Enthalpy Relaxation Phenomenon of Heavy Ice

Abstract

The heat capacities of quenched and annealed heavy ice Ih were measured in the temperature range 14 to 300 K by an adiabatic calorimeter. A relaxational thermal anomaly was found at around 115K and this phenomenon was ascribed to the onset of deuteron ordering in the crystal. The average activation enthalpy of the relaxational process was determined to be (26±5) kJ mol^–1. Residual entropies of the crystal were recalculated on the basis of the present heat-capacity data combined with the revised values for enthalpy of vapourization, saturated vapour pressure, and spectroscopic entropy. They are (3.47±0.41) J K^–1mol^–1for the quenched crystal and (3.44±0.41) J K^–1mol^–1for the crystal annealed at 102–106 K for 264 h. The characteristics and the origin of the anomaly are discussed in comparison with that of ordinary ice.