New phase in the water–hydrogen system

A new hydrogen hydrate denoted C0 is discovered in the H2O–H2 system at −20 °C and hydrogen pressures near 5 kbar. Powder samples of the hydrate were quenched under high pressure to the liquid N2 temperature and recovered to ambient pressure. Hot extraction in vacuum demonstrated that the hydrogen release from the quenched samples started immediately on heating above 77 K and completed at about 170 K. The total amount of the released hydrogen reached a molecular ratio of H2/H2O ≈ 0.13. An X-ray diffraction investigation at 80 K showed that the samples were single-phase and had a trigonal structure with a = 6.33 Å and c = 6.20 Å on hexagonal axes.

Research highlights▶ A new hydrogen hydrate denoted C0 is discovered in the H2O-H2 system. ▶ The C0 phase is shown to be stable at hydrogen pressures from 3.5 to 8 kbar. ▶ The hydrogen content of the C0 phase reaches a molecular ratio of H2/H2O ≥ 0.13. ▶ The C0 phase has a trigonal structure never observed in other gas hydrates.