Phase transitions and equilibrium hydrogen content of phases in the water-hydrogen system at pressures to 1.8 kbar

Using a volumetric technique, the hydrogen solubility X in liquid water (L), low-pressure hexagonal ice (I-h), and high-pressure cubic clathrate ice (sII) is studied at hydrogen pressures up to 1.8 kbar and temperatures from -36 to +20 degrees C. The triple point of the L + I-h + s(II) equilibrium is located at P = 1.07(5) kbar and T = - 10(1) degrees C. The hydrogen concentrations of phases at the triple point are X-L = 0.17(5), X-Ih = 0.10(5) and X-SII = 2.3(1) wt.% H-2. The thermal stability and the process of decomposition of the clathrate phase at ambient pressure are studied by neutron diffraction.