In order to investigate the causes of a spontaneous temperature increase upon annealing pure hexagonal and cubic ices, and of a sigmoidal increase in the heat capacity (Cp) upon heating the annealed samples, the enthalpy and entropy decrease during the annealing of pure cubic ice were determined. The decrease was much higher than that expected from the orientational relaxation of H2O molecules, and that calculated from a spontaneous decrease in Bjerrum or orientational defect concentrations. Combining this with the observation that dopants decreased the relaxation time of ices and modified the shape of the Cp-increase, it was concluded that the spontaneous temperature rise upon annealing occurred when some of the H2O molecules attained a preferential orientation. This was equivalent to partial proton ordering. The sigmoidal Cp change that was observed upon heating the annealed samples was attributed to time- and temperature-dependent recovery of their random orientations or complete proton disorder. This was very different to the structural relaxation in glasses, where the entropy of disorder increased upon heating. The known temperature dependence of the self-diffusion coefficient of water was used to calculate the size of the cooperatively rearranging regions (4.7 molecules at 150K) and the temperature-invariant energy (7.4kJ/mol).

On the Origin of the Heat Capacity Feature of Annealed Ices and Ice Clathrates, and Interpreting Water’s Diffusivity in Terms of the Entropy. G.P.Johari: Chemical Physics, 2000, 258[2-3], 277-90