geophysical Laboratory and Center for High Pressure Research, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington DC 20015

Contributed by Ho-kwang Mao, November 10, 2001

A high-pressure investigation of the Xe�H2O chemical system wasconducted by using diamond-anvil cell techniques combined within situ Raman spectroscopy, synchrotron x-ray diffraction, and laserheating. Structure I xenon clathrate was observed to be stable upto 1.8 GPa, at which pressure it transforms to a new Xe clathratephase stable up to 2.5 GPa before breaking down to ice VII plussolid xenon. The bulk modulus and structure of both phases weredetermined: 9 � 1 GPa for Xe clathrate A with structure I (cubic, a �

11.595 � 0.003 Å, V � 1,558.9 � 1.2 Å3 at 1.1 GPa) and 45 � 5 GPafor Xe clathrate B (tetragonal, a � 8.320 � 0.004 Å, c � 10.287 �

0.007 Å, V � 712.1 � 1.2 Å3 at 2.2 GPa). The extended pressurestability field of Xe clathrate structure I (A) and the discovery of asecond Xe clathrate (B) above 1.8 GPa have implications for xenonin terrestrial and planetary interiors.

W ith CH4 or CO2, xenon is among the gases that stabilizeclathrate hydrates structure I through van der Waals

interactions. The unit cell of such clathrates contains two kindsof cages: two pentagonal dodecahedra (12-sided polyhedron)and six tetrakaidecahedra (14-sided polyhedron). In the caseof Xe hydrates, all cavities are filled so that its formula is idealwith 46 H2O molecules and eight guest molecules (1). Com-pression experiments in a piston-cylinder apparatus haveshown 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�H2Ocompounds have been reported at high pressure. At ambientpressure, the water molecules forming the cavities do notinteract specifically with the encaged molecules, so that thestructure of Xe hydrates is the same as CH4 hydrates. Onincreasing pressure, the water compresses leading eventuallyto structural changes and expulsion of the guest molecules.Consequently, the pressure evolution of the clathrates dependson the size of their guest species (4.4 Å for Xe versus 4.1 Å forCH4) and might differ between CH4 hydrates and Xe hydrates.It is therefore useful to compare it to the behavior of CH4hydrates under pressure (4–9).

Experiments were therefore conducted in diamond-anvil cellsto examine the incorporation and possible high pressure–temperature chemical reactions of Xe with H2O. A 250-�m holein a rhenium gasket was half-filled with distilled deionized water,and xenon was then loaded cryogenically on top of it, in an inertN2 atmosphere. Contamination of the samples by N2 duringloading can be disregarded by the absence of N2 vibron in theRaman spectra. Pressure was determined with the ruby fluores-cence technique (10). After closing the diamond-anvil cell atabout 0.5 GPa, a defocused CO2 laser beam was used to heat thesample, and therefore completely homogenize the Xe � H2Osample by melting both phases. Homogenization of the startingproducts is essential because the clathrate hydrates are formedby a surface reaction in which the hydrate structure grows andencages guest molecules occupying the partially formed cavities(11). An alternative and conventional method to synthesizeclathrates is to stir powdered ice under a certain pressure of gasuntil it reacts with ice to give gas hydrates (2, 12).

Raman spectra were collected by a single-grating ISA HR-460spectrometer equipped with holographic notch filters and acharge-coupled device detector. The 488-nm argon laser exci-tation was selected and spectra were recorded by using a 300grooves per mm grating mode, best suited for very broad spectral

features. Synchrotron x-ray diffraction experiments were alsocarried out on beamline X17C at the National Synchrotron LightSource (Brookhaven, NY) by using an energy dispersive setupwith 2� � 8.0078° (13). Typical recording time for diffractionexperiments was 30 min, and the cell was continuously rockedaround a vertical axis from �8° to �8° with respect to the x-raybeam direction.

Growth of crystals from the initial Xe � H2O mixture wasobserved by increasing pressure to 0.7-0.8 GPa at room temper-ature (Fig. 1 Left). Raman spectra then revealed broad featuresin the OOH stretching vibrations region above 3,000 cm�1 (Fig.2). These features are distinct from the Raman signature of anyH2O phases but are very similar to those reported for Arclathrates (14). The structure of this Xe�H2O phase (referred toas Xe clathrate A in this paper) was determined to be a primitivecubic cell by using the TREOR program (15) to index thesynchrotron x-ray data (a � 11.595 Å, V � 1558.9 Å3 at 1.1 GPa;see Fig. 3 and Table 1). Such a structure is also consistent withthe low temperature-ambient pressure clathrate structure I (1).

