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JETP Letters, Vol. 82, No. 7, 2005, pp. 413­415. Translated from Pis'ma v Zhurnal èksperimental'nooe i Teoreticheskooe Fiziki, Vol. 82, No. 7, 2005, pp. 464­466. Original Russian Text Copyright © 2005 by Barkalov, Klyamkin, Efimchenko, Antonov.

Formation and Composition of the Clathrate Phase in the H2O­H2 System at Pressures to 1.8 kbar
O. I. Barkalova, S. N. Klyamkinb, V. S. Efimchenkoa, and V. E. Antonova
a

Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow region, 142432 Russia e-mail: efimchen@issp.ac.ru b Moscow State University, Vorob'evy gory, Moscow, 119992 Russia
Received July 14, 2005

The transition of the hexagonal ice phase Ih to the clathrate phase sII has been found in the H2O­H2 system at a pressure of about 1 kbar under conditions of an excess of gaseous hydrogen. The pressures of the Ih sII and sII Ih transitions have been determined over a temperature range from ­36 to ­18°C, and the pressure dependence of the synthesis temperature of the clathrate phase from a liquid at pressures from 1.0 to 1.8 kbar has been constructed. The solubility of hydrogen in the Ih and sII phases and in liquid water has been measured. The concentration of hydrogen in the clathrate phase sII is about 1.2 wt % (10 mol %) near the boundary of the sII Ih transition, and it increases to 2 wt % (16 mol %) at a pressure of 1.8 kbar. © 2005 Pleiades Publishing, Inc. PACS numbers: 05.70.Fh

Dyadin et al. [1] studied the H2O­H2 system by differential thermal analysis and found a region with anomalous behaviors of the melting temperature and the kinetics of melting of ice in an atmosphere of hydrogen at pressures from 1 to 3.6 kbar. They hypothesized that the clathrate phase of hydrogen hydrate was formed in this pressure range. Indeed, the clathrate phase sII of hydrogen hydrate was synthesized more recently [2, 3] from a liquid at a pressure of about 2 kbar and at ­24°C. Lokshin et al. [3] studied the composition of this phase by neutron-diffraction analysis and found that H2/H2O = (32 + X)/136, where X varied from 0 to 16 depending on pressure and temperature. The value of X = 16 corresponds to the dissolution of 3.8 wt % hydrogen in ice. Because of such a high concentration of hydrogen in ice at the pressure P 2 kbar, which is reachable within ice planet satellites, a study of the H2O­H2 system at P 2 kbar is of considerable interest in both high-pressure physics and chemistry and planetology [2]. The aim of this work was to study experimentally the transition of the hexagonal phase Ih of low-pressure ice to the clathrate phase sII and vice versa and to study the transition from the liquid phase to the clathrate phase. The measurements were performed under conditions of an excess of molecular hydrogen. The transitions were determined from changes in the solubility of hydrogen in H2O at pressures from 0.2 to 1.8 kbar. The experimental setup has been described elsewhere [4]. High-purity hydrogen (99.9999%) was prepared by the thermal decomposition of the hydride of the Ti­Fe­V alloy. The amount of hydrogen absorbed by a sample was determined volumetrically by measur-

ing pressure and temperature in calibrated volumes of the measuring system. A modified van der Waals equation for strongly compressed hydrogen was used as the equation of state for the gaseous phase; this equation took into account the temperature and pressure dependence of coefficients [5]. Published data [6] on the pressure and temperature dependence of the molar volumes of liquid water and ice Ih were used. The molar volume of the clathrate phase sII was evaluated based on the published value [2] of the lattice parameter a 17.05 å of this cubic phase at P = 2.2 kbar and T = ­39°C. The sample of H2O had a volume of about 1 cm3 and consisted of individual segments with characteristic sizes of about 5 mm. The steady-state pressure reached after changing temperature or total hydrogen amount in an autoclave was measured in the experiments. The drift of pressure lasted about 5 min, about 1 h, or 3­ 5 min in the absence of phase transitions, in the Ih sII and sII Ih transitions, or in the synthesis of the sII phase from a liquid, respectively. With consideration for errors in the determination of various phase volumes in the H2O­H2 system, the concentration of hydrogen in condensed phases was determined to within ±0.05 wt %. Figure 1 shows a typical solubility isotherm of hydrogen in water ice. The concentration of hydrogen in the Ih phase at T = ­22°C increased from 0.1 wt % at P = 0.5 kbar to 0.3 wt % at P = 1 kbar. A transition to the clathrate phase sII occurred at a higher pressure, and the amount of dissolved hydrogen increased to 1.2 wt %. As the pressure was increased to 1.8 kbar, the solubility of hydrogen in the clathrate phase monotonically increased to approximately 2 wt %.

0021-3640/05/8207-0413$26.00 © 2005 Pleiades Publishing, Inc.


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BARKALOV et al.

