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T–P–X Phase Diagram of the Water–Hydrogen System at Pressures up to 10 kbar

Kuzovnikov, M. A., Tkacz, M.
Journal of physical chemistry 2019 v.123 no.6 pp. 3696-3702
hydrogen, liquids, melting, temperature, thermodynamics
The stability field of the C₀ hydrogen hydrate in the pressure–temperature phase diagram of the water–hydrogen system is studied by volumetry under hydrogen excess. This stability field is confined by the sII hydrate stability field at the low-pressure side; by the liquid stability field at the high-temperature side; and by the C₁ hydrate stability field at the high-pressure side. The pressure of the C₀ ↔ C₁ phase equilibrium is temperature-independent in the studied temperature range from −20 to +18 °C. The corresponding equilibrium line in the phase diagram terminates at the nonvariant quadruple point “C₀ hydrate + C₁ hydrate + liquid H₂O + H₂ gas” (or Q₃) at 7.7(3) kbar and +22(2) °C. The volume change accompanying the C₀ → C₁ phase transition, and the dTdPH2 slopes of the melting lines of the C₀ and C₁ hydrates are measured by volumetry. Thermodynamic considerations are used to estimate the hydrogen content H₂/H₂O ≈ 0.53(5) and 0.23(5) of the C₀ and C₁ hydrates, respectively, near the Q₃ point.