Temperature-Pressure Phase Diagram of a D2O-D2 System at Pressures to 1.8 kbar

Oct 26, 2026

Using a volumetric technique, deuterium solubility X in heavy water L, low-pressure hexagonal ice Ih, and high-pressure cubic clathrate ice sII is measured for deuterium pressures up to 1.8 kbar and temperatures from −40 to +5 °C. The L+Ih+sII triple point is determined at P=1.07(3) kbar and T=−4.5(8) °C, with molar ratios XL=0.020(5), XIh=0.012(5), and XsII=0.207(5). Phase-transition lines in the D2O-D2 system are constructed and compared for consistency with earlier H2O-H2 results.

LanguageEnglish

CategoryResearch & Report

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Pages6

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Introduction

Experimental Details

Results and Discussion

7026 J. Phys. Chem. B 2008, 112, 7026–7031 Temperature-Pressure Phase Diagram of a D2O-D2 System at Pressures to 1.8 kbar Vadim S. Eﬁmchenko,*,† Vladimir E. Antonov,† Oleg I. Barkalov,† and Semen N. Klyamkin‡ Institute of Solid State Physics, Russian Academy of Sciences, 142432 ChernogoloVka, Moscow District, Russia, and Department of Chemistry, Moscow State UniVersity, 119991 Moscow, Russia ReceiVed: January 17, 2008; In Final Form: March 24, 2008 Using a volumetric technique, the deuterium solubility, X, in heavy water (L), low-pressure hexagonal ice (Ih), and high-pressure cubic clathrate ice (sII) is studied at deuterium pressures up to 1.8 kbar and temperatures from-40 to +5 °C. The triple point of the L + Ih + sII equilibrium is located at P ) 1.07(3) kbar and T ) -4.5(8)°C. The molar ratios D2/D2O of phases at the triple point are XL ) 0.020(5), XIh ) 0.012(5), and XsII ) 0.207(5) . Introduction Phase transformations in the water-hydrogen system have recently attracted close attention due to the discovery of new hydrogen clathrate hydrate.1–3 The cubic unit cell of the sIItype2 crystal structure of this clathrate phase is composed of 136 H2O molecules and can accommodate from 27 .2 4 to 48 3 guest molecules of H2, which gives a molar ratio of H2/H2O ) 0.200-0.353. The clathrate with these compositions is formed at hydrogen pressures of 1-3 .6 kbar.1 If quenched under high pressure, the clathrate is metastable in vacuum at temperatures up to 145 K.2 The rather high hydrogen capacity makes the sII clathrate a promising candidate for hydrogen storage applications.2,5 The clathrate is of signiﬁcant interest for planetary science and astrophysics because its formation pressure is low enough tobe reached inside small icy satellites and because hydrogen trapped in interstellar ices can be in the form of this clathrate due to its thermal stability at low pressure.2 Some signiﬁcant properties of the hydrogen clathrate hydrate, such as the crystal structure,3 the maximum3 and minimum4 concentrations of the guest hydrogen molecules, and the formation kinetics,3 were studied by neutron diffraction on deuterium-substituted D2O-D2 samples. At the same time, the temperature-pressure (T-P) phase diagram of the D2O-D2 system has never been examined. In the present paper, we construct the lines of phase transitions between the liquid phase (L), low-pressure hexagonal ice (Ih), and clathrate phase (sII) and determine the deuterium content of these phases in the vicinity of the L-Ih-sII triple point in the D2O-D2 system using a volumetric technique. The feasibility of the obtained data was additionally tested and improved by comparing with earlier results for the H2O-H2 system.4,6 Experimental Details The experimental setup is described in ref 7 . The measurements were carried out in an atmosphere of molecular deuterium taken in excess. The D2 gas with isotopic purity 99.9% was prepared by thermal decomposition of a LaNi5-based deuteride. The heavy water had an atomic ratio of H/D ) 10-3. The amount of deuterium absorbed by the sample was determined volumetrically from the pressure and temperature * To whom correspondence should be addressed. E-mail: eﬁmchen@ [issp.ac.ru](issp.ac.ru). † Institute of Solid State Physics, Russian Academy of Sciences.‡ Department of Chemistry, Moscow State University. in the calibrated volume of the measuring system. The molar ratio X ) D2/D2O of the absorbed deuterium was calculated with an accuracy of (0.005 by using published data on the pressure and temperature dependences of the molar volumes of deuterium-substituted liquid water and ice, Ih,8 the sII phase2–4 and gaseous deuterium.9 The solubility of water in gaseous hydrogen was earlier shown to be vanishingly small at 27 °Cand pressures up to 6 kbar,10 so we neglected the solubility of heavy water in the deuterium gas in our experiments at pressuresup to 1.8 kbar. The deuterium dissolved in heavy water and ice Ih was assume

FAQs

What phases and ranges are studied in the D2O-D2 system?

Deuterium solubility is studied in heavy water (L), low-pressure hexagonal ice (Ih), and high-pressure cubic clathrate ice (sII). Measurements cover pressures up to 1.8 kbar and temperatures from −40 to +5 °C.

Where is the L + Ih + sII triple point located?

The triple point occurs at P=1.07(3) kbar and T=−4.5(8) °C. The corresponding molar ratios are XL=0.020(5), XIh=0.012(5), and XsII=0.207(5).

How were the transition lines and phase equilibria determined?

The T–P diagram lines for equilibria between L, Ih, and sII were constructed using experimental melting/formation and decomposition points, with additional feasibility testing by comparison to earlier H2O-H2 results.

Oct 26, 2026

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