Pure samples of H2O (D2O) ice IV and of H2O (D2O) ice XII were prepared by isobaric heating of high-density amorphous ice at a rate of ≈0.5 K min−1 up to ≈175 K and a pressure of 0.81 GPa for ice IV, and at a heating rate of ≈12 K min−1 and a pressure of 1.21 GPa for ice XII. The crystalline phases recovered at 77 K and 1 bar were characterized by X-ray diffraction and differential scanning calorimetry (DSC). X-Ray diffractograms of D2O ice IV show that it transforms directly into cubic ice, and not via some amorphous phase as intermediate. The DSC scans of all four types of crystalline phases recorded on heating revealed three thermal features in this order: a reversible endothermic step, an intense exotherm from the phase transition to cubic ice, and a weak exotherm from the cubic → hexagonal ice phase transition. For H2O (D2O) ice IV the onset and end temperature of the endothermic step (Tonset and Tend) are at 139.8 ± 0.3 and 147.1 ± 0.2 K (at 146.2 ± 0.8 and 150.2 ± 0.8 K) on heating at 30 K min−1, and the increase in heat capacity (ΔCp) is 1.2 ± 0.2 (0.5 ± 0.1) J K−1 mol−1. On heating at 5 K min−1, the extrapolated peak onset and peak minimum temperature (Te and Tmin) of the phase transition to cubic ice is at 148.7 ± 0.3 and 151.1 ± 0.1 K (at 152.7 ± 0.1 and 155.3 ± 0.2 K), and the enthalpy of the phase transition to cubic ice is −938 ± 14 (−986 ± 14) J mol−1. The corresponding values for H2O (D2O) ice XII are for the endothermic step on heating at 30 K min−1: Tonset and Tend at 131.4 ± 0.8 and 139.5 ± 0.7 K (at 138.1 ± 0.4 and 145.0 ± 0.5 K), and ΔCp is 1.5 ± 0.2 (1.4 ± 0.2) J K−1 mol−1. On heating at 5 K min−1, Te and Tmin of the phase transition to cubic ice is at 149.9 ± 0.3 and 153.0 ± 0.2 K (at 154.0 ± 1.2 and 156.9 ± 0.9 K), and the enthalpy of the phase transition to cubic ice is −1233 ± 23 (−1408 ± 8) J mol−1. Therefore, ΔH for the H2O (D2O) ice XII to H2O (D2O) ice IV phase transition is −295 (−422) J mol−1, and ice XII is more metastable than ice IV at ≈150 K and 1 bar. For H2O (D2O) ice XII the endothermic step is separated by a plateau region from the beginning of crystallization, whereas for H2O (D2O) ice IV these two thermal effects overlap because its Tonset is higher by ≈8 K. Tonset and Te are separated by 4.9 K for D2O ice IV and 7.3 K for H2O ice IV. Thus overlap is more pronounced for the former, the endothermic step being cut off by the beginning of crystallization, which is consistent with the lower ΔCp value for D2O ice IV. The endothermic DSC features are interpreted as before for H2O ice XII by Salzmann et al.
(Phys. Chem. Chem. Phys., 2003, 5, 3507), in terms of relaxation of the frozen-in proton order–disorder toward equilibrium via proton order → disorder transition. When H2O (D2O) cubic ice is obtained on heating ice IV, its phase transition to hexagonal ice has a ΔH value of −20 ± 7 (−15 ± 5) J mol−1 which is lower than most of the values reported in the literature. We further present new low-frequency Raman spectra of ice XII and show that a peak at 127 cm−1 has its counterpart in the Raman spectra of Chou et al.'s new “High-Pressure Phase of H2O Ice”
(Science, 1998, 281, 809). This supports our previous conclusion (J. Phys. Chem. B, 2002, 106, 1) that the Chou phase is in fact ice XII. We then conjecture on the solid–liquid phase boundaries of both ice IV and ice XII, and on a metastable triple point where ice IV, ice XII and liquid water are in metastable equilibrium.