­22°C

Fig. 1. Solubility of hydrogen in ice at ­22°C. Closed and open triangles refer to increasing and decreasing pressure, respectively.

Fig. 2. Solubility of hydrogen in H2O in the course of decreasing temperature at a pressure of 1.3 kbar.

A smooth decrease in the concentration of hydrogen in the clathrate phase was observed in the course of decreasing pressure at all of the test temperatures. For example, as can be seen in Fig. 1, at T = ­22°C, the composition decreased from 2 wt % at P = 1.8 kbar to 1.2 wt % at P 0.8 kbar; thereafter, a transition to the ice phase Ih occurred. Figure 2 shows the isobar of hydrogen solubility in liquid water and the sII phase at P 1.3 kbar. The amount of hydrogen dissolved in the liquid increased from 0.2 wt % at +16°C to 0.4 wt % at ­18°C. After the transition to the clathrate phase, the solubility of hydrogen increased to 1.9­2.0 wt %. The phase transition points in the H2O­H2 system determined in this work were plotted in the T­P diagram, which is shown in Fig. 3. The pressure of the transition of ice Ih to the clathrate phase sII is about 1 kbar, and it only slightly depends on temperature. The pressure of the reverse transition of the sII phase to ice Ih decreased with temperature from 0.9 kbar at ­18°C to 0.5 kbar at ­36°C. The synthesis of the clathrate phase from the liquid occurred at a temperature 5­7°C lower than the melting temperature of ice Ih at the same pressure in the absence of hydrogen. In the tested range of temperatures to ­36°C, the maximum concentration of hydrogen in ice Ih was about 0.3 wt %; this concentration was reached at the pressure P 1 kbar near the boundary of a transition to the clathrate phase sII (closed circles in Fig. 3). The concentration of hydrogen in the sII phase changed from approximately 1.2 wt % near the boundary of a transition to ice Ih (to the right of open circles in Fig. 3) to 2 wt % at a pressure of 1.8 kbar. The solubility of hydrogen in the Ih and sII phases over the given temperature and pressure ranges was not studied previously. Lokshin et al. [3] evaluated the hydrogen content of the sII phase at temperatures lower than ­73°C based on the most likely interpretation of

the neutron-diffraction patterns of D2O­D2 polycrystals measured at the two pressures 1 bar and 2 kbar over a temperature range from ­233 to ­73°C. According to this evaluation, the concentration of hydrogen in the sII phase of the H2O­H2 system can vary within a range from 2.5 wt % (H2/H2O = 32/136) to 3.8 wt % (H2/H2O = 48/136). The minimum value of 1.2 wt % obtained in this work for the concentration of hydrogen in the sII phase is lower than a value of 2.5 wt % by a factor of about 2; according to Lokshin et al. [3], the latter value is required for the stability of the clathrate structure sII. The linear extrapolation of the solubility isotherms of

Ih

Fig. 3. T­P diagram of the H2O­H2 system. Closed and open circles refer to Ih sII and sII Ih phase transition points, respectively. Closed triangles indicate the conditions of the synthesis of the sII phase from a liquid with decreasing temperature (this work) and the dashed horizontal segment refers to published data [2]. The dashed curve shows the melting line of the clathrate phase sII determined by Dyadin et al. [1] and the solid line shows the melting temperature of ice Ih in the absence of hydrogen [7]. JETP LETTERS Vol. 82 No. 7 2005


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hydrogen in the sII phase (e.g., see Fig. 1) to a maximum pressure of 3.6 kbar (according to Dyadin et al. [1], this is the maximum pressure at which this phase can exist) gave a value of about 4 wt %. This value is close to an estimated value of 3.8 wt % found by Lokshin et al. [3] for the maximally possible hydrogen concentration in the sII phase. This work was supported by the Russian Academy of Sciences (the program "The Physics and Mechanics of Strongly Compressed Matter"), the Russian Foundation for Basic Research (project no. 05-02-17733), and the Foundation for Support of Russian Science. REFERENCES
1. Yu. A. Dyadin, è. G. Larionov, and A. Yu. Manakov, Zh. Strukt. Khim. 40, 974 (1999).

2. W. L. Mao, H. Mao, A. F. Goncharov, et al., Science 297, 2247 (2002). 3. K. A. Lokshin, Y. Zhao, D. He, et al., Phys. Rev. Lett. 93, 125503 (2004). 4. S. N. Klyamkin and V. N. Verbetsky, J. Alloys Compd. 194, 41 (1993). 5. H. Hemmes, A. Driessen, and R. Griessen, J. Phys. C 19, 3571 (1986). 6. W. B. Bridgman, J. Chem. Phys. 3, 597 (1935). 7. E. Yu. Tonkov, High Pressure Phase Transformations: A Handbook (Metallurgiya, Moscow, 1988; Gordon and Breach, Philadelphia, 1992), Vol. 2, p. 448.

Translated by V. Makhlyarchuk

JETP LETTERS

Vol. 82

No. 7

2005