Thermal properties of metastable ice XII

Samples of ice XII containing varying amounts of high-density amorphous water (HDA) were prepared by pressurizing hexagonal ice up to 1.8 GPa at 77 K, and they were characterized by X-ray diffraction and differential scanning calorimetry (DSC). Only exotherms were observed in the DSC curves on heating at a rate of 10 K min(-1) up to the melting point: conversion of HDA to low-density amorphous water (LDA) at 124 K, LDA-->cubic ice transition at 166 K, and at 155 K from ice XII to cubic ice. Thus, unlike ice VIII, ice XII converts directly into cubic ice, and not via LDA. The enthalpy of the ice XII-->cubic ice transition is -1.27 +/- 0.05 kJ mol(-1). From comparison with the reported enthalpy value of the ice V-->cubic ice transition of -0.915 kJ mol(-1) determined at similar temperature (Handa, Y. P.; Klug, D. D.; Whalley, E. Can. J. Chem. 1988, 66, 919), we conclude that ice XII is metastable with respect to ice V in the investigated temperature range.