With increasing pressure, the Raman-stretching modes shift tolower frequencies at about �128 cm�1 per GPa (Fig. 2). As forthe cell volume evolution, a fit to a second-order Birch–Murnaghan equation of state gives an isothermal bulk modulusof 9 �1 GPa (Fig. 4). Xe clathrate A evolves with pressuresimilarly to structure I CH4 clathrate as reported (8). Wetherefore propose that structure I clathrate is also stable withxenon as a guest for pressures ranging from 0.8 � 0.1 GPa to1.8 � 0.05 GPa at room temperature, with 8 xenon atoms and 46H2O molecules per unit cell.

At 1.8 GPa, though no morphological or color change wasvisually observed in the sample, modifications in the Ramanspectra (Fig. 2) clearly indicate that Xe clathrate A trans-formed to a new phase (Xe clathrate B). This phase transitionwas confirmed by the x-ray data (Fig. 3) and is accompaniedby the transition of the excess H2O to ice VI. The x-ray patternof 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 � 1.2Å3 at 2.2 GPa; Fig. 3 and Table 1), though single-crystal x-raydiffraction experiments would be needed to reach firmerconclusions. Such displacive phase transitions are quite com-mon among zeolites and clathrates; these transitions arenonquenchable and single crystals can be preserved throughthe transition. For example, the orthorhombic or tetragonalanalcites become monoclinic at 0.4 GPa (16) and on increasingtemperature above 65°C, the tetragonal melanophlogopitebecomes cubic (17). The bulk modulus of phase B, as extractedfrom a second-order Birch–Murnaghan equation of state is45 � 5 GPa (Fig. 4), and no distinct Raman shift is observed(Fig. 2). It is worth noticing that clathrate B is stiffer than bothice VI and ice VII, the bulk modulus values of which are 17.8and 27.8 GPa, respectively (18, 19); this probably results fromthe very tight packing of xenon inside the H2O network ofclathrate B.

*To whom reprints should be requested. E-mail: hemley@gl.ciw.edu.

The publication costs of this article were defrayed in part by page charge payment. Thisarticle must therefore be hereby marked “advertisement” in accordance with 18 U.S.C.§1734 solely to indicate this fact.

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The chemical formula per unit-cell of Xe clathrate B can beapproximated from the following observations. (i) The Xe�H2O content of Xe clathrate B can be estimated relative to Xeclathrate A from the ratio of Xe clathrate diffraction peaksheights (plane [210] of clathrate A and plane [110] of clathrateB) to xenon f luorescence peak heights. For both sets of xenonf luorescence bands the resulting Xe�H2O measured ratio istwice that of Xe clathrate B. (ii) The orientation of theclathrates cristallites does not change on phase transition,therefore the (110) diffraction peak of phase B should be twicemore intense than the (210) diffraction peak of phase A. (iii)No xenon is released from the clathrate, which is consistentwith the absence of any crystalline xenon diffraction peak, andit is assumed than the volume of free H2O does not changeeither. It follows that the Xe�H2O molecular ratio is the samefor both clathrates. Because the cell volume of phase B is abouthalf that of phase A, phase B contains four xenon moleculesper unit cell.

Combining the above observations leads to a 4Xe�24H2Oformula per unit cell in Xe clathrate B. Knowing the numberof molecules for both phases, one can trace the pressure

evolution of the mean molecular volume of both clathratephases along with ice VI and VII as observed in x-ray data (Fig.4). In contrast to CH4-clathrates, the guest�host molecularratio does not increase with pressure and no Xe clathrate hasbeen observed beyond 2.55 GPa, whereas up to three phasetransitions have been reported for CH4 clathrates (5, 6, 8).

Fig. 1. Sample photographs taken in transmitted light. In this case, the excessxenon surrounds a droplet of water in which crystals of Xe clathrates aregrowing. Note that the darkening in the right picture is simultaneous with therelease of xenon from the clathrate and the transition of ice VI to ice VII.

Fig. 2. (Left) Raman spectra after laser heating at 0.6 GPa and for thedifferent phases (pressure indicated in GPa for each spectrum, letters A and Bstand for clathrate A and B, respectively). (Right) Pressure shift of the mostintense OOH 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 isfrom ref. 28 for ice VII.

Fig. 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 Xeclathrates A and B

Index hkl

d spacing, Å

�,* ÅObserved Fitted

Xe clathrate A†

200 5.80 5.803 �0.003210 5.19 5.194 �0.004211 4.73 4.726 0.004222 3.36 3.372 �0.012320 3.21 3.205 0.005321 3.10 3.101 �0.001400 2.90 2.901 �0.001410 2.82 2.827 �0.007421 2.53 2.530 0.000332 2.47 2.468 0.002520 2.15 2.147 0.003530 1.99 1.991 �0.001600 1.93 1.928 0.002611 1.88 1.879 0.001630 1.73 1.731 �0.001721 1.578 1.578 0.00Xe clathrate B‡

110 5.94 5.996 �0.056111 5.10 5.093 0.007202 3.23 3.226 0.004220 2.94 2.938 0.002104 2.46 2.462 �0.002320 2.31 2.312 �0.002420 1.86 1.860 0.0215 1.80 1.800 0.0

*� � d spacingobs � d spacingsfit.†Cubic unit cell; a � 11.595 � 0.003 Å; V � 1158.88 � 1.2 Å3; P � 1.1 GPa.‡Tetragonal unit cell; a � 8.32 � 0.004 Å; c � 10.287 � 0.007 Å; V � 712.21 �1.2 Å3; P � 2.2 GPa.

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Indeed, with further increase in pressure above 2.5 GPa, bothRaman and x-ray features of the Xe clathrate B vanish withsimultaneous appearance of solid xenon and ice VII peaks inx-ray data (Fig. 3). Experiments were repeated up to 9 GPa andalways only solid xenon and ice VII were observed.

We noted that the K� and K�1 xenon fluorescence peaks arenot only very intense in spectra from the Xe clathrate zones ofthe samples but are also present in patterns obtained from boththe liquid H2O and ice VI regions (Fig. 5). It is known that xenonreadily dissolves in water and has the highest water solubilityamong hydrate-forming species (20), with a solubility of 110.9 (inc.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 mightapply as well for solid H2O phases such as ice VI. The hypothesisthat a significant amount of xenon exists in microinclusions in iceVI can be ruled out because the stability field of the phase isexpanded. The larger stability field of both liquid H2O (transi-tion to ice VI at 1.8 GPa instead of 0.9 GPa for pure H2O) andice VI (transition to ice VII at 2.5 GPa instead of 2.2 GPa) canthen be explained by the solubility of xenon in these phases andthe lack of xenon incorporation in ice VII.

Examining the extent to which xenon can be hosted in Earthmaterials is an important geochemical problem. Xenon is de-pleted by a factor of 20 in the atmospheres of Earth and Marsrelative to the other rare gases neon, argon, and krypton (22);this is the so-called ‘‘missing Xe problem.’’ One possible expla-nation is the incorporation of xenon within rocks and minerals,perhaps under pressure. In fact, xenon compounds have beensynthesized at ambient pressure (23, 24), but none apparently sofar with major terrestrial materials without photolysis (ref. 24;see also ref. 25). In the ideal Xe clathrate I structure, each cageis occupied by 1 xenon atom. Meanwhile, in natural occurrencesof clathrates, the maximum reported concentration of xenon isonly �2 parts per million (e.g., for CH4 clathrates obtained from

the southeast coast of the United States; ref. 26). Although Xeclathrates have been thought to be a potentially important sinkof xenon on both Earth and Mars (27, 28), there appears to beinsufficient xenon in the sampled hydrates to account for itsdepletion in the terrestrial atmosphere. Similarly, field studieshave confirmed that polar ices do not form an important sink forxenon on Earth (29).

The stability of Xe clathrate B at high pressure shown herereopens the discussion. Pressures of 1.8–2.5 GPa correspondto depths of 50–75 km, that is the upper mantle of Earth. It isuseful to consider the uptake of terrestrial xenon in hydratedhigh-pressure rocks in the upper mantle rather than in marinesediment clathrates. Because such depths correspond to atemperature range of 540–750°C, the thermal stability of Xeclathrate B needs to be determined to reach a firmer conclu-sion. As for the depletion of martian xenon, the storage depthwould have to be translated from 160 to 225 km because thegravity field is one-third the terrestrial value. Why then is onlyxenon depleted in terrestrial and martian atmospheres becauseargon also enhances the formation of clathrates up to 0.6 GPaat room temperature and up to 3.0 GPa at 140°C (14)?Previous studies of phase equilibria in rare gas–water systemsunder pressure (3, 30) have led to the conclusion that thehydrate stability diminishes from xenon to neon. Argon canenter 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 areexpected to be thermodynamically more stable than Ar clath-rates at high pressures and temperatures, but this needs to beexplored directly.

We acknowledge helpful comments from I. M. Chou and Y. A. Dyadin.This work has been financially supported by the National ScienceFoundation, the National Aeronautics and Space Administration,the Department of Energy, Carnegie Canada, and the W. M. KeckFoundation.

Fig. 4. Pressure dependence of the mean molecular volume (unit cell volumedivided by the total number of molecules) for Xe clathrate A (F, this work; E,CH4-clathrates data from ref. 8) and Xe clathrate B [Œ, this work; Œ, ice VI datafrom P. Dera (personal communication)] and ice VII (}, this work; {, data fromref. 33). All lines are second-order Birch–Murnaghan equations of state for Xeclathrates A and B (this work) and using the published KT0 for ice VI (18) andice VII (19, 34). (Inset) Xe clathrates phase transitions superimposed on the H2Ophase diagram.

Fig. 5. Energy-dispersive x-ray data collected in the H2O-rich regions of thesamples as a function of pressure. Vertical lines show the position of the Xe K�

K�1 fluorescence peaks (esc., escape peaks); major ice VI and ice VII diffractionpeaks are off-scale.

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