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 I _h 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 I _h → sII and sII → I _h 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 I _h 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 → I _h transition, and it increases to 2 wt % (16 mol %) at a pressure of 1.8 kbar.