Phase transitions in the water-hydrogen system at pressures up to 4.7 kbar.

Using a volumetric technique, a T-P diagram of phase transformations between the hydrogen-rich clathrate hydrate (sII phase), hydrogen-filled ice II (C(1) phase), and the liquid (L) is studied in the H(2)O-H(2) system at pressures up to 4.7 kbar and temperatures from -22 to +15 degrees C. The volume and entropy effects of these transformations are established in the vicinity of the triple point of the L + sII + C(1) equilibrium located at P = 3.6(1) kbar and T = +1(1) degrees C. The estimated molar ratios H(2)/H(2)O of phases at the triple point are X(L) = 0.04(2), X(sII) = 0.32(2), and X(C1) = 0.10(2).