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

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 d T d P H 2 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.