A calorimetric study on the low temperature dynamics of doped ice V and its reversible phase transition to hydrogen ordered ice XIII

Abstract

Doped ice V samples made from solutions containing 0.01 M HCl (DCl), HF (DF), or KOH (KOD) in H₂O (D₂O) were slow-cooled from 250 to 77 K at 0.5 GPa. The effect of the dopant on the hydrogen disorder → order transition and formation of hydrogen ordered ice XIII was studied by differential scanning calorimetry (DSC) with samples recovered at 77 K. DSC scans of acid-doped samples are consistent with a reversible ice XIII ↔ ice V phase transition at ambient pressure, showing an endothermic peak on heating due to the hydrogen ordered ice XIII → disordered ice V phase transition, and an exothermic peak on subsequent cooling due to the ice V → ice XIII phase transition. The equilibrium temperature (T_o) for the ice V ↔ ice XIII phase transition is 112 K for both HCl doped H₂O and DCl doped D₂O. From the maximal enthalpy change of 250 J mol⁻¹ on the ice XIII → ice V phase transition and T_o of 112 K, the change in configurational entropy for the ice XIII → ice V transition is calculated as 2.23 J mol⁻¹K⁻¹ which is 66% of the Pauling entropy. For HCl, the most effective dopant, the influence of HCl concentration on the formation of ice XIII was determined: on decreasing the concentration of HCl from 0.01 to 0.001 M, its effectiveness is only slightly lowered. However, further HCl decrease to 0.0001 M drastically lowered its effectiveness. HF (DF) doping is less effective in inducing formation of ice XIII than HCl (DCl) doping. On heating at a rate of 5 K min⁻¹, kinetic unfreezing starts in pure ice V at ∼132 K, whereas in acid doped ice XIII it starts at about 105 K due to acceleration of reorientation of water molecules. KOH doping does not lead to formation of hydrogen ordered ice XIII, a result which is consistent with our powder neutron diffraction study (C. G. Salzmann, P. G. Radaelli, A. Hallbrucker, E. Mayer, J. L. Finney, Science, 2006, 311, 1758). We further conjecture whether or not ice XIII has a stable region in the water/ice phase diagram, and on a metastable triple point where ice XIII, ice V and ice II are in equilibrium.