Formation and composition of the clathrate phase in the H2O-H2 system at pressures to 1.8 kbar

The transition of the hexagonal ice phase Ih to the clathrate phase sII has been found in the H2O-H2 system at a pressure of about 1 kbar under conditions of an excess of gaseous hydrogen. The pressures of the Ih → sII and sII → Ih transitions have been determined over a temperature range from −36 to −18°C, and the pressure dependence of the synthesis temperature of the clathrate phase from a liquid at pressures from 1.0 to 1.8 kbar has been constructed. The solubility of hydrogen in the Ih and sII phases and in liquid water has been measured. The concentration of hydrogen in the clathrate phase sII is about 1.2 wt % (10 mol %) near the boundary of the sII → Ih transition, and it increases to 2 wt % (16 mol %) at a pressure of 1.8 kbar.