The low-temperature dynamics of recovered ice XII as studied by differential scanning calorimetry: a comparison with ice V

Samples of ice XII made on isobaric heating of high-density amorphous ice (HDA) were recovered at 77 K and characterized by X-ray diffraction. Studies by differential scanning calorimetry (DSC) revealed, in addition to the intense exotherm from the ice XII --> cubic ice transition, on heating at 30 K min(-1) a reversible endothermic step with an onset temperature (T(onset)) of 130 +/- 1 K, an increase in heat capacity (DeltaC(p)) of 1.9 +/- 0.2 J K(-1) mol(-1), and a width of 10 +/- 1 K. The effects of the annealing temperature (T(anneal)) for a fixed annealing time (t(anneal)), and of t(anneal) for a fixed T(anneal) on the enthalpy and entropy relaxations and recovery have been ascertained, and these are phenomenologically similar to those of a glass. Ice XII samples made at 77 K on pressurizing of hexagonal ice via HDA show an additional thermal effect, namely an irreversible exotherm centered at 133 (125) K on heating at 30 (10) K min(-1), which is attributed to stress/strain release. For comparison, recovered ice V samples were studied by DSC in the same manner. These also show, in addition to the exotherm from the ice V --> cubic ice transition, on first heating at 30 K min(-1) a reversible endothermic step with Tonset of 130 +/- 1 K, DeltaC(p) of 1.7 +/- 0.2 J K(-1) mol(-1), and width of 20 +/- 1 K, and annealing effects similar to those of ice XII. For ice V the existence of a temperature-dependent equilibrium between proton order and disorder, with proton order increasing with decreasing temperature, had been ascertained by Lobban et al. (J. Chem. Phys., 2000, 112, 7169). We thus interpret the endothermic step in unannealed ice V to kinetic unfreezing of proton order-disorder, the equilibrium line being attained by proton order --> disorder transition, and the effects of annealing to enthalpy and entropy loss during annealing by approaching the proton order disorder equilibrium, and on their subsequent recovery on reheating, in line with Handa et al. ( J. Phys. 1987, 48, C1- 435). The endothermic DSC features of ice XII are interpreted in the same manner, by relaxation of the frozen-in proton order-disorder towards equilibrium via proton order! disorder transition. For ice XII ( ice V) the maximal value of recovered configurational entropy corresponds to 0.17 J K(-1) mol(-1) (0.22 J K(-1) mol(-1)) after annealing at 115 K for 120 min (111 K for 90 min) which is 5.0% (6.5%) of the maximal value of 3.37 J K(-1) mol(-1) for complete proton order --> disorder transition. We further report the DSC features of low-density amorphous ice recorded on heating at 30 K min(-1), with T(onset) of an endothermic step at 134 +/- 2 K DeltaC(p) of 0.7 +/- 0.1 J K(-1) mol(-1), and we show the similarities of this endothermic feature to those of ice XII.