Thermal properties of metastable ices IV and XII: comparison, isotope effects and relative stabilities

Abstract

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 (T_onset and T_end) 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 (ΔC_p) 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 (T_e and T_min) 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: T_onset and T_end at 131.4 ± 0.8 and 139.5 ± 0.7 K (at 138.1 ± 0.4 and 145.0 ± 0.5 K), and ΔC_p is 1.5 ± 0.2 (1.4 ± 0.2) J K⁻¹ mol⁻¹. On heating at 5 K min⁻¹, T_e and T_min 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 T_onset is higher by ≈8 K. T_onset and T_e 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 ΔC_p 